sys
— System-specific parameters and functions¶This module provides access to some variables used or maintained by the interpreter and to functions that interact strongly with the interpreter. It is always available.
sys.
abiflags
¶On POSIX systems where Python was built with the standard configure
script, this contains the ABI flags as specified by PEP 3149.
New in version 3.2.
sys.
argv
¶The list of command line arguments passed to a Python script. argv[0]
is the
script name (it is operating system dependent whether this is a full pathname or
not). If the command was executed using the -c
command line option to
the interpreter, argv[0]
is set to the string '-c'
. If no script name
was passed to the Python interpreter, argv[0]
is the empty string.
To loop over the standard input, or the list of files given on the
command line, see the fileinput
module.
sys.
base_exec_prefix
¶Set during Python startup, before site.py
is run, to the same value as
exec_prefix
. If not running in a
virtual environment, the values will stay the same; if
site.py
finds that a virtual environment is in use, the values of
prefix
and exec_prefix
will be changed to point to the
virtual environment, whereas base_prefix
and
base_exec_prefix
will remain pointing to the base Python
installation (the one which the virtual environment was created from).
New in version 3.3.
sys.
base_prefix
¶Set during Python startup, before site.py
is run, to the same value as
prefix
. If not running in a virtual environment, the values
will stay the same; if site.py
finds that a virtual environment is in
use, the values of prefix
and exec_prefix
will be changed to
point to the virtual environment, whereas base_prefix
and
base_exec_prefix
will remain pointing to the base Python
installation (the one which the virtual environment was created from).
New in version 3.3.
sys.
byteorder
¶An indicator of the native byte order. This will have the value 'big'
on
big-endian (most-significant byte first) platforms, and 'little'
on
little-endian (least-significant byte first) platforms.
sys.
builtin_module_names
¶A tuple of strings giving the names of all modules that are compiled into this
Python interpreter. (This information is not available in any other way —
modules.keys()
only lists the imported modules.)
sys.
call_tracing
(func, args)¶Call func(*args)
, while tracing is enabled. The tracing state is saved,
and restored afterwards. This is intended to be called from a debugger from
a checkpoint, to recursively debug some other code.
sys.
copyright
¶A string containing the copyright pertaining to the Python interpreter.
sys.
_clear_type_cache
()¶Clear the internal type cache. The type cache is used to speed up attribute and method lookups. Use the function only to drop unnecessary references during reference leak debugging.
This function should be used for internal and specialized purposes only.
sys.
_current_frames
()¶Return a dictionary mapping each thread’s identifier to the topmost stack frame
currently active in that thread at the time the function is called. Note that
functions in the traceback
module can build the call stack given such a
frame.
This is most useful for debugging deadlock: this function does not require the deadlocked threads’ cooperation, and such threads’ call stacks are frozen for as long as they remain deadlocked. The frame returned for a non-deadlocked thread may bear no relationship to that thread’s current activity by the time calling code examines the frame.
This function should be used for internal and specialized purposes only.
sys.
_debugmallocstats
()¶Print low-level information to stderr about the state of CPython’s memory allocator.
If Python is configured –with-pydebug, it also performs some expensive internal consistency checks.
New in version 3.3.
CPython implementation detail: This function is specific to CPython. The exact output format is not defined here, and may change.
sys.
dllhandle
¶Integer specifying the handle of the Python DLL. Availability: Windows.
sys.
displayhook
(value)¶If value is not None
, this function prints repr(value)
to
sys.stdout
, and saves value in builtins._
. If repr(value)
is
not encodable to sys.stdout.encoding
with sys.stdout.errors
error
handler (which is probably 'strict'
), encode it to
sys.stdout.encoding
with 'backslashreplace'
error handler.
sys.displayhook
is called on the result of evaluating an expression
entered in an interactive Python session. The display of these values can be
customized by assigning another one-argument function to sys.displayhook
.
Pseudo-code:
def displayhook(value):
if value is None:
return
# Set '_' to None to avoid recursion
builtins._ = None
text = repr(value)
try:
sys.stdout.write(text)
except UnicodeEncodeError:
bytes = text.encode(sys.stdout.encoding, 'backslashreplace')
if hasattr(sys.stdout, 'buffer'):
sys.stdout.buffer.write(bytes)
else:
text = bytes.decode(sys.stdout.encoding, 'strict')
sys.stdout.write(text)
sys.stdout.write("\n")
builtins._ = value
Changed in version 3.2: Use 'backslashreplace'
error handler on UnicodeEncodeError
.
sys.
dont_write_bytecode
¶If this is true, Python won’t try to write .pyc
files on the
import of source modules. This value is initially set to True
or
False
depending on the -B
command line option and the
PYTHONDONTWRITEBYTECODE
environment variable, but you can set it
yourself to control bytecode file generation.
sys.
excepthook
(type, value, traceback)¶This function prints out a given traceback and exception to sys.stderr
.
When an exception is raised and uncaught, the interpreter calls
sys.excepthook
with three arguments, the exception class, exception
instance, and a traceback object. In an interactive session this happens just
before control is returned to the prompt; in a Python program this happens just
before the program exits. The handling of such top-level exceptions can be
customized by assigning another three-argument function to sys.excepthook
.
sys.
__displayhook__
¶sys.
__excepthook__
¶These objects contain the original values of displayhook
and excepthook
at the start of the program. They are saved so that displayhook
and
excepthook
can be restored in case they happen to get replaced with broken
objects.
sys.
exc_info
()¶This function returns a tuple of three values that give information about the exception that is currently being handled. The information returned is specific both to the current thread and to the current stack frame. If the current stack frame is not handling an exception, the information is taken from the calling stack frame, or its caller, and so on until a stack frame is found that is handling an exception. Here, “handling an exception” is defined as “executing an except clause.” For any stack frame, only information about the exception being currently handled is accessible.
If no exception is being handled anywhere on the stack, a tuple containing
three None
values is returned. Otherwise, the values returned are
(type, value, traceback)
. Their meaning is: type gets the type of the
exception being handled (a subclass of BaseException
); value gets
the exception instance (an instance of the exception type); traceback gets
a traceback object (see the Reference Manual) which encapsulates the call
stack at the point where the exception originally occurred.
sys.
exec_prefix
¶A string giving the site-specific directory prefix where the platform-dependent
Python files are installed; by default, this is also '/usr/local'
. This can
be set at build time with the --exec-prefix
argument to the
configure script. Specifically, all configuration files (e.g. the
pyconfig.h
header file) are installed in the directory
exec_prefix/lib/pythonX.Y/config
, and shared library modules are
installed in exec_prefix/lib/pythonX.Y/lib-dynload
, where X.Y
is the version number of Python, for example 3.2
.
Note
If a virtual environment is in effect, this
value will be changed in site.py
to point to the virtual environment.
The value for the Python installation will still be available, via
base_exec_prefix
.
sys.
executable
¶A string giving the absolute path of the executable binary for the Python
interpreter, on systems where this makes sense. If Python is unable to retrieve
the real path to its executable, sys.executable
will be an empty string
or None
.
sys.
exit
([arg])¶Exit from Python. This is implemented by raising the SystemExit
exception, so cleanup actions specified by finally clauses of try
statements are honored, and it is possible to intercept the exit attempt at
an outer level.
The optional argument arg can be an integer giving the exit status
(defaulting to zero), or another type of object. If it is an integer, zero
is considered “successful termination” and any nonzero value is considered
“abnormal termination” by shells and the like. Most systems require it to be
in the range 0–127, and produce undefined results otherwise. Some systems
have a convention for assigning specific meanings to specific exit codes, but
these are generally underdeveloped; Unix programs generally use 2 for command
line syntax errors and 1 for all other kind of errors. If another type of
object is passed, None
is equivalent to passing zero, and any other
object is printed to stderr
and results in an exit code of 1. In
particular, sys.exit("some error message")
is a quick way to exit a
program when an error occurs.
Since exit()
ultimately “only” raises an exception, it will only exit
the process when called from the main thread, and the exception is not
intercepted.
Changed in version 3.6: If an error occurs in the cleanup after the Python interpreter
has caught SystemExit
(such as an error flushing buffered data
in the standard streams), the exit status is changed to 120.
sys.
flags
¶The struct sequence flags exposes the status of command line flags. The attributes are read only.
attribute | flag |
---|---|
debug |
-d |
inspect |
-i |
interactive |
-i |
optimize |
-O or -OO |
dont_write_bytecode |
-B |
no_user_site |
-s |
no_site |
-S |
ignore_environment |
-E |
verbose |
-v |
bytes_warning |
-b |
quiet |
-q |
hash_randomization |
-R |
Changed in version 3.2: Added quiet
attribute for the new -q
flag.
New in version 3.2.3: The hash_randomization
attribute.
Changed in version 3.3: Removed obsolete division_warning
attribute.
sys.
float_info
¶A struct sequence holding information about the float type. It
contains low level information about the precision and internal
representation. The values correspond to the various floating-point
constants defined in the standard header file float.h
for the ‘C’
programming language; see section 5.2.4.2.2 of the 1999 ISO/IEC C standard
[C99], ‘Characteristics of floating types’, for details.
attribute | float.h macro | explanation |
---|---|---|
epsilon |
DBL_EPSILON | difference between 1 and the least value greater than 1 that is representable as a float |
dig |
DBL_DIG | maximum number of decimal digits that can be faithfully represented in a float; see below |
mant_dig |
DBL_MANT_DIG | float precision: the number of base-radix
digits in the significand of a float |
max |
DBL_MAX | maximum representable finite float |
max_exp |
DBL_MAX_EXP | maximum integer e such that radix**(e-1) is
a representable finite float |
max_10_exp |
DBL_MAX_10_EXP | maximum integer e such that 10**e is in the
range of representable finite floats |
min |
DBL_MIN | minimum positive normalized float |
min_exp |
DBL_MIN_EXP | minimum integer e such that radix**(e-1) is
a normalized float |
min_10_exp |
DBL_MIN_10_EXP | minimum integer e such that 10**e is a
normalized float |
radix |
FLT_RADIX | radix of exponent representation |
rounds |
FLT_ROUNDS | integer constant representing the rounding mode used for arithmetic operations. This reflects the value of the system FLT_ROUNDS macro at interpreter startup time. See section 5.2.4.2.2 of the C99 standard for an explanation of the possible values and their meanings. |
The attribute sys.float_info.dig
needs further explanation. If
s
is any string representing a decimal number with at most
sys.float_info.dig
significant digits, then converting s
to a
float and back again will recover a string representing the same decimal
value:
>>> import sys
>>> sys.float_info.dig
15
>>> s = '3.14159265358979' # decimal string with 15 significant digits
>>> format(float(s), '.15g') # convert to float and back -> same value
'3.14159265358979'
But for strings with more than sys.float_info.dig
significant digits,
this isn’t always true:
>>> s = '9876543211234567' # 16 significant digits is too many!
>>> format(float(s), '.16g') # conversion changes value
'9876543211234568'
sys.
float_repr_style
¶A string indicating how the repr()
function behaves for
floats. If the string has value 'short'
then for a finite
float x
, repr(x)
aims to produce a short string with the
property that float(repr(x)) == x
. This is the usual behaviour
in Python 3.1 and later. Otherwise, float_repr_style
has value
'legacy'
and repr(x)
behaves in the same way as it did in
versions of Python prior to 3.1.
New in version 3.1.
sys.
getallocatedblocks
()¶Return the number of memory blocks currently allocated by the interpreter,
regardless of their size. This function is mainly useful for tracking
and debugging memory leaks. Because of the interpreter’s internal
caches, the result can vary from call to call; you may have to call
_clear_type_cache()
and gc.collect()
to get more
predictable results.
If a Python build or implementation cannot reasonably compute this
information, getallocatedblocks()
is allowed to return 0 instead.
New in version 3.4.
sys.
getcheckinterval
()¶Return the interpreter’s “check interval”; see setcheckinterval()
.
Deprecated since version 3.2: Use getswitchinterval()
instead.
sys.
getdefaultencoding
()¶Return the name of the current default string encoding used by the Unicode implementation.
sys.
getdlopenflags
()¶Return the current value of the flags that are used for
dlopen()
calls. Symbolic names for the flag values can be
found in the os
module (RTLD_xxx
constants, e.g.
os.RTLD_LAZY
). Availability: Unix.
sys.
getfilesystemencoding
()¶Return the name of the encoding used to convert between Unicode filenames and bytes filenames. For best compatibility, str should be used for filenames in all cases, although representing filenames as bytes is also supported. Functions accepting or returning filenames should support either str or bytes and internally convert to the system’s preferred representation.
This encoding is always ASCII-compatible.
os.fsencode()
and os.fsdecode()
should be used to ensure that
the correct encoding and errors mode are used.
'utf-8'
.'utf-8'
or 'mbcs'
, depending
on user configuration.Changed in version 3.2: getfilesystemencoding()
result cannot be None
anymore.
Changed in version 3.6: Windows is no longer guaranteed to return 'mbcs'
. See PEP 529
and _enablelegacywindowsfsencoding()
for more information.
sys.
getfilesystemencodeerrors
()¶Return the name of the error mode used to convert between Unicode filenames
and bytes filenames. The encoding name is returned from
getfilesystemencoding()
.
os.fsencode()
and os.fsdecode()
should be used to ensure that
the correct encoding and errors mode are used.
New in version 3.6.
sys.
getrefcount
(object)¶Return the reference count of the object. The count returned is generally one
higher than you might expect, because it includes the (temporary) reference as
an argument to getrefcount()
.
sys.
getrecursionlimit
()¶Return the current value of the recursion limit, the maximum depth of the Python
interpreter stack. This limit prevents infinite recursion from causing an
overflow of the C stack and crashing Python. It can be set by
setrecursionlimit()
.
sys.
getsizeof
(object[, default])¶Return the size of an object in bytes. The object can be any type of object. All built-in objects will return correct results, but this does not have to hold true for third-party extensions as it is implementation specific.
Only the memory consumption directly attributed to the object is accounted for, not the memory consumption of objects it refers to.
If given, default will be returned if the object does not provide means to
retrieve the size. Otherwise a TypeError
will be raised.
getsizeof()
calls the object’s __sizeof__
method and adds an
additional garbage collector overhead if the object is managed by the garbage
collector.
See recursive sizeof recipe
for an example of using getsizeof()
recursively to find the size of
containers and all their contents.
sys.
getswitchinterval
()¶Return the interpreter’s “thread switch interval”; see
setswitchinterval()
.
New in version 3.2.
sys.
_getframe
([depth])¶Return a frame object from the call stack. If optional integer depth is
given, return the frame object that many calls below the top of the stack. If
that is deeper than the call stack, ValueError
is raised. The default
for depth is zero, returning the frame at the top of the call stack.
CPython implementation detail: This function should be used for internal and specialized purposes only. It is not guaranteed to exist in all implementations of Python.
sys.
getprofile
()¶Get the profiler function as set by setprofile()
.
sys.
gettrace
()¶Get the trace function as set by settrace()
.
CPython implementation detail: The gettrace()
function is intended only for implementing debuggers,
profilers, coverage tools and the like. Its behavior is part of the
implementation platform, rather than part of the language definition, and
thus may not be available in all Python implementations.
sys.
getwindowsversion
()¶Return a named tuple describing the Windows version
currently running. The named elements are major, minor,
build, platform, service_pack, service_pack_minor,
service_pack_major, suite_mask, product_type and
platform_version. service_pack contains a string,
platform_version a 3-tuple and all other values are
integers. The components can also be accessed by name, so
sys.getwindowsversion()[0]
is equivalent to
sys.getwindowsversion().major
. For compatibility with prior
versions, only the first 5 elements are retrievable by indexing.
platform will be 2 (VER_PLATFORM_WIN32_NT)
.
product_type may be one of the following values:
Constant | Meaning |
---|---|
1 (VER_NT_WORKSTATION) |
The system is a workstation. |
2 (VER_NT_DOMAIN_CONTROLLER) |
The system is a domain controller. |
3 (VER_NT_SERVER) |
The system is a server, but not a domain controller. |
This function wraps the Win32 GetVersionEx()
function; see the
Microsoft documentation on OSVERSIONINFOEX()
for more information
about these fields.
platform_version returns the accurate major version, minor version and build number of the current operating system, rather than the version that is being emulated for the process. It is intended for use in logging rather than for feature detection.
Availability: Windows.
Changed in version 3.2: Changed to a named tuple and added service_pack_minor, service_pack_major, suite_mask, and product_type.
Changed in version 3.6: Added platform_version
sys.
get_asyncgen_hooks
()¶Returns an asyncgen_hooks object, which is similar to a
namedtuple
of the form (firstiter, finalizer),
where firstiter and finalizer are expected to be either None
or
functions which take an asynchronous generator iterator as an
argument, and are used to schedule finalization of an asychronous
generator by an event loop.
New in version 3.6: See PEP 525 for more details.
Note
This function has been added on a provisional basis (see PEP 411 for details.)
sys.
get_coroutine_wrapper
()¶Returns None
, or a wrapper set by set_coroutine_wrapper()
.
New in version 3.5: See PEP 492 for more details.
Note
This function has been added on a provisional basis (see PEP 411 for details.) Use it only for debugging purposes.
sys.
hash_info
¶A struct sequence giving parameters of the numeric hash implementation. For more details about hashing of numeric types, see Hashing of numeric types.
attribute | explanation |
---|---|
width |
width in bits used for hash values |
modulus |
prime modulus P used for numeric hash scheme |
inf |
hash value returned for a positive infinity |
nan |
hash value returned for a nan |
imag |
multiplier used for the imaginary part of a complex number |
algorithm |
name of the algorithm for hashing of str, bytes, and memoryview |
hash_bits |
internal output size of the hash algorithm |
seed_bits |
size of the seed key of the hash algorithm |
New in version 3.2.
Changed in version 3.4: Added algorithm, hash_bits and seed_bits
sys.
hexversion
¶The version number encoded as a single integer. This is guaranteed to increase with each version, including proper support for non-production releases. For example, to test that the Python interpreter is at least version 1.5.2, use:
if sys.hexversion >= 0x010502F0:
# use some advanced feature
...
else:
# use an alternative implementation or warn the user
...
This is called hexversion
since it only really looks meaningful when viewed
as the result of passing it to the built-in hex()
function. The
struct sequence sys.version_info
may be used for a more
human-friendly encoding of the same information.
More details of hexversion
can be found at apiabiversion.
sys.
implementation
¶An object containing information about the implementation of the currently running Python interpreter. The following attributes are required to exist in all Python implementations.
name is the implementation’s identifier, e.g. 'cpython'
. The actual
string is defined by the Python implementation, but it is guaranteed to be
lower case.
version is a named tuple, in the same format as
sys.version_info
. It represents the version of the Python
implementation. This has a distinct meaning from the specific
version of the Python language to which the currently running
interpreter conforms, which sys.version_info
represents. For
example, for PyPy 1.8 sys.implementation.version
might be
sys.version_info(1, 8, 0, 'final', 0)
, whereas sys.version_info
would be sys.version_info(2, 7, 2, 'final', 0)
. For CPython they
are the same value, since it is the reference implementation.
hexversion is the implementation version in hexadecimal format, like
sys.hexversion
.
cache_tag is the tag used by the import machinery in the filenames of
cached modules. By convention, it would be a composite of the
implementation’s name and version, like 'cpython-33'
. However, a
Python implementation may use some other value if appropriate. If
cache_tag
is set to None
, it indicates that module caching should
be disabled.
sys.implementation
may contain additional attributes specific to
the Python implementation. These non-standard attributes must start with
an underscore, and are not described here. Regardless of its contents,
sys.implementation
will not change during a run of the interpreter,
nor between implementation versions. (It may change between Python
language versions, however.) See PEP 421 for more information.
New in version 3.3.
sys.
int_info
¶A struct sequence that holds information about Python’s internal representation of integers. The attributes are read only.
Attribute | Explanation |
---|---|
bits_per_digit |
number of bits held in each digit. Python
integers are stored internally in base
2**int_info.bits_per_digit |
sizeof_digit |
size in bytes of the C type used to represent a digit |
New in version 3.1.
sys.
__interactivehook__
¶When this attribute exists, its value is automatically called (with no
arguments) when the interpreter is launched in interactive mode. This is done after the PYTHONSTARTUP
file is
read, so that you can set this hook there. The site
module
sets this.
New in version 3.4.
sys.
intern
(string)¶Enter string in the table of “interned” strings and return the interned string – which is string itself or a copy. Interning strings is useful to gain a little performance on dictionary lookup – if the keys in a dictionary are interned, and the lookup key is interned, the key comparisons (after hashing) can be done by a pointer compare instead of a string compare. Normally, the names used in Python programs are automatically interned, and the dictionaries used to hold module, class or instance attributes have interned keys.
Interned strings are not immortal; you must keep a reference to the return
value of intern()
around to benefit from it.
sys.
is_finalizing
()¶Return True
if the Python interpreter is
shutting down, False
otherwise.
New in version 3.5.
sys.
last_type
¶sys.
last_value
¶sys.
last_traceback
¶These three variables are not always defined; they are set when an exception is
not handled and the interpreter prints an error message and a stack traceback.
Their intended use is to allow an interactive user to import a debugger module
and engage in post-mortem debugging without having to re-execute the command
that caused the error. (Typical use is import pdb; pdb.pm()
to enter the
post-mortem debugger; see pdb
module for
more information.)
The meaning of the variables is the same as that of the return values from
exc_info()
above.
sys.
maxsize
¶An integer giving the maximum value a variable of type Py_ssize_t
can
take. It’s usually 2**31 - 1
on a 32-bit platform and 2**63 - 1
on a
64-bit platform.
sys.
maxunicode
¶An integer giving the value of the largest Unicode code point,
i.e. 1114111
(0x10FFFF
in hexadecimal).
Changed in version 3.3: Before PEP 393, sys.maxunicode
used to be either 0xFFFF
or 0x10FFFF
, depending on the configuration option that specified
whether Unicode characters were stored as UCS-2 or UCS-4.
sys.
meta_path
¶A list of meta path finder objects that have their
find_spec()
methods called to see if one
of the objects can find the module to be imported. The
find_spec()
method is called with at
least the absolute name of the module being imported. If the module to be
imported is contained in a package, then the parent package’s __path__
attribute is passed in as a second argument. The method returns a
module spec, or None
if the module cannot be found.
See also
importlib.abc.MetaPathFinder
meta_path
.importlib.machinery.ModuleSpec
find_spec()
should return
instances of.Changed in version 3.4: Module specs were introduced in Python 3.4, by
PEP 451. Earlier versions of Python looked for a method called
find_module()
.
This is still called as a fallback if a meta_path
entry doesn’t
have a find_spec()
method.
sys.
modules
¶This is a dictionary that maps module names to modules which have already been loaded. This can be manipulated to force reloading of modules and other tricks. However, replacing the dictionary will not necessarily work as expected and deleting essential items from the dictionary may cause Python to fail.
sys.
path
¶A list of strings that specifies the search path for modules. Initialized from
the environment variable PYTHONPATH
, plus an installation-dependent
default.
As initialized upon program startup, the first item of this list, path[0]
,
is the directory containing the script that was used to invoke the Python
interpreter. If the script directory is not available (e.g. if the interpreter
is invoked interactively or if the script is read from standard input),
path[0]
is the empty string, which directs Python to search modules in the
current directory first. Notice that the script directory is inserted before
the entries inserted as a result of PYTHONPATH
.
A program is free to modify this list for its own purposes. Only strings
and bytes should be added to sys.path
; all other data types are
ignored during import.
sys.
path_hooks
¶A list of callables that take a path argument to try to create a
finder for the path. If a finder can be created, it is to be
returned by the callable, else raise ImportError
.
Originally specified in PEP 302.
sys.
path_importer_cache
¶A dictionary acting as a cache for finder objects. The keys are
paths that have been passed to sys.path_hooks
and the values are
the finders that are found. If a path is a valid file system path but no
finder is found on sys.path_hooks
then None
is
stored.
Originally specified in PEP 302.
Changed in version 3.3: None
is stored instead of imp.NullImporter
when no finder
is found.
sys.
platform
¶This string contains a platform identifier that can be used to append
platform-specific components to sys.path
, for instance.
For Unix systems, except on Linux, this is the lowercased OS name as
returned by uname -s
with the first part of the version as returned by
uname -r
appended, e.g. 'sunos5'
or 'freebsd8'
, at the time
when Python was built. Unless you want to test for a specific system
version, it is therefore recommended to use the following idiom:
if sys.platform.startswith('freebsd'):
# FreeBSD-specific code here...
elif sys.platform.startswith('linux'):
# Linux-specific code here...
For other systems, the values are:
System | platform value |
---|---|
Linux | 'linux' |
Windows | 'win32' |
Windows/Cygwin | 'cygwin' |
Mac OS X | 'darwin' |
Changed in version 3.3: On Linux, sys.platform
doesn’t contain the major version anymore.
It is always 'linux'
, instead of 'linux2'
or 'linux3'
. Since
older Python versions include the version number, it is recommended to
always use the startswith
idiom presented above.
See also
os.name
has a coarser granularity. os.uname()
gives
system-dependent version information.
The platform
module provides detailed checks for the
system’s identity.
sys.
prefix
¶A string giving the site-specific directory prefix where the platform
independent Python files are installed; by default, this is the string
'/usr/local'
. This can be set at build time with the --prefix
argument to the configure script. The main collection of Python
library modules is installed in the directory prefix/lib/pythonX.Y
while the platform independent header files (all except pyconfig.h
) are
stored in prefix/include/pythonX.Y
, where X.Y is the version
number of Python, for example 3.2
.
Note
If a virtual environment is in effect, this
value will be changed in site.py
to point to the virtual
environment. The value for the Python installation will still be
available, via base_prefix
.
sys.
ps1
¶sys.
ps2
¶Strings specifying the primary and secondary prompt of the interpreter. These
are only defined if the interpreter is in interactive mode. Their initial
values in this case are '>>> '
and '... '
. If a non-string object is
assigned to either variable, its str()
is re-evaluated each time the
interpreter prepares to read a new interactive command; this can be used to
implement a dynamic prompt.
sys.
setcheckinterval
(interval)¶Set the interpreter’s “check interval”. This integer value determines how often
the interpreter checks for periodic things such as thread switches and signal
handlers. The default is 100
, meaning the check is performed every 100
Python virtual instructions. Setting it to a larger value may increase
performance for programs using threads. Setting it to a value <=
0 checks
every virtual instruction, maximizing responsiveness as well as overhead.
Deprecated since version 3.2: This function doesn’t have an effect anymore, as the internal logic for
thread switching and asynchronous tasks has been rewritten. Use
setswitchinterval()
instead.
sys.
setdlopenflags
(n)¶Set the flags used by the interpreter for dlopen()
calls, such as when
the interpreter loads extension modules. Among other things, this will enable a
lazy resolving of symbols when importing a module, if called as
sys.setdlopenflags(0)
. To share symbols across extension modules, call as
sys.setdlopenflags(os.RTLD_GLOBAL)
. Symbolic names for the flag values
can be found in the os
module (RTLD_xxx
constants, e.g.
os.RTLD_LAZY
).
Availability: Unix.
sys.
setprofile
(profilefunc)¶Set the system’s profile function, which allows you to implement a Python source
code profiler in Python. See chapter The Python Profilers for more information on the
Python profiler. The system’s profile function is called similarly to the
system’s trace function (see settrace()
), but it isn’t called for each
executed line of code (only on call and return, but the return event is reported
even when an exception has been set). The function is thread-specific, but
there is no way for the profiler to know about context switches between threads,
so it does not make sense to use this in the presence of multiple threads. Also,
its return value is not used, so it can simply return None
.
sys.
setrecursionlimit
(limit)¶Set the maximum depth of the Python interpreter stack to limit. This limit prevents infinite recursion from causing an overflow of the C stack and crashing Python.
The highest possible limit is platform-dependent. A user may need to set the limit higher when they have a program that requires deep recursion and a platform that supports a higher limit. This should be done with care, because a too-high limit can lead to a crash.
If the new limit is too low at the current recursion depth, a
RecursionError
exception is raised.
Changed in version 3.5.1: A RecursionError
exception is now raised if the new limit is too
low at the current recursion depth.
sys.
setswitchinterval
(interval)¶Set the interpreter’s thread switch interval (in seconds). This floating-point value determines the ideal duration of the “timeslices” allocated to concurrently running Python threads. Please note that the actual value can be higher, especially if long-running internal functions or methods are used. Also, which thread becomes scheduled at the end of the interval is the operating system’s decision. The interpreter doesn’t have its own scheduler.
New in version 3.2.
sys.
settrace
(tracefunc)¶Set the system’s trace function, which allows you to implement a Python
source code debugger in Python. The function is thread-specific; for a
debugger to support multiple threads, it must be registered using
settrace()
for each thread being debugged.
Trace functions should have three arguments: frame, event, and
arg. frame is the current stack frame. event is a string: 'call'
,
'line'
, 'return'
, 'exception'
, 'c_call'
, 'c_return'
, or
'c_exception'
. arg depends on the event type.
The trace function is invoked (with event set to 'call'
) whenever a new
local scope is entered; it should return a reference to a local trace
function to be used that scope, or None
if the scope shouldn’t be traced.
The local trace function should return a reference to itself (or to another
function for further tracing in that scope), or None
to turn off tracing
in that scope.
The events have the following meaning:
'call'
None
; the return value
specifies the local trace function.'line'
None
; the return value specifies the new local trace function. See
Objects/lnotab_notes.txt
for a detailed explanation of how this
works.'return'
None
if the event is caused by an exception being raised. The trace function’s
return value is ignored.'exception'
(exception, value, traceback)
; the return value specifies the
new local trace function.'c_call'
'c_return'
'c_exception'
Note that as an exception is propagated down the chain of callers, an
'exception'
event is generated at each level.
For more information on code and frame objects, refer to The standard type hierarchy.
CPython implementation detail: The settrace()
function is intended only for implementing debuggers,
profilers, coverage tools and the like. Its behavior is part of the
implementation platform, rather than part of the language definition, and
thus may not be available in all Python implementations.
sys.
set_asyncgen_hooks
(firstiter, finalizer)¶Accepts two optional keyword arguments which are callables that accept an asynchronous generator iterator as an argument. The firstiter callable will be called when an asynchronous generator is iterated for the first time. The finalizer will be called when an asynchronous generator is about to be garbage collected.
New in version 3.6: See PEP 525 for more details, and for a reference example of a
finalizer method see the implementation of
asyncio.Loop.shutdown_asyncgens
in
Lib/asyncio/base_events.py
Note
This function has been added on a provisional basis (see PEP 411 for details.)
sys.
set_coroutine_wrapper
(wrapper)¶Allows intercepting creation of coroutine objects (only ones that
are created by an async def
function; generators decorated with
types.coroutine()
or asyncio.coroutine()
will not be
intercepted).
The wrapper argument must be either:
None
, to reset the wrapper.If called twice, the new wrapper replaces the previous one. The function is thread-specific.
The wrapper callable cannot define new coroutines directly or indirectly:
def wrapper(coro):
async def wrap(coro):
return await coro
return wrap(coro)
sys.set_coroutine_wrapper(wrapper)
async def foo():
pass
# The following line will fail with a RuntimeError, because
# ``wrapper`` creates a ``wrap(coro)`` coroutine:
foo()
See also get_coroutine_wrapper()
.
New in version 3.5: See PEP 492 for more details.
Note
This function has been added on a provisional basis (see PEP 411 for details.) Use it only for debugging purposes.
sys.
_enablelegacywindowsfsencoding
()¶Changes the default filesystem encoding and errors mode to ‘mbcs’ and ‘replace’ respectively, for consistency with versions of Python prior to 3.6.
This is equivalent to defining the PYTHONLEGACYWINDOWSFSENCODING
environment variable before launching Python.
Availability: Windows
New in version 3.6: See PEP 529 for more details.
sys.
stdin
¶sys.
stdout
¶sys.
stderr
¶File objects used by the interpreter for standard input, output and errors:
stdin
is used for all interactive input (including calls to
input()
);stdout
is used for the output of print()
and expression
statements and for the prompts of input()
;stderr
.These streams are regular text files like those
returned by the open()
function. Their parameters are chosen as
follows:
The character encoding is platform-dependent. Under Windows, if the stream
is interactive (that is, if its isatty()
method returns True
), the
console codepage is used, otherwise the ANSI code page. Under other
platforms, the locale encoding is used (see locale.getpreferredencoding()
).
Under all platforms though, you can override this value by setting the
PYTHONIOENCODING
environment variable before starting Python.
When interactive, standard streams are line-buffered. Otherwise, they
are block-buffered like regular text files. You can override this
value with the -u
command-line option.
Note
To write or read binary data from/to the standard streams, use the
underlying binary buffer
object. For example, to
write bytes to stdout
, use sys.stdout.buffer.write(b'abc')
.
However, if you are writing a library (and do not control in which
context its code will be executed), be aware that the standard streams
may be replaced with file-like objects like io.StringIO
which
do not support the buffer
attribute.
sys.
__stdin__
¶sys.
__stdout__
¶sys.
__stderr__
¶These objects contain the original values of stdin
, stderr
and
stdout
at the start of the program. They are used during finalization,
and could be useful to print to the actual standard stream no matter if the
sys.std*
object has been redirected.
It can also be used to restore the actual files to known working file objects in case they have been overwritten with a broken object. However, the preferred way to do this is to explicitly save the previous stream before replacing it, and restore the saved object.
Note
Under some conditions stdin
, stdout
and stderr
as well as the
original values __stdin__
, __stdout__
and __stderr__
can be
None
. It is usually the case for Windows GUI apps that aren’t connected
to a console and Python apps started with pythonw.
sys.
thread_info
¶A struct sequence holding information about the thread implementation.
Attribute | Explanation |
---|---|
name |
Name of the thread implementation:
|
lock |
Name of the lock implementation:
|
version |
Name and version of the thread library. It is a string,
or None if these informations are unknown. |
New in version 3.3.
sys.
tracebacklimit
¶When this variable is set to an integer value, it determines the maximum number
of levels of traceback information printed when an unhandled exception occurs.
The default is 1000
. When set to 0
or less, all traceback information
is suppressed and only the exception type and value are printed.
sys.
version
¶A string containing the version number of the Python interpreter plus additional
information on the build number and compiler used. This string is displayed
when the interactive interpreter is started. Do not extract version information
out of it, rather, use version_info
and the functions provided by the
platform
module.
sys.
api_version
¶The C API version for this interpreter. Programmers may find this useful when debugging version conflicts between Python and extension modules.
sys.
version_info
¶A tuple containing the five components of the version number: major, minor,
micro, releaselevel, and serial. All values except releaselevel are
integers; the release level is 'alpha'
, 'beta'
, 'candidate'
, or
'final'
. The version_info
value corresponding to the Python version 2.0
is (2, 0, 0, 'final', 0)
. The components can also be accessed by name,
so sys.version_info[0]
is equivalent to sys.version_info.major
and so on.
Changed in version 3.1: Added named component attributes.
sys.
warnoptions
¶This is an implementation detail of the warnings framework; do not modify this
value. Refer to the warnings
module for more information on the warnings
framework.
sys.
winver
¶The version number used to form registry keys on Windows platforms. This is
stored as string resource 1000 in the Python DLL. The value is normally the
first three characters of version
. It is provided in the sys
module for informational purposes; modifying this value has no effect on the
registry keys used by Python. Availability: Windows.
sys.
_xoptions
¶A dictionary of the various implementation-specific flags passed through
the -X
command-line option. Option names are either mapped to
their values, if given explicitly, or to True
. Example:
$ ./python -Xa=b -Xc
Python 3.2a3+ (py3k, Oct 16 2010, 20:14:50)
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys._xoptions
{'a': 'b', 'c': True}
CPython implementation detail: This is a CPython-specific way of accessing options passed through
-X
. Other implementations may export them through other
means, or not at all.
New in version 3.2.
Citations
[C99] | ISO/IEC 9899:1999. “Programming languages – C.” A public draft of this standard is available at http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf. |