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.
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.
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.
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.
If value is not None, this function prints it to sys.stdout, and saves it in builtins._.
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.
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.
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.
Warning
Assigning the traceback return value to a local variable in a function that is handling an exception will cause a circular reference. 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.
Such cycles are normally automatically reclaimed when garbage collection is enabled and they become unreachable, but it remains more efficient to avoid creating cycles.
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.
The struct sequence flags exposes the status of command line flags. The attributes are read only.
| attribute | flag |
|---|---|
| debug | -d |
| division_warning | -Q |
| 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 |
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 | constant representing rounding mode used for arithmetic operations |
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'
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.
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:
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.
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.
Get the profiler function as set by setprofile().
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.
Return a tuple containing five components, describing the Windows version currently running. The elements are major, minor, build, platform, and text. text contains a string while all other values are integers.
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 |
This function wraps the Win32 GetVersionEx() function; see the Microsoft documentation for more information about these fields.
Availability: Windows.
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.
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.
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.
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.
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.
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.
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.
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 explicit finder is found on sys.path_hooks then None is stored to represent the implicit default finder should be used. If the path is not an existing path then imp.NullImporter is set.
Originally specified in PEP 302.
This string contains a platform identifier that can be used to append platform-specific components to sys.path, for instance.
For Unix systems, 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 'linux2', at the time when Python was built. For other systems, the values are:
| System | platform value |
|---|---|
| Windows | 'win32' |
| Windows/Cygwin | 'cygwin' |
| Mac OS X | 'darwin' |
| OS/2 | 'os2' |
| OS/2 EMX | 'os2emx' |
| AtheOS | 'atheos' |
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.
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.
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 she has 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.
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:
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.
File objects corresponding to the interpreter’s standard input, output and error streams. stdin is used for all interpreter input except for scripts but including calls to input(). stdout is used for the output of print() and expression statements and for the prompts of input(). The interpreter’s own prompts and (almost all of) its error messages go to stderr. stdout and stderr needn’t be built-in file objects: any object is acceptable as long as it has a write() method that takes a string argument. (Changing these objects doesn’t affect the standard I/O streams of processes executed by os.popen(), os.system() or the exec*() family of functions in the os module.)
The standard streams are in text mode by default. To write or read binary data to these, use the underlying binary buffer. For example, to write bytes to stdout, use sys.stdout.buffer.write(b'abc'). Using io.TextIOBase.detach() streams can be made binary by default. This function sets stdin and stdout to binary:
def make_streams_binary():
sys.stdin = sys.stdin.detach()
sys.stdout = sys.stdout.detach()
Note that the streams can be replaced with objects (like io.StringIO) that do not support the buffer attribute or the detach() method and can raise AttributeError or io.UnsupportedOperation.
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.
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.
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 . |