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26. Python Runtime Services

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26.2. __builtin__ — Built-in objects

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26.1. 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.


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.


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.


A triple (repo, branch, version) representing the Subversion information of the Python interpreter. repo is the name of the repository, 'CPython'. branch is a string of one of the forms 'trunk', 'branches/name' or 'tags/name'. version is the output of svnversion, if the interpreter was built from a Subversion checkout; it contains the revision number (range) and possibly a trailing ‘M’ if there were local modifications. If the tree was exported (or svnversion was not available), it is the revision of Include/patchlevel.h if the branch is a tag. Otherwise, it is None.

New in version 2.5.


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.)


A string containing the copyright pertaining to the Python interpreter.


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.


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.


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


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.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.


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.


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.


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/pythonversion/config', and shared library modules are installed in exec_prefix + '/lib/pythonversion/lib-dynload', where version is equal to version[:3].


A string giving the name of the executable binary for the Python interpreter, on systems where this makes sense.


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 sys.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.


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.


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_site -S
ignore_environment -E
tabcheck -t or -tt
verbose -v
unicode -U

New in version 2.6.


A structseq holding information about the float type. It contains low level information about the precision and internal representation. Please study your system’s float.h for more information.

attribute explanation
epsilon Difference between 1 and the next representable floating point number
dig digits (see float.h)
mant_dig mantissa digits (see float.h)
max maximum representable finite float
max_exp maximum int e such that radix**(e-1) is in the range of finite representable floats
max_10_exp maximum int e such that 10**e is in the range of finite representable floats
min Minimum positive normalizer float
min_exp minimum int e such that radix**(e-1) is a normalized float
min_10_exp minimum int e such that 10**e is a normalized float
radix radix of exponent
rounds addition rounds (see float.h)

Note: The information in the table is simplified.

New in version 2.6.


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

New in version 2.3.


Return the name of the current default string encoding used by the Unicode implementation.

New in version 2.0.


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.


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 Windows 9x, the encoding is “mbcs”.
  • 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.

New in version 2.3.


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().


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.

The default argument allows to define a value which will be returned if the object type does not provide means to retrieve the size and would cause a TypeError.

func: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.


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.

This function should be used for internal and specialized purposes only.


Get the profiler function as set by setprofile().

New in version 2.6.


Get the trace function as set by settrace().

Note: 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.


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
2 (VER_PLATFORM_WIN32_NT) Windows NT/2000/XP/x64

This function wraps the Win32 GetVersionEx() function; see the Microsoft documentation for more information about these fields.

Availability: Windows.

New in version 2.3.


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 ...
# 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.

New in version 1.5.2.

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; 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.)


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.


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.


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.


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.


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.


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.

See also:

Module site This describes how to use .pth files to extend sys.path.


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'
RiscOS 'riscos'
AtheOS 'atheos'


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/pythonversion' while the platform independent header files (all except pyconfig.h) are stored in prefix + '/include/pythonversion', where version is equal to version[:3].

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.


Bool containing the status of the Python 3.0 warning flag. It’s True when Python is started with the -3 option. (This should be considered read-only; setting it to a different value doesn’t have an effect on Python 3.0 warnings.)

New in version 2.6.


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.


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.


Set the current default string encoding used by the Unicode implementation. If name does not match any available encoding, LookupError is raised. This function is only intended to be used by the site module implementation and, where needed, by sitecustomize. Once used by the site module, it is removed from the sys module’s namespace.

New in version 2.0.


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(dl.RTLD_NOW | dl.RTLD_GLOBAL). Symbolic names for the flag modules can be either found in the dl module, or in the DLFCN module. If DLFCN is not available, it can be generated from /usr/include/dlfcn.h using the h2py script. Availability: Unix.

New in version 2.2.


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:

A function is called (or some other code block entered). The global trace function is called; arg is None; the return value specifies the local trace function.
The interpreter is about to execute a new line of code (sometimes multiple line events on one line exist). The local trace function is called; arg is None; the return value specifies the new local trace function.
A function (or other code block) is about to return. The local trace function is called; arg is the value that will be returned. The trace function’s return value is ignored.
An exception has occurred. The local trace function is called; arg is a tuple (exception, value, traceback); the return value specifies the new local trace function.
A C function is about to be called. This may be an extension function or a builtin. arg is the C function object.
A C function has returned. arg is None.
A C function has thrown an exception. arg is None.

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.

Note: 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.


Activate dumping of VM measurements using the Pentium timestamp counter, if on_flag is true. Deactivate these dumps if on_flag is off. The function is available only if Python was compiled with –with-tsc. To understand the output of this dump, read Python/ceval.c in the Python sources.

New in version 2.4.

sys.stdin sys.stdout sys.stderr

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() and raw_input(). stdout is used for the output of print and expression statements and for the prompts of input() and raw_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.)

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.


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.


A string containing the version number of the Python interpreter plus additional information on the build number and compiler used. It has a value of the form 'version (#build_number, build_date, build_time) [compiler]'. The first three characters are used to identify the version in the installation directories (where appropriate on each platform). An example:

>>> import sys
>>> sys.version
'1.5.2 (#0 Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)]'


The C API version for this interpreter. Programmers may find this useful when debugging version conflicts between Python and extension modules.

New in version 2.3.


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).

New in version 2.0.


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.


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.