doctest — Test interactive Python examples¶Source code: Lib/doctest.py
The doctest module searches for pieces of text that look like interactive
Python sessions, and then executes those sessions to verify that they work
exactly as shown. There are several common ways to use doctest:
Here’s a complete but small example module:
"""
This is the "example" module.
The example module supplies one function, factorial(). For example,
>>> factorial(5)
120
"""
def factorial(n):
"""Return the factorial of n, an exact integer >= 0.
>>> [factorial(n) for n in range(6)]
[1, 1, 2, 6, 24, 120]
>>> factorial(30)
265252859812191058636308480000000
>>> factorial(-1)
Traceback (most recent call last):
...
ValueError: n must be >= 0
Factorials of floats are OK, but the float must be an exact integer:
>>> factorial(30.1)
Traceback (most recent call last):
...
ValueError: n must be exact integer
>>> factorial(30.0)
265252859812191058636308480000000
It must also not be ridiculously large:
>>> factorial(1e100)
Traceback (most recent call last):
...
OverflowError: n too large
"""
import math
if not n >= 0:
raise ValueError("n must be >= 0")
if math.floor(n) != n:
raise ValueError("n must be exact integer")
if n+1 == n: # catch a value like 1e300
raise OverflowError("n too large")
result = 1
factor = 2
while factor <= n:
result *= factor
factor += 1
return result
if __name__ == "__main__":
import doctest
doctest.testmod()
If you run example.py directly from the command line, doctest
works its magic:
$ python example.py
$
There’s no output! That’s normal, and it means all the examples worked. Pass
-v to the script, and doctest prints a detailed log of what
it’s trying, and prints a summary at the end:
$ python example.py -v
Trying:
factorial(5)
Expecting:
120
ok
Trying:
[factorial(n) for n in range(6)]
Expecting:
[1, 1, 2, 6, 24, 120]
ok
And so on, eventually ending with:
Trying:
factorial(1e100)
Expecting:
Traceback (most recent call last):
...
OverflowError: n too large
ok
2 items passed all tests:
1 tests in __main__
8 tests in __main__.factorial
9 tests in 2 items.
9 passed and 0 failed.
Test passed.
$
That’s all you need to know to start making productive use of doctest!
Jump in. The following sections provide full details. Note that there are many
examples of doctests in the standard Python test suite and libraries.
Especially useful examples can be found in the standard test file
Lib/test/test_doctest.py.
The simplest way to start using doctest (but not necessarily the way you’ll
continue to do it) is to end each module M with:
if __name__ == "__main__":
import doctest
doctest.testmod()
doctest then examines docstrings in module M.
Running the module as a script causes the examples in the docstrings to get executed and verified:
python M.py
This won’t display anything unless an example fails, in which case the failing
example(s) and the cause(s) of the failure(s) are printed to stdout, and the
final line of output is ***Test Failed*** N failures., where N is the
number of examples that failed.
Run it with the -v switch instead:
python M.py -v
and a detailed report of all examples tried is printed to standard output, along with assorted summaries at the end.
You can force verbose mode by passing verbose=True to testmod(), or
prohibit it by passing verbose=False. In either of those cases,
sys.argv is not examined by testmod() (so passing -v or not
has no effect).
There is also a command line shortcut for running testmod(). You can
instruct the Python interpreter to run the doctest module directly from the
standard library and pass the module name(s) on the command line:
python -m doctest -v example.py
This will import example.py as a standalone module and run
testmod() on it. Note that this may not work correctly if the file is
part of a package and imports other submodules from that package.
Another simple application of doctest is testing interactive examples in a text
file. This can be done with the testfile() function:
import doctest
doctest.testfile("example.txt")
That short script executes and verifies any interactive Python examples
contained in the file example.txt. The file content is treated as if it
were a single giant docstring; the file doesn’t need to contain a Python
program! For example, perhaps example.txt contains this:
The ``example`` module
======================
Using ``factorial``
-------------------
This is an example text file in reStructuredText format. First import
``factorial`` from the ``example`` module:
>>> from example import factorial
Now use it:
>>> factorial(6)
120
Running doctest.testfile("example.txt") then finds the error in this
documentation:
File "./example.txt", line 14, in example.txt
Failed example:
factorial(6)
Expected:
120
Got:
720
As with testmod(), testfile() won’t display anything unless an
example fails. If an example does fail, then the failing example(s) and the
cause(s) of the failure(s) are printed to stdout, using the same format as
testmod().
By default, testfile() looks for files in the calling module’s directory.
See section Basic API for a description of the optional arguments
that can be used to tell it to look for files in other locations.
Like testmod(), testfile()‘s verbosity can be set with the
-v command-line switch or with the optional keyword argument
verbose.
There is also a command line shortcut for running testfile(). You can
instruct the Python interpreter to run the doctest module directly from the
standard library and pass the file name(s) on the command line:
python -m doctest -v example.txt
Because the file name does not end with .py, doctest infers that
it must be run with testfile(), not testmod().
For more information on testfile(), see section Basic API.
This section examines in detail how doctest works: which docstrings it looks at, how it finds interactive examples, what execution context it uses, how it handles exceptions, and how option flags can be used to control its behavior. This is the information that you need to know to write doctest examples; for information about actually running doctest on these examples, see the following sections.
The module docstring, and all function, class and method docstrings are searched. Objects imported into the module are not searched.
In addition, if M.__test__ exists and “is true”, it must be a dict, and each
entry maps a (string) name to a function object, class object, or string.
Function and class object docstrings found from M.__test__ are searched, and
strings are treated as if they were docstrings. In output, a key K in
M.__test__ appears with name
<name of M>.__test__.K
Any classes found are recursively searched similarly, to test docstrings in their contained methods and nested classes.
CPython implementation detail: Prior to version 3.4, extension modules written in C were not fully searched by doctest.
In most cases a copy-and-paste of an interactive console session works fine, but doctest isn’t trying to do an exact emulation of any specific Python shell.
>>> # comments are ignored
>>> x = 12
>>> x
12
>>> if x == 13:
... print("yes")
... else:
... print("no")
... print("NO")
... print("NO!!!")
...
no
NO
NO!!!
>>>
Any expected output must immediately follow the final '>>> ' or '... '
line containing the code, and the expected output (if any) extends to the next
'>>> ' or all-whitespace line.
The fine print:
Expected output cannot contain an all-whitespace line, since such a line is
taken to signal the end of expected output. If expected output does contain a
blank line, put <BLANKLINE> in your doctest example each place a blank line
is expected.
All hard tab characters are expanded to spaces, using 8-column tab stops.
Tabs in output generated by the tested code are not modified. Because any
hard tabs in the sample output are expanded, this means that if the code
output includes hard tabs, the only way the doctest can pass is if the
NORMALIZE_WHITESPACE option or directive
is in effect.
Alternatively, the test can be rewritten to capture the output and compare it
to an expected value as part of the test. This handling of tabs in the
source was arrived at through trial and error, and has proven to be the least
error prone way of handling them. It is possible to use a different
algorithm for handling tabs by writing a custom DocTestParser class.
Output to stdout is captured, but not output to stderr (exception tracebacks are captured via a different means).
If you continue a line via backslashing in an interactive session, or for any other reason use a backslash, you should use a raw docstring, which will preserve your backslashes exactly as you type them:
>>> def f(x):
... r'''Backslashes in a raw docstring: m\n'''
>>> print(f.__doc__)
Backslashes in a raw docstring: m\n
Otherwise, the backslash will be interpreted as part of the string. For example,
the \n above would be interpreted as a newline character. Alternatively, you
can double each backslash in the doctest version (and not use a raw string):
>>> def f(x):
... '''Backslashes in a raw docstring: m\\n'''
>>> print(f.__doc__)
Backslashes in a raw docstring: m\n
The starting column doesn’t matter:
>>> assert "Easy!"
>>> import math
>>> math.floor(1.9)
1
and as many leading whitespace characters are stripped from the expected output
as appeared in the initial '>>> ' line that started the example.
By default, each time doctest finds a docstring to test, it uses a
shallow copy of M‘s globals, so that running tests doesn’t change the
module’s real globals, and so that one test in M can’t leave behind
crumbs that accidentally allow another test to work. This means examples can
freely use any names defined at top-level in M, and names defined earlier
in the docstring being run. Examples cannot see names defined in other
docstrings.
You can force use of your own dict as the execution context by passing
globs=your_dict to testmod() or testfile() instead.
No problem, provided that the traceback is the only output produced by the example: just paste in the traceback. [1] Since tracebacks contain details that are likely to change rapidly (for example, exact file paths and line numbers), this is one case where doctest works hard to be flexible in what it accepts.
Simple example:
>>> [1, 2, 3].remove(42)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ValueError: list.remove(x): x not in list
That doctest succeeds if ValueError is raised, with the list.remove(x):
x not in list detail as shown.
The expected output for an exception must start with a traceback header, which may be either of the following two lines, indented the same as the first line of the example:
Traceback (most recent call last):
Traceback (innermost last):
The traceback header is followed by an optional traceback stack, whose contents are ignored by doctest. The traceback stack is typically omitted, or copied verbatim from an interactive session.
The traceback stack is followed by the most interesting part: the line(s) containing the exception type and detail. This is usually the last line of a traceback, but can extend across multiple lines if the exception has a multi-line detail:
>>> raise ValueError('multi\n line\ndetail')
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ValueError: multi
line
detail
The last three lines (starting with ValueError) are compared against the
exception’s type and detail, and the rest are ignored.
Best practice is to omit the traceback stack, unless it adds significant documentation value to the example. So the last example is probably better as:
>>> raise ValueError('multi\n line\ndetail')
Traceback (most recent call last):
...
ValueError: multi
line
detail
Note that tracebacks are treated very specially. In particular, in the
rewritten example, the use of ... is independent of doctest’s
ELLIPSIS option. The ellipsis in that example could be left out, or
could just as well be three (or three hundred) commas or digits, or an indented
transcript of a Monty Python skit.
Some details you should read once, but won’t need to remember:
Doctest can’t guess whether your expected output came from an exception
traceback or from ordinary printing. So, e.g., an example that expects
ValueError: 42 is prime will pass whether ValueError is actually
raised or if the example merely prints that traceback text. In practice,
ordinary output rarely begins with a traceback header line, so this doesn’t
create real problems.
Each line of the traceback stack (if present) must be indented further than the first line of the example, or start with a non-alphanumeric character. The first line following the traceback header indented the same and starting with an alphanumeric is taken to be the start of the exception detail. Of course this does the right thing for genuine tracebacks.
When the IGNORE_EXCEPTION_DETAIL doctest option is specified,
everything following the leftmost colon and any module information in the
exception name is ignored.
The interactive shell omits the traceback header line for some
SyntaxErrors. But doctest uses the traceback header line to
distinguish exceptions from non-exceptions. So in the rare case where you need
to test a SyntaxError that omits the traceback header, you will need to
manually add the traceback header line to your test example.
For some SyntaxErrors, Python displays the character position of the
syntax error, using a ^ marker:
>>> 1 1
File "<stdin>", line 1
1 1
^
SyntaxError: invalid syntax
Since the lines showing the position of the error come before the exception type
and detail, they are not checked by doctest. For example, the following test
would pass, even though it puts the ^ marker in the wrong location:
>>> 1 1
File "<stdin>", line 1
1 1
^
SyntaxError: invalid syntax
A number of option flags control various aspects of doctest’s behavior.
Symbolic names for the flags are supplied as module constants, which can be
bitwise ORed together and passed to various functions.
The names can also be used in doctest directives,
and may be passed to the doctest command line interface via the -o option.
New in version 3.4: The -o command line option.
The first group of options define test semantics, controlling aspects of how doctest decides whether actual output matches an example’s expected output:
doctest.DONT_ACCEPT_TRUE_FOR_1¶By default, if an expected output block contains just 1, an actual output
block containing just 1 or just True is considered to be a match, and
similarly for 0 versus False. When DONT_ACCEPT_TRUE_FOR_1 is
specified, neither substitution is allowed. The default behavior caters to that
Python changed the return type of many functions from integer to boolean;
doctests expecting “little integer” output still work in these cases. This
option will probably go away, but not for several years.
doctest.DONT_ACCEPT_BLANKLINE¶By default, if an expected output block contains a line containing only the
string <BLANKLINE>, then that line will match a blank line in the actual
output. Because a genuinely blank line delimits the expected output, this is
the only way to communicate that a blank line is expected. When
DONT_ACCEPT_BLANKLINE is specified, this substitution is not allowed.
doctest.NORMALIZE_WHITESPACE¶When specified, all sequences of whitespace (blanks and newlines) are treated as
equal. Any sequence of whitespace within the expected output will match any
sequence of whitespace within the actual output. By default, whitespace must
match exactly. NORMALIZE_WHITESPACE is especially useful when a line of
expected output is very long, and you want to wrap it across multiple lines in
your source.
doctest.ELLIPSIS¶When specified, an ellipsis marker (...) in the expected output can match
any substring in the actual output. This includes substrings that span line
boundaries, and empty substrings, so it’s best to keep usage of this simple.
Complicated uses can lead to the same kinds of “oops, it matched too much!”
surprises that .* is prone to in regular expressions.
doctest.IGNORE_EXCEPTION_DETAIL¶When specified, an example that expects an exception passes if an exception of
the expected type is raised, even if the exception detail does not match. For
example, an example expecting ValueError: 42 will pass if the actual
exception raised is ValueError: 3*14, but will fail, e.g., if
TypeError is raised.
It will also ignore the module name used in Python 3 doctest reports. Hence both of these variations will work with the flag specified, regardless of whether the test is run under Python 2.7 or Python 3.2 (or later versions):
>>> raise CustomError('message')
Traceback (most recent call last):
CustomError: message
>>> raise CustomError('message')
Traceback (most recent call last):
my_module.CustomError: message
Note that ELLIPSIS can also be used to ignore the
details of the exception message, but such a test may still fail based
on whether or not the module details are printed as part of the
exception name. Using IGNORE_EXCEPTION_DETAIL and the details
from Python 2.3 is also the only clear way to write a doctest that doesn’t
care about the exception detail yet continues to pass under Python 2.3 or
earlier (those releases do not support doctest directives and ignore them as irrelevant comments). For example:
>>> (1, 2)[3] = 'moo'
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: object doesn't support item assignment
passes under Python 2.3 and later Python versions with the flag specified, even though the detail changed in Python 2.4 to say “does not” instead of “doesn’t”.
Changed in version 3.2: IGNORE_EXCEPTION_DETAIL now also ignores any information relating
to the module containing the exception under test.
doctest.SKIP¶When specified, do not run the example at all. This can be useful in contexts where doctest examples serve as both documentation and test cases, and an example should be included for documentation purposes, but should not be checked. E.g., the example’s output might be random; or the example might depend on resources which would be unavailable to the test driver.
The SKIP flag can also be used for temporarily “commenting out” examples.
doctest.COMPARISON_FLAGS¶A bitmask or’ing together all the comparison flags above.
The second group of options controls how test failures are reported:
doctest.REPORT_UDIFF¶When specified, failures that involve multi-line expected and actual outputs are displayed using a unified diff.
doctest.REPORT_CDIFF¶When specified, failures that involve multi-line expected and actual outputs will be displayed using a context diff.
doctest.REPORT_NDIFF¶When specified, differences are computed by difflib.Differ, using the same
algorithm as the popular ndiff.py utility. This is the only method that
marks differences within lines as well as across lines. For example, if a line
of expected output contains digit 1 where actual output contains letter
l, a line is inserted with a caret marking the mismatching column positions.
doctest.REPORT_ONLY_FIRST_FAILURE¶When specified, display the first failing example in each doctest, but suppress
output for all remaining examples. This will prevent doctest from reporting
correct examples that break because of earlier failures; but it might also hide
incorrect examples that fail independently of the first failure. When
REPORT_ONLY_FIRST_FAILURE is specified, the remaining examples are
still run, and still count towards the total number of failures reported; only
the output is suppressed.
doctest.FAIL_FAST¶When specified, exit after the first failing example and don’t attempt to run the remaining examples. Thus, the number of failures reported will be at most 1. This flag may be useful during debugging, since examples after the first failure won’t even produce debugging output.
The doctest command line accepts the option -f as a shorthand for -o
FAIL_FAST.
New in version 3.4.
doctest.REPORTING_FLAGS¶A bitmask or’ing together all the reporting flags above.
There is also a way to register new option flag names, though this isn’t
useful unless you intend to extend doctest internals via subclassing:
doctest.register_optionflag(name)¶Create a new option flag with a given name, and return the new flag’s integer
value. register_optionflag() can be used when subclassing
OutputChecker or DocTestRunner to create new options that are
supported by your subclasses. register_optionflag() should always be
called using the following idiom:
MY_FLAG = register_optionflag('MY_FLAG')
Doctest directives may be used to modify the option flags for an individual example. Doctest directives are special Python comments following an example’s source code:
directive ::= "#" "doctest:"directive_optionsdirective_options ::=directive_option(","directive_option)\* directive_option ::=on_or_offdirective_option_nameon_or_off ::= "+" \| "-" directive_option_name ::= "DONT_ACCEPT_BLANKLINE" \| "NORMALIZE_WHITESPACE" \| ...
Whitespace is not allowed between the + or - and the directive option
name. The directive option name can be any of the option flag names explained
above.
An example’s doctest directives modify doctest’s behavior for that single
example. Use + to enable the named behavior, or - to disable it.
For example, this test passes:
>>> print(list(range(20))) # doctest: +NORMALIZE_WHITESPACE
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
Without the directive it would fail, both because the actual output doesn’t have two blanks before the single-digit list elements, and because the actual output is on a single line. This test also passes, and also requires a directive to do so:
>>> print(list(range(20))) # doctest: +ELLIPSIS
[0, 1, ..., 18, 19]
Multiple directives can be used on a single physical line, separated by commas:
>>> print(list(range(20))) # doctest: +ELLIPSIS, +NORMALIZE_WHITESPACE
[0, 1, ..., 18, 19]
If multiple directive comments are used for a single example, then they are combined:
>>> print(list(range(20))) # doctest: +ELLIPSIS
... # doctest: +NORMALIZE_WHITESPACE
[0, 1, ..., 18, 19]
As the previous example shows, you can add ... lines to your example
containing only directives. This can be useful when an example is too long for
a directive to comfortably fit on the same line:
>>> print(list(range(5)) + list(range(10, 20)) + list(range(30, 40)))
... # doctest: +ELLIPSIS
[0, ..., 4, 10, ..., 19, 30, ..., 39]
Note that since all options are disabled by default, and directives apply only
to the example they appear in, enabling options (via + in a directive) is
usually the only meaningful choice. However, option flags can also be passed to
functions that run doctests, establishing different defaults. In such cases,
disabling an option via - in a directive can be useful.
doctest is serious about requiring exact matches in expected output. If
even a single character doesn’t match, the test fails. This will probably
surprise you a few times, as you learn exactly what Python does and doesn’t
guarantee about output. For example, when printing a dict, Python doesn’t
guarantee that the key-value pairs will be printed in any particular order, so a
test like
>>> foo()
{"Hermione": "hippogryph", "Harry": "broomstick"}
is vulnerable! One workaround is to do
>>> foo() == {"Hermione": "hippogryph", "Harry": "broomstick"}
True
instead. Another is to do
>>> d = sorted(foo().items())
>>> d
[('Harry', 'broomstick'), ('Hermione', 'hippogryph')]
There are others, but you get the idea.
Another bad idea is to print things that embed an object address, like
>>> id(1.0) # certain to fail some of the time
7948648
>>> class C: pass
>>> C() # the default repr() for instances embeds an address
<__main__.C instance at 0x00AC18F0>
The ELLIPSIS directive gives a nice approach for the last example:
>>> C() #doctest: +ELLIPSIS
<__main__.C instance at 0x...>
Floating-point numbers are also subject to small output variations across platforms, because Python defers to the platform C library for float formatting, and C libraries vary widely in quality here.
>>> 1./7 # risky
0.14285714285714285
>>> print(1./7) # safer
0.142857142857
>>> print(round(1./7, 6)) # much safer
0.142857
Numbers of the form I/2.**J are safe across all platforms, and I often
contrive doctest examples to produce numbers of that form:
>>> 3./4 # utterly safe
0.75
Simple fractions are also easier for people to understand, and that makes for better documentation.
The functions testmod() and testfile() provide a simple interface to
doctest that should be sufficient for most basic uses. For a less formal
introduction to these two functions, see sections Simple Usage: Checking Examples in Docstrings
and Simple Usage: Checking Examples in a Text File.
doctest.testfile(filename, module_relative=True, name=None, package=None, globs=None, verbose=None, report=True, optionflags=0, extraglobs=None, raise_on_error=False, parser=DocTestParser(), encoding=None)¶All arguments except filename are optional, and should be specified in keyword form.
Test examples in the file named filename. Return (failure_count,
test_count).
Optional argument module_relative specifies how the filename should be interpreted:
True (the default), then filename specifies an
OS-independent module-relative path. By default, this path is relative to the
calling module’s directory; but if the package argument is specified, then it
is relative to that package. To ensure OS-independence, filename should use
/ characters to separate path segments, and may not be an absolute path
(i.e., it may not begin with /).False, then filename specifies an OS-specific
path. The path may be absolute or relative; relative paths are resolved with
respect to the current working directory.Optional argument name gives the name of the test; by default, or if None,
os.path.basename(filename) is used.
Optional argument package is a Python package or the name of a Python package
whose directory should be used as the base directory for a module-relative
filename. If no package is specified, then the calling module’s directory is
used as the base directory for module-relative filenames. It is an error to
specify package if module_relative is False.
Optional argument globs gives a dict to be used as the globals when executing
examples. A new shallow copy of this dict is created for the doctest, so its
examples start with a clean slate. By default, or if None, a new empty dict
is used.
Optional argument extraglobs gives a dict merged into the globals used to
execute examples. This works like dict.update(): if globs and
extraglobs have a common key, the associated value in extraglobs appears in
the combined dict. By default, or if None, no extra globals are used. This
is an advanced feature that allows parameterization of doctests. For example, a
doctest can be written for a base class, using a generic name for the class,
then reused to test any number of subclasses by passing an extraglobs dict
mapping the generic name to the subclass to be tested.
Optional argument verbose prints lots of stuff if true, and prints only
failures if false; by default, or if None, it’s true if and only if '-v'
is in sys.argv.
Optional argument report prints a summary at the end when true, else prints nothing at the end. In verbose mode, the summary is detailed, else the summary is very brief (in fact, empty if all tests passed).
Optional argument optionflags (default value 0) takes the bitwise OR of option flags. See section Option Flags.
Optional argument raise_on_error defaults to false. If true, an exception is raised upon the first failure or unexpected exception in an example. This allows failures to be post-mortem debugged. Default behavior is to continue running examples.
Optional argument parser specifies a DocTestParser (or subclass) that
should be used to extract tests from the files. It defaults to a normal parser
(i.e., DocTestParser()).
Optional argument encoding specifies an encoding that should be used to convert the file to unicode.
doctest.testmod(m=None, name=None, globs=None, verbose=None, report=True, optionflags=0, extraglobs=None, raise_on_error=False, exclude_empty=False)¶All arguments are optional, and all except for m should be specified in keyword form.
Test examples in docstrings in functions and classes reachable from module m
(or module __main__ if m is not supplied or is None), starting with
m.__doc__.
Also test examples reachable from dict m.__test__, if it exists and is not
None. m.__test__ maps names (strings) to functions, classes and
strings; function and class docstrings are searched for examples; strings are
searched directly, as if they were docstrings.
Only docstrings attached to objects belonging to module m are searched.
Return (failure_count, test_count).
Optional argument name gives the name of the module; by default, or if
None, m.__name__ is used.
Optional argument exclude_empty defaults to false. If true, objects for which
no doctests are found are excluded from consideration. The default is a backward
compatibility hack, so that code still using doctest.master.summarize() in
conjunction with testmod() continues to get output for objects with no
tests. The exclude_empty argument to the newer DocTestFinder
constructor defaults to true.
Optional arguments extraglobs, verbose, report, optionflags,
raise_on_error, and globs are the same as for function testfile()
above, except that globs defaults to m.__dict__.
doctest.run_docstring_examples(f, globs, verbose=False, name="NoName", compileflags=None, optionflags=0)¶Test examples associated with object f; for example, f may be a string, a module, a function, or a class object.
A shallow copy of dictionary argument globs is used for the execution context.
Optional argument name is used in failure messages, and defaults to
"NoName".
If optional argument verbose is true, output is generated even if there are no failures. By default, output is generated only in case of an example failure.
Optional argument compileflags gives the set of flags that should be used by
the Python compiler when running the examples. By default, or if None,
flags are deduced corresponding to the set of future features found in globs.
Optional argument optionflags works as for function testfile() above.
As your collection of doctest’ed modules grows, you’ll want a way to run all
their doctests systematically. doctest provides two functions that can
be used to create unittest test suites from modules and text files
containing doctests. To integrate with unittest test discovery, include
a load_tests() function in your test module:
import unittest
import doctest
import my_module_with_doctests
def load_tests(loader, tests, ignore):
tests.addTests(doctest.DocTestSuite(my_module_with_doctests))
return tests
There are two main functions for creating unittest.TestSuite instances
from text files and modules with doctests:
doctest.DocFileSuite(*paths, module_relative=True, package=None, setUp=None, tearDown=None, globs=None, optionflags=0, parser=DocTestParser(), encoding=None)¶Convert doctest tests from one or more text files to a
unittest.TestSuite.
The returned unittest.TestSuite is to be run by the unittest framework
and runs the interactive examples in each file. If an example in any file
fails, then the synthesized unit test fails, and a failureException
exception is raised showing the name of the file containing the test and a
(sometimes approximate) line number.
Pass one or more paths (as strings) to text files to be examined.
Options may be provided as keyword arguments:
Optional argument module_relative specifies how the filenames in paths should be interpreted:
True (the default), then each filename in
paths specifies an OS-independent module-relative path. By default, this
path is relative to the calling module’s directory; but if the package
argument is specified, then it is relative to that package. To ensure
OS-independence, each filename should use / characters to separate path
segments, and may not be an absolute path (i.e., it may not begin with
/).False, then each filename in paths specifies
an OS-specific path. The path may be absolute or relative; relative paths
are resolved with respect to the current working directory.Optional argument package is a Python package or the name of a Python
package whose directory should be used as the base directory for
module-relative filenames in paths. If no package is specified, then the
calling module’s directory is used as the base directory for module-relative
filenames. It is an error to specify package if module_relative is
False.
Optional argument setUp specifies a set-up function for the test suite.
This is called before running the tests in each file. The setUp function
will be passed a DocTest object. The setUp function can access the
test globals as the globs attribute of the test passed.
Optional argument tearDown specifies a tear-down function for the test
suite. This is called after running the tests in each file. The tearDown
function will be passed a DocTest object. The setUp function can
access the test globals as the globs attribute of the test passed.
Optional argument globs is a dictionary containing the initial global variables for the tests. A new copy of this dictionary is created for each test. By default, globs is a new empty dictionary.
Optional argument optionflags specifies the default doctest options for the
tests, created by or-ing together individual option flags. See section
Option Flags. See function set_unittest_reportflags() below
for a better way to set reporting options.
Optional argument parser specifies a DocTestParser (or subclass)
that should be used to extract tests from the files. It defaults to a normal
parser (i.e., DocTestParser()).
Optional argument encoding specifies an encoding that should be used to convert the file to unicode.
The global __file__ is added to the globals provided to doctests loaded
from a text file using DocFileSuite().
doctest.DocTestSuite(module=None, globs=None, extraglobs=None, test_finder=None, setUp=None, tearDown=None, checker=None)¶Convert doctest tests for a module to a unittest.TestSuite.
The returned unittest.TestSuite is to be run by the unittest framework
and runs each doctest in the module. If any of the doctests fail, then the
synthesized unit test fails, and a failureException exception is raised
showing the name of the file containing the test and a (sometimes approximate)
line number.
Optional argument module provides the module to be tested. It can be a module object or a (possibly dotted) module name. If not specified, the module calling this function is used.
Optional argument globs is a dictionary containing the initial global variables for the tests. A new copy of this dictionary is created for each test. By default, globs is a new empty dictionary.
Optional argument extraglobs specifies an extra set of global variables, which is merged into globs. By default, no extra globals are used.
Optional argument test_finder is the DocTestFinder object (or a
drop-in replacement) that is used to extract doctests from the module.
Optional arguments setUp, tearDown, and optionflags are the same as for
function DocFileSuite() above.
This function uses the same search technique as testmod().
Changed in version 3.5: DocTestSuite() returns an empty unittest.TestSuite if module
contains no docstrings instead of raising ValueError.
Under the covers, DocTestSuite() creates a unittest.TestSuite out
of doctest.DocTestCase instances, and DocTestCase is a
subclass of unittest.TestCase. DocTestCase isn’t documented
here (it’s an internal detail), but studying its code can answer questions about
the exact details of unittest integration.
Similarly, DocFileSuite() creates a unittest.TestSuite out of
doctest.DocFileCase instances, and DocFileCase is a subclass
of DocTestCase.
So both ways of creating a unittest.TestSuite run instances of
DocTestCase. This is important for a subtle reason: when you run
doctest functions yourself, you can control the doctest options in
use directly, by passing option flags to doctest functions. However, if
you’re writing a unittest framework, unittest ultimately controls
when and how tests get run. The framework author typically wants to control
doctest reporting options (perhaps, e.g., specified by command line
options), but there’s no way to pass options through unittest to
doctest test runners.
For this reason, doctest also supports a notion of doctest
reporting flags specific to unittest support, via this function:
doctest.set_unittest_reportflags(flags)¶Set the doctest reporting flags to use.
Argument flags takes the bitwise OR of option flags. See section Option Flags. Only “reporting flags” can be used.
This is a module-global setting, and affects all future doctests run by module
unittest: the runTest() method of DocTestCase looks at
the option flags specified for the test case when the DocTestCase
instance was constructed. If no reporting flags were specified (which is the
typical and expected case), doctest‘s unittest reporting flags are
bitwise ORed into the option flags, and the option flags
so augmented are passed to the DocTestRunner instance created to
run the doctest. If any reporting flags were specified when the
DocTestCase instance was constructed, doctest‘s
unittest reporting flags are ignored.
The value of the unittest reporting flags in effect before the function
was called is returned by the function.
The basic API is a simple wrapper that’s intended to make doctest easy to use. It is fairly flexible, and should meet most users’ needs; however, if you require more fine-grained control over testing, or wish to extend doctest’s capabilities, then you should use the advanced API.
The advanced API revolves around two container classes, which are used to store the interactive examples extracted from doctest cases:
Example: A single Python statement, paired with its expected
output.DocTest: A collection of Examples, typically extracted
from a single docstring or text file.Additional processing classes are defined to find, parse, and run, and check doctest examples:
DocTestFinder: Finds all docstrings in a given module, and uses a
DocTestParser to create a DocTest from every docstring that
contains interactive examples.DocTestParser: Creates a DocTest object from a string (such
as an object’s docstring).DocTestRunner: Executes the examples in a DocTest, and uses
an OutputChecker to verify their output.OutputChecker: Compares the actual output from a doctest example with
the expected output, and decides whether they match.The relationships among these processing classes are summarized in the following diagram:
list of:
+------+ +---------+
|module| --DocTestFinder-> | DocTest | --DocTestRunner-> results
+------+ | ^ +---------+ | ^ (printed)
| | | Example | | |
v | | ... | v |
DocTestParser | Example | OutputChecker
+---------+
doctest.DocTest(examples, globs, name, filename, lineno, docstring)¶A collection of doctest examples that should be run in a single namespace. The constructor arguments are used to initialize the attributes of the same names.
DocTest defines the following attributes. They are initialized by
the constructor, and should not be modified directly.
examples¶A list of Example objects encoding the individual interactive Python
examples that should be run by this test.
globs¶The namespace (aka globals) that the examples should be run in. This is a
dictionary mapping names to values. Any changes to the namespace made by the
examples (such as binding new variables) will be reflected in globs
after the test is run.
name¶A string name identifying the DocTest. Typically, this is the name
of the object or file that the test was extracted from.
filename¶The name of the file that this DocTest was extracted from; or
None if the filename is unknown, or if the DocTest was not
extracted from a file.
lineno¶The line number within filename where this DocTest begins, or
None if the line number is unavailable. This line number is zero-based
with respect to the beginning of the file.
docstring¶The string that the test was extracted from, or None if the string is
unavailable, or if the test was not extracted from a string.
doctest.Example(source, want, exc_msg=None, lineno=0, indent=0, options=None)¶A single interactive example, consisting of a Python statement and its expected output. The constructor arguments are used to initialize the attributes of the same names.
Example defines the following attributes. They are initialized by
the constructor, and should not be modified directly.
source¶A string containing the example’s source code. This source code consists of a single Python statement, and always ends with a newline; the constructor adds a newline when necessary.
want¶The expected output from running the example’s source code (either from
stdout, or a traceback in case of exception). want ends with a
newline unless no output is expected, in which case it’s an empty string. The
constructor adds a newline when necessary.
exc_msg¶The exception message generated by the example, if the example is expected to
generate an exception; or None if it is not expected to generate an
exception. This exception message is compared against the return value of
traceback.format_exception_only(). exc_msg ends with a newline
unless it’s None. The constructor adds a newline if needed.
lineno¶The line number within the string containing this example where the example begins. This line number is zero-based with respect to the beginning of the containing string.
indent¶The example’s indentation in the containing string, i.e., the number of space characters that precede the example’s first prompt.
options¶A dictionary mapping from option flags to True or False, which is used
to override default options for this example. Any option flags not contained
in this dictionary are left at their default value (as specified by the
DocTestRunner‘s optionflags). By default, no options are set.
doctest.DocTestFinder(verbose=False, parser=DocTestParser(), recurse=True, exclude_empty=True)¶A processing class used to extract the DocTests that are relevant to
a given object, from its docstring and the docstrings of its contained objects.
DocTests can be extracted from modules, classes, functions,
methods, staticmethods, classmethods, and properties.
The optional argument verbose can be used to display the objects searched by
the finder. It defaults to False (no output).
The optional argument parser specifies the DocTestParser object (or a
drop-in replacement) that is used to extract doctests from docstrings.
If the optional argument recurse is false, then DocTestFinder.find()
will only examine the given object, and not any contained objects.
If the optional argument exclude_empty is false, then
DocTestFinder.find() will include tests for objects with empty docstrings.
DocTestFinder defines the following method:
find(obj[, name][, module][, globs][, extraglobs])¶Return a list of the DocTests that are defined by obj‘s
docstring, or by any of its contained objects’ docstrings.
The optional argument name specifies the object’s name; this name will be
used to construct names for the returned DocTests. If name is
not specified, then obj.__name__ is used.
The optional parameter module is the module that contains the given object.
If the module is not specified or is None, then the test finder will attempt
to automatically determine the correct module. The object’s module is used:
If module is False, no attempt to find the module will be made. This is
obscure, of use mostly in testing doctest itself: if module is False, or
is None but cannot be found automatically, then all objects are considered
to belong to the (non-existent) module, so all contained objects will
(recursively) be searched for doctests.
The globals for each DocTest is formed by combining globs and
extraglobs (bindings in extraglobs override bindings in globs). A new
shallow copy of the globals dictionary is created for each DocTest.
If globs is not specified, then it defaults to the module’s __dict__, if
specified, or {} otherwise. If extraglobs is not specified, then it
defaults to {}.
doctest.DocTestParser¶A processing class used to extract interactive examples from a string, and use
them to create a DocTest object.
DocTestParser defines the following methods:
get_doctest(string, globs, name, filename, lineno)¶Extract all doctest examples from the given string, and collect them into a
DocTest object.
globs, name, filename, and lineno are attributes for the new
DocTest object. See the documentation for DocTest for more
information.
doctest.DocTestRunner(checker=None, verbose=None, optionflags=0)¶A processing class used to execute and verify the interactive examples in a
DocTest.
The comparison between expected outputs and actual outputs is done by an
OutputChecker. This comparison may be customized with a number of
option flags; see section Option Flags for more information. If the
option flags are insufficient, then the comparison may also be customized by
passing a subclass of OutputChecker to the constructor.
The test runner’s display output can be controlled in two ways. First, an output
function can be passed to TestRunner.run(); this function will be called
with strings that should be displayed. It defaults to sys.stdout.write. If
capturing the output is not sufficient, then the display output can be also
customized by subclassing DocTestRunner, and overriding the methods
report_start(), report_success(),
report_unexpected_exception(), and report_failure().
The optional keyword argument checker specifies the OutputChecker
object (or drop-in replacement) that should be used to compare the expected
outputs to the actual outputs of doctest examples.
The optional keyword argument verbose controls the DocTestRunner‘s
verbosity. If verbose is True, then information is printed about each
example, as it is run. If verbose is False, then only failures are
printed. If verbose is unspecified, or None, then verbose output is used
iff the command-line switch -v is used.
The optional keyword argument optionflags can be used to control how the test runner compares expected output to actual output, and how it displays failures. For more information, see section Option Flags.
DocTestParser defines the following methods:
report_start(out, test, example)¶Report that the test runner is about to process the given example. This method
is provided to allow subclasses of DocTestRunner to customize their
output; it should not be called directly.
example is the example about to be processed. test is the test
containing example. out is the output function that was passed to
DocTestRunner.run().
report_success(out, test, example, got)¶Report that the given example ran successfully. This method is provided to
allow subclasses of DocTestRunner to customize their output; it
should not be called directly.
example is the example about to be processed. got is the actual output
from the example. test is the test containing example. out is the
output function that was passed to DocTestRunner.run().
report_failure(out, test, example, got)¶Report that the given example failed. This method is provided to allow
subclasses of DocTestRunner to customize their output; it should not
be called directly.
example is the example about to be processed. got is the actual output
from the example. test is the test containing example. out is the
output function that was passed to DocTestRunner.run().
report_unexpected_exception(out, test, example, exc_info)¶Report that the given example raised an unexpected exception. This method is
provided to allow subclasses of DocTestRunner to customize their
output; it should not be called directly.
example is the example about to be processed. exc_info is a tuple
containing information about the unexpected exception (as returned by
sys.exc_info()). test is the test containing example. out is the
output function that was passed to DocTestRunner.run().
run(test, compileflags=None, out=None, clear_globs=True)¶Run the examples in test (a DocTest object), and display the
results using the writer function out.
The examples are run in the namespace test.globs. If clear_globs is
true (the default), then this namespace will be cleared after the test runs,
to help with garbage collection. If you would like to examine the namespace
after the test completes, then use clear_globs=False.
compileflags gives the set of flags that should be used by the Python compiler when running the examples. If not specified, then it will default to the set of future-import flags that apply to globs.
The output of each example is checked using the DocTestRunner‘s
output checker, and the results are formatted by the
DocTestRunner.report_*() methods.
summarize(verbose=None)¶Print a summary of all the test cases that have been run by this DocTestRunner,
and return a named tuple TestResults(failed, attempted).
The optional verbose argument controls how detailed the summary is. If the
verbosity is not specified, then the DocTestRunner‘s verbosity is
used.
doctest.OutputChecker¶A class used to check the whether the actual output from a doctest example
matches the expected output. OutputChecker defines two methods:
check_output(), which compares a given pair of outputs, and returns true
if they match; and output_difference(), which returns a string describing
the differences between two outputs.
OutputChecker defines the following methods:
check_output(want, got, optionflags)¶Return True iff the actual output from an example (got) matches the
expected output (want). These strings are always considered to match if
they are identical; but depending on what option flags the test runner is
using, several non-exact match types are also possible. See section
Option Flags for more information about option flags.
output_difference(example, got, optionflags)¶Return a string describing the differences between the expected output for a given example (example) and the actual output (got). optionflags is the set of option flags used to compare want and got.
Doctest provides several mechanisms for debugging doctest examples:
Several functions convert doctests to executable Python programs, which can be
run under the Python debugger, pdb.
The DebugRunner class is a subclass of DocTestRunner that
raises an exception for the first failing example, containing information about
that example. This information can be used to perform post-mortem debugging on
the example.
The unittest cases generated by DocTestSuite() support the
debug() method defined by unittest.TestCase.
You can add a call to pdb.set_trace() in a doctest example, and you’ll
drop into the Python debugger when that line is executed. Then you can inspect
current values of variables, and so on. For example, suppose a.py
contains just this module docstring:
"""
>>> def f(x):
... g(x*2)
>>> def g(x):
... print(x+3)
... import pdb; pdb.set_trace()
>>> f(3)
9
"""
Then an interactive Python session may look like this:
>>> import a, doctest
>>> doctest.testmod(a)
--Return--
> <doctest a[1]>(3)g()->None
-> import pdb; pdb.set_trace()
(Pdb) list
1 def g(x):
2 print(x+3)
3 -> import pdb; pdb.set_trace()
[EOF]
(Pdb) p x
6
(Pdb) step
--Return--
> <doctest a[0]>(2)f()->None
-> g(x*2)
(Pdb) list
1 def f(x):
2 -> g(x*2)
[EOF]
(Pdb) p x
3
(Pdb) step
--Return--
> <doctest a[2]>(1)?()->None
-> f(3)
(Pdb) cont
(0, 3)
>>>
Functions that convert doctests to Python code, and possibly run the synthesized code under the debugger:
doctest.script_from_examples(s)¶Convert text with examples to a script.
Argument s is a string containing doctest examples. The string is converted to a Python script, where doctest examples in s are converted to regular code, and everything else is converted to Python comments. The generated script is returned as a string. For example,
import doctest
print(doctest.script_from_examples(r"""
Set x and y to 1 and 2.
>>> x, y = 1, 2
Print their sum:
>>> print(x+y)
3
"""))
displays:
# Set x and y to 1 and 2.
x, y = 1, 2
#
# Print their sum:
print(x+y)
# Expected:
## 3
This function is used internally by other functions (see below), but can also be useful when you want to transform an interactive Python session into a Python script.
doctest.testsource(module, name)¶Convert the doctest for an object to a script.
Argument module is a module object, or dotted name of a module, containing the
object whose doctests are of interest. Argument name is the name (within the
module) of the object with the doctests of interest. The result is a string,
containing the object’s docstring converted to a Python script, as described for
script_from_examples() above. For example, if module a.py
contains a top-level function f(), then
import a, doctest
print(doctest.testsource(a, "a.f"))
prints a script version of function f()‘s docstring, with doctests
converted to code, and the rest placed in comments.
doctest.debug(module, name, pm=False)¶Debug the doctests for an object.
The module and name arguments are the same as for function
testsource() above. The synthesized Python script for the named object’s
docstring is written to a temporary file, and then that file is run under the
control of the Python debugger, pdb.
A shallow copy of module.__dict__ is used for both local and global
execution context.
Optional argument pm controls whether post-mortem debugging is used. If pm
has a true value, the script file is run directly, and the debugger gets
involved only if the script terminates via raising an unhandled exception. If
it does, then post-mortem debugging is invoked, via pdb.post_mortem(),
passing the traceback object from the unhandled exception. If pm is not
specified, or is false, the script is run under the debugger from the start, via
passing an appropriate exec() call to pdb.run().
doctest.debug_src(src, pm=False, globs=None)¶Debug the doctests in a string.
This is like function debug() above, except that a string containing
doctest examples is specified directly, via the src argument.
Optional argument pm has the same meaning as in function debug() above.
Optional argument globs gives a dictionary to use as both local and global
execution context. If not specified, or None, an empty dictionary is used.
If specified, a shallow copy of the dictionary is used.
The DebugRunner class, and the special exceptions it may raise, are of
most interest to testing framework authors, and will only be sketched here. See
the source code, and especially DebugRunner‘s docstring (which is a
doctest!) for more details:
doctest.DebugRunner(checker=None, verbose=None, optionflags=0)¶A subclass of DocTestRunner that raises an exception as soon as a
failure is encountered. If an unexpected exception occurs, an
UnexpectedException exception is raised, containing the test, the
example, and the original exception. If the output doesn’t match, then a
DocTestFailure exception is raised, containing the test, the example, and
the actual output.
For information about the constructor parameters and methods, see the
documentation for DocTestRunner in section Advanced API.
There are two exceptions that may be raised by DebugRunner instances:
doctest.DocTestFailure(test, example, got)¶An exception raised by DocTestRunner to signal that a doctest example’s
actual output did not match its expected output. The constructor arguments are
used to initialize the attributes of the same names.
DocTestFailure defines the following attributes:
DocTestFailure.got¶The example’s actual output.
doctest.UnexpectedException(test, example, exc_info)¶An exception raised by DocTestRunner to signal that a doctest
example raised an unexpected exception. The constructor arguments are used
to initialize the attributes of the same names.
UnexpectedException defines the following attributes:
UnexpectedException.exc_info¶A tuple containing information about the unexpected exception, as returned by
sys.exc_info().
As mentioned in the introduction, doctest has grown to have three primary
uses:
These uses have different requirements, and it is important to distinguish them. In particular, filling your docstrings with obscure test cases makes for bad documentation.
When writing a docstring, choose docstring examples with care. There’s an art to
this that needs to be learned—it may not be natural at first. Examples should
add genuine value to the documentation. A good example can often be worth many
words. If done with care, the examples will be invaluable for your users, and
will pay back the time it takes to collect them many times over as the years go
by and things change. I’m still amazed at how often one of my doctest
examples stops working after a “harmless” change.
Doctest also makes an excellent tool for regression testing, especially if you don’t skimp on explanatory text. By interleaving prose and examples, it becomes much easier to keep track of what’s actually being tested, and why. When a test fails, good prose can make it much easier to figure out what the problem is, and how it should be fixed. It’s true that you could write extensive comments in code-based testing, but few programmers do. Many have found that using doctest approaches instead leads to much clearer tests. Perhaps this is simply because doctest makes writing prose a little easier than writing code, while writing comments in code is a little harder. I think it goes deeper than just that: the natural attitude when writing a doctest-based test is that you want to explain the fine points of your software, and illustrate them with examples. This in turn naturally leads to test files that start with the simplest features, and logically progress to complications and edge cases. A coherent narrative is the result, instead of a collection of isolated functions that test isolated bits of functionality seemingly at random. It’s a different attitude, and produces different results, blurring the distinction between testing and explaining.
Regression testing is best confined to dedicated objects or files. There are several options for organizing tests:
testfile() or DocFileSuite(). This is recommended,
although is easiest to do for new projects, designed from the start to use
doctest._regrtest_topic that consist of single docstrings,
containing test cases for the named topics. These functions can be included in
the same file as the module, or separated out into a separate test file.__test__ dictionary mapping from regression test topics to
docstrings containing test cases.When you have placed your tests in a module, the module can itself be the test runner. When a test fails, you can arrange for your test runner to re-run only the failing doctest while you debug the problem. Here is a minimal example of such a test runner:
if __name__ == '__main__':
import doctest
flags = doctest.REPORT_NDIFF|doctest.FAIL_FAST
if len(sys.argv) > 1:
name = sys.argv[1]
if name in globals():
obj = globals()[name]
else:
obj = __test__[name]
doctest.run_docstring_examples(obj, globals(), name=name,
optionflags=flags)
else:
fail, total = doctest.testmod(optionflags=flags)
print("{} failures out of {} tests".format(fail, total))
Footnotes
| [1] | Examples containing both expected output and an exception are not supported. Trying to guess where one ends and the other begins is too error-prone, and that also makes for a confusing test. |