28.1. sys — System-specific parameters and functions

Python 2.7.13

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28.1. `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.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.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.

New in version 2.0.

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.

New in version 2.6.

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.

New in version 2.5.

sys.dllhandle: Integer specifying the handle of the Python DLL. Availability: Windows.

sys.displayhook(value)

If value is not None, this function prints it to sys.stdout, and saves it in __builtin__._.

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.

sys.dont_write_bytecode: If this is true, Python won’t try to write .pyc or .pyo 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.

New in version 2.6.

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 or having executed an except clause.” For any stack frame, only information about the most recently handled exception 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 exception type of the exception being handled (a class object); value gets the exception parameter (its associated value or the second argument to raise, which is always a class instance if the exception type is a class object); traceback gets a traceback object (see the Reference Manual) which encapsulates the call stack at the point where the exception originally occurred.

If exc_clear() is called, this function will return three None values until either another exception is raised in the current thread or the execution stack returns to a frame where another exception is being handled.

Warning

Assigning the traceback return value to a local variable in a function that is handling an exception will cause a circular reference. This will prevent anything referenced by a local variable in the same function or by the traceback from being garbage collected. Since most functions don’t need access to the traceback, the best solution is to use something like exctype, value = sys.exc_info()[:2] to extract only the exception type and value. If you do need the traceback, make sure to delete it after use (best done with a try ... finally statement) or to call exc_info() in a function that does not itself handle an exception.

Note

Beginning with Python 2.2, such cycles are automatically reclaimed when garbage collection is enabled and they become unreachable, but it remains more efficient to avoid creating cycles.

sys.exc_clear()

This function clears all information relating to the current or last exception that occurred in the current thread. After calling this function, exc_info() will return three None values until another exception is raised in the current thread or the execution stack returns to a frame where another exception is being handled.

This function is only needed in only a few obscure situations. These include logging and error handling systems that report information on the last or current exception. This function can also be used to try to free resources and trigger object finalization, though no guarantee is made as to what objects will be freed, if any.

New in version 2.3.

sys.exc_type
sys.exc_value
sys.exc_traceback: Deprecated since version 1.5: Use exc_info() instead.

Since they are global variables, they are not specific to the current thread, so their use is not safe in a multi-threaded program. When no exception is being handled, exc_type is set to None and the other two are undefined.

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 2.7.

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.

sys.exitfunc: This value is not actually defined by the module, but can be set by the user (or by a program) to specify a clean-up action at program exit. When set, it should be a parameterless function. This function will be called when the interpreter exits. Only one function may be installed in this way; to allow multiple functions which will be called at termination, use the atexit module.

Note

The exit function is not called when the program is killed by a signal, when a Python fatal internal error is detected, or when os._exit() is called.

Deprecated since version 2.4: Use atexit instead.

sys.flags

The struct sequence flags exposes the status of command line flags. The attributes are read only.

attribute	flag
`debug`	`-d`
`py3k_warning`	`-3`
`division_warning`	`-Q`
`division_new`	`-Qnew`
`inspect`	`-i`
`interactive`	`-i`
`optimize`	`-O` or `-OO`
`dont_write_bytecode`	`-B`
`no_user_site`	`-s`
`no_site`	`-S`
`ignore_environment`	`-E`
`tabcheck`	`-t` or `-tt`
`verbose`	`-v`
`unicode`	`-U`
`bytes_warning`	`-b`
`hash_randomization`	`-R`

New in version 2.6.

New in version 2.7.3: The hash_randomization attribute.

sys.float_info

A structseq 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'

New in version 2.6.

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 2.7 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 2.7.

New in version 2.7.

sys.getcheckinterval(): Return the interpreter’s “check interval”; see setcheckinterval().

New in version 2.3.

sys.getdefaultencoding(): Return the name of the current default string encoding used by the Unicode implementation.

New in version 2.0.

sys.getdlopenflags(): Return the current value of the flags that are used for dlopen() calls. The flag constants are defined in the dl and DLFCN modules. Availability: Unix.

New in version 2.2.

sys.getfilesystemencoding()

Return the name of the encoding used to convert Unicode filenames into system file names, or None if the system default encoding is used. The result value depends on the operating system:

On Mac OS X, the encoding is 'utf-8'.
On Unix, the encoding is the user’s preference according to the result of nl_langinfo(CODESET), or None if the nl_langinfo(CODESET) failed.
On Windows NT+, file names are Unicode natively, so no conversion is performed. getfilesystemencoding() still returns 'mbcs', as this is the encoding that applications should use when they explicitly want to convert Unicode strings to byte strings that are equivalent when used as file names.
On Windows 9x, the encoding is 'mbcs'.

New in version 2.3.

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.

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.

New in version 2.6.

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().

New in version 2.6.

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.

New in version 2.6.

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, and product_type. service_pack contains a string while 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 may be one of the following values:

Constant	Platform
`0 (VER_PLATFORM_WIN32s)`	Win32s on Windows 3.1
`1 (VER_PLATFORM_WIN32_WINDOWS)`	Windows 95/98/ME
`2 (VER_PLATFORM_WIN32_NT)`	Windows NT/2000/XP/x64
`3 (VER_PLATFORM_WIN32_CE)`	Windows CE

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.

Availability: Windows.

New in version 2.3.

Changed in version 2.7: Changed to a named tuple and added service_pack_minor, service_pack_major, suite_mask, and product_type.

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 version_info value may be used for a more human-friendly encoding of the same information.

The hexversion is a 32-bit number with the following layout:

Bits (big endian order)	Meaning
`1-8`	`PY_MAJOR_VERSION` (the `2` in `2.1.0a3`)
`9-16`	`PY_MINOR_VERSION` (the `1` in `2.1.0a3`)
`17-24`	`PY_MICRO_VERSION` (the `0` in `2.1.0a3`)
`25-28`	`PY_RELEASE_LEVEL` (`0xA` for alpha, `0xB` for beta, `0xC` for release candidate and `0xF` for final)
`29-32`	`PY_RELEASE_SERIAL` (the `3` in `2.1.0a3`, zero for final releases)

Thus 2.1.0a3 is hexversion 0x020100a3.

New in version 1.5.2.

sys.long_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**long_info.bits_per_digit`
`sizeof_digit`	size in bytes of the C type used to represent a digit

New in version 2.7.

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 chapter pdb — The Python Debugger for more information.)

The meaning of the variables is the same as that of the return values from exc_info() above. (Since there is only one interactive thread, thread-safety is not a concern for these variables, unlike for exc_type etc.)

sys.maxint: The largest positive integer supported by Python’s regular integer type. This is at least 2**31-1. The largest negative integer is -maxint-1 — the asymmetry results from the use of 2’s complement binary arithmetic.

sys.maxsize: The largest positive integer supported by the platform’s Py_ssize_t type, and thus the maximum size lists, strings, dicts, and many other containers can have.

sys.maxunicode: An integer giving the largest supported code point for a Unicode character. The value of this depends on the configuration option that specifies whether Unicode characters are stored as UCS-2 or UCS-4.

sys.meta_path

A list of finder objects that have their find_module() methods called to see if one of the objects can find the module to be imported. The find_module() method is called at least with the absolute name of the module being imported. If the module to be imported is contained in package then the parent package’s __path__ attribute is passed in as a second argument. The method returns None if the module cannot be found, else returns a loader.

sys.meta_path is searched before any implicit default finders or sys.path.

See PEP 302 for the original specification.

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. Note that removing a module from this dictionary is not the same as calling reload() on the corresponding module object.

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.

Changed in version 2.3: Unicode strings are no longer ignored.

System	`platform` value
Linux (2.x and 3.x)	`'linux2'`
Windows	`'win32'`
Windows/Cygwin	`'cygwin'`
Mac OS X	`'darwin'`
OS/2	`'os2'`
OS/2 EMX	`'os2emx'`
RiscOS	`'riscos'`
AtheOS	`'atheos'`