pdb
— The Python Debugger¶Source code: Lib/pdb.py
The module pdb
defines an interactive source code debugger for Python
programs. It supports setting (conditional) breakpoints and single stepping at
the source line level, inspection of stack frames, source code listing, and
evaluation of arbitrary Python code in the context of any stack frame. It also
supports post-mortem debugging and can be called under program control.
The debugger is extensible – it is actually defined as the class Pdb
.
This is currently undocumented but easily understood by reading the source. The
extension interface uses the modules bdb
and cmd
.
The debugger’s prompt is (Pdb)
. Typical usage to run a program under control
of the debugger is:
>>> import pdb
>>> import mymodule
>>> pdb.run('mymodule.test()')
> <string>(0)?()
(Pdb) continue
> <string>(1)?()
(Pdb) continue
NameError: 'spam'
> <string>(1)?()
(Pdb)
Changed in version 3.3: Tab-completion via the readline
module is available for commands and
command arguments, e.g. the current global and local names are offered as
arguments of the p
command.
pdb.py
can also be invoked as a script to debug other scripts. For
example:
python3 -m pdb myscript.py
When invoked as a script, pdb will automatically enter post-mortem debugging if the program being debugged exits abnormally. After post-mortem debugging (or after normal exit of the program), pdb will restart the program. Automatic restarting preserves pdb’s state (such as breakpoints) and in most cases is more useful than quitting the debugger upon program’s exit.
New in version 3.2: pdb.py
now accepts a -c
option that executes commands as if given
in a .pdbrc
file, see Debugger Commands.
The typical usage to break into the debugger from a running program is to insert
import pdb; pdb.set_trace()
at the location you want to break into the debugger. You can then step through
the code following this statement, and continue running without the debugger
using the continue
command.
The typical usage to inspect a crashed program is:
>>> import pdb
>>> import mymodule
>>> mymodule.test()
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "./mymodule.py", line 4, in test
test2()
File "./mymodule.py", line 3, in test2
print(spam)
NameError: spam
>>> pdb.pm()
> ./mymodule.py(3)test2()
-> print(spam)
(Pdb)
The module defines the following functions; each enters the debugger in a slightly different way:
pdb.
run
(statement, globals=None, locals=None)¶Execute the statement (given as a string or a code object) under debugger
control. The debugger prompt appears before any code is executed; you can
set breakpoints and type continue
, or you can step through the
statement using step
or next
(all these commands are
explained below). The optional globals and locals arguments specify the
environment in which the code is executed; by default the dictionary of the
module __main__
is used. (See the explanation of the built-in
exec()
or eval()
functions.)
pdb.
runeval
(expression, globals=None, locals=None)¶Evaluate the expression (given as a string or a code object) under debugger
control. When runeval()
returns, it returns the value of the
expression. Otherwise this function is similar to run()
.
pdb.
runcall
(function, *args, **kwds)¶Call the function (a function or method object, not a string) with the
given arguments. When runcall()
returns, it returns whatever the
function call returned. The debugger prompt appears as soon as the function
is entered.
pdb.
set_trace
()¶Enter the debugger at the calling stack frame. This is useful to hard-code a breakpoint at a given point in a program, even if the code is not otherwise being debugged (e.g. when an assertion fails).
pdb.
post_mortem
(traceback=None)¶Enter post-mortem debugging of the given traceback object. If no traceback is given, it uses the one of the exception that is currently being handled (an exception must be being handled if the default is to be used).
pdb.
pm
()¶Enter post-mortem debugging of the traceback found in
sys.last_traceback
.
The run*
functions and set_trace()
are aliases for instantiating the
Pdb
class and calling the method of the same name. If you want to
access further features, you have to do this yourself:
pdb.
Pdb
(completekey='tab', stdin=None, stdout=None, skip=None, nosigint=False, readrc=True)¶Pdb
is the debugger class.
The completekey, stdin and stdout arguments are passed to the
underlying cmd.Cmd
class; see the description there.
The skip argument, if given, must be an iterable of glob-style module name patterns. The debugger will not step into frames that originate in a module that matches one of these patterns. [1]
By default, Pdb sets a handler for the SIGINT signal (which is sent when the
user presses Ctrl-C
on the console) when you give a continue
command.
This allows you to break into the debugger again by pressing Ctrl-C
. If you
want Pdb not to touch the SIGINT handler, set nosigint to true.
The readrc argument defaults to true and controls whether Pdb will load .pdbrc files from the filesystem.
Example call to enable tracing with skip:
import pdb; pdb.Pdb(skip=['django.*']).set_trace()
New in version 3.1: The skip argument.
New in version 3.2: The nosigint argument. Previously, a SIGINT handler was never set by Pdb.
Changed in version 3.6: The readrc argument.
The commands recognized by the debugger are listed below. Most commands can be
abbreviated to one or two letters as indicated; e.g. h(elp)
means that
either h
or help
can be used to enter the help command (but not he
or hel
, nor H
or Help
or HELP
). Arguments to commands must be
separated by whitespace (spaces or tabs). Optional arguments are enclosed in
square brackets ([]
) in the command syntax; the square brackets must not be
typed. Alternatives in the command syntax are separated by a vertical bar
(|
).
Entering a blank line repeats the last command entered. Exception: if the last
command was a list
command, the next 11 lines are listed.
Commands that the debugger doesn’t recognize are assumed to be Python statements
and are executed in the context of the program being debugged. Python
statements can also be prefixed with an exclamation point (!
). This is a
powerful way to inspect the program being debugged; it is even possible to
change a variable or call a function. When an exception occurs in such a
statement, the exception name is printed but the debugger’s state is not
changed.
The debugger supports aliases. Aliases can have parameters which allows one a certain level of adaptability to the context under examination.
Multiple commands may be entered on a single line, separated by ;;
. (A
single ;
is not used as it is the separator for multiple commands in a line
that is passed to the Python parser.) No intelligence is applied to separating
the commands; the input is split at the first ;;
pair, even if it is in the
middle of a quoted string.
If a file .pdbrc
exists in the user’s home directory or in the current
directory, it is read in and executed as if it had been typed at the debugger
prompt. This is particularly useful for aliases. If both files exist, the one
in the home directory is read first and aliases defined there can be overridden
by the local file.
Changed in version 3.2: .pdbrc
can now contain commands that continue debugging, such as
continue
or next
. Previously, these commands had no
effect.
h(elp)
[command]
¶Without argument, print the list of available commands. With a command as
argument, print help about that command. help pdb
displays the full
documentation (the docstring of the pdb
module). Since the command
argument must be an identifier, help exec
must be entered to get help on
the !
command.
w(here)
¶Print a stack trace, with the most recent frame at the bottom. An arrow indicates the current frame, which determines the context of most commands.
d(own)
[count]
¶Move the current frame count (default one) levels down in the stack trace (to a newer frame).
u(p)
[count]
¶Move the current frame count (default one) levels up in the stack trace (to an older frame).
b(reak)
[([filename:]lineno | function) [, condition]]
¶With a lineno argument, set a break there in the current file. With a
function argument, set a break at the first executable statement within
that function. The line number may be prefixed with a filename and a colon,
to specify a breakpoint in another file (probably one that hasn’t been loaded
yet). The file is searched on sys.path
. Note that each breakpoint
is assigned a number to which all the other breakpoint commands refer.
If a second argument is present, it is an expression which must evaluate to true before the breakpoint is honored.
Without argument, list all breaks, including for each breakpoint, the number of times that breakpoint has been hit, the current ignore count, and the associated condition if any.
tbreak
[([filename:]lineno | function) [, condition]]
¶Temporary breakpoint, which is removed automatically when it is first hit.
The arguments are the same as for break
.
cl(ear)
[filename:lineno | bpnumber [bpnumber ...]]
¶With a filename:lineno argument, clear all the breakpoints at this line. With a space separated list of breakpoint numbers, clear those breakpoints. Without argument, clear all breaks (but first ask confirmation).
disable
[bpnumber [bpnumber ...]]
¶Disable the breakpoints given as a space separated list of breakpoint numbers. Disabling a breakpoint means it cannot cause the program to stop execution, but unlike clearing a breakpoint, it remains in the list of breakpoints and can be (re-)enabled.
enable
[bpnumber [bpnumber ...]]
¶Enable the breakpoints specified.
ignore
bpnumber [count]
¶Set the ignore count for the given breakpoint number. If count is omitted, the ignore count is set to 0. A breakpoint becomes active when the ignore count is zero. When non-zero, the count is decremented each time the breakpoint is reached and the breakpoint is not disabled and any associated condition evaluates to true.
condition
bpnumber [condition]
¶Set a new condition for the breakpoint, an expression which must evaluate to true before the breakpoint is honored. If condition is absent, any existing condition is removed; i.e., the breakpoint is made unconditional.
commands
[bpnumber]
¶Specify a list of commands for breakpoint number bpnumber. The commands
themselves appear on the following lines. Type a line containing just
end
to terminate the commands. An example:
(Pdb) commands 1
(com) p some_variable
(com) end
(Pdb)
To remove all commands from a breakpoint, type commands and follow it
immediately with end
; that is, give no commands.
With no bpnumber argument, commands refers to the last breakpoint set.
You can use breakpoint commands to start your program up again. Simply use the continue command, or step, or any other command that resumes execution.
Specifying any command resuming execution (currently continue, step, next, return, jump, quit and their abbreviations) terminates the command list (as if that command was immediately followed by end). This is because any time you resume execution (even with a simple next or step), you may encounter another breakpoint—which could have its own command list, leading to ambiguities about which list to execute.
If you use the ‘silent’ command in the command list, the usual message about stopping at a breakpoint is not printed. This may be desirable for breakpoints that are to print a specific message and then continue. If none of the other commands print anything, you see no sign that the breakpoint was reached.
s(tep)
¶Execute the current line, stop at the first possible occasion (either in a function that is called or on the next line in the current function).
n(ext)
¶Continue execution until the next line in the current function is reached or
it returns. (The difference between next
and step
is
that step
stops inside a called function, while next
executes called functions at (nearly) full speed, only stopping at the next
line in the current function.)
unt(il)
[lineno]
¶Without argument, continue execution until the line with a number greater than the current one is reached.
With a line number, continue execution until a line with a number greater or equal to that is reached. In both cases, also stop when the current frame returns.
Changed in version 3.2: Allow giving an explicit line number.
r(eturn)
¶Continue execution until the current function returns.
c(ont(inue))
¶Continue execution, only stop when a breakpoint is encountered.
j(ump)
lineno
¶Set the next line that will be executed. Only available in the bottom-most frame. This lets you jump back and execute code again, or jump forward to skip code that you don’t want to run.
It should be noted that not all jumps are allowed – for instance it is not
possible to jump into the middle of a for
loop or out of a
finally
clause.
l(ist)
[first[, last]]
¶List source code for the current file. Without arguments, list 11 lines
around the current line or continue the previous listing. With .
as
argument, list 11 lines around the current line. With one argument,
list 11 lines around at that line. With two arguments, list the given range;
if the second argument is less than the first, it is interpreted as a count.
The current line in the current frame is indicated by ->
. If an
exception is being debugged, the line where the exception was originally
raised or propagated is indicated by >>
, if it differs from the current
line.
New in version 3.2: The >>
marker.
ll
| longlist
¶List all source code for the current function or frame. Interesting lines
are marked as for list
.
New in version 3.2.
a(rgs)
¶Print the argument list of the current function.
p
expression
¶Evaluate the expression in the current context and print its value.
Note
print()
can also be used, but is not a debugger command — this executes the
Python print()
function.
pp
expression
¶Like the p
command, except the value of the expression is
pretty-printed using the pprint
module.
whatis
expression
¶Print the type of the expression.
source
expression
¶Try to get source code for the given object and display it.
New in version 3.2.
display
[expression]
¶Display the value of the expression if it changed, each time execution stops in the current frame.
Without expression, list all display expressions for the current frame.
New in version 3.2.
undisplay
[expression]
¶Do not display the expression any more in the current frame. Without expression, clear all display expressions for the current frame.
New in version 3.2.
interact
¶Start an interactive interpreter (using the code
module) whose global
namespace contains all the (global and local) names found in the current
scope.
New in version 3.2.
alias
[name [command]]
¶Create an alias called name that executes command. The command must
not be enclosed in quotes. Replaceable parameters can be indicated by
%1
, %2
, and so on, while %*
is replaced by all the parameters.
If no command is given, the current alias for name is shown. If no
arguments are given, all aliases are listed.
Aliases may be nested and can contain anything that can be legally typed at the pdb prompt. Note that internal pdb commands can be overridden by aliases. Such a command is then hidden until the alias is removed. Aliasing is recursively applied to the first word of the command line; all other words in the line are left alone.
As an example, here are two useful aliases (especially when placed in the
.pdbrc
file):
# Print instance variables (usage "pi classInst")
alias pi for k in %1.__dict__.keys(): print("%1.",k,"=",%1.__dict__[k])
# Print instance variables in self
alias ps pi self
unalias
name
¶Delete the specified alias.
!
statement
¶Execute the (one-line) statement in the context of the current stack frame.
The exclamation point can be omitted unless the first word of the statement
resembles a debugger command. To set a global variable, you can prefix the
assignment command with a global
statement on the same line,
e.g.:
(Pdb) global list_options; list_options = ['-l']
(Pdb)
run
[args ...]
¶restart
[args ...]
¶Restart the debugged Python program. If an argument is supplied, it is split
with shlex
and the result is used as the new sys.argv
.
History, breakpoints, actions and debugger options are preserved.
restart
is an alias for run
.
q(uit)
¶Quit from the debugger. The program being executed is aborted.
Footnotes
[1] | Whether a frame is considered to originate in a certain module
is determined by the __name__ in the frame globals. |