30.1. `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.abiflags¶: On POSIX systems where Python was built with the standard configure script, this contains the ABI flags as specified by PEP 3149.

3.2 版新加入.

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).

3.3 版新加入.

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).

3.3 版新加入.

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.breakpointhook()¶

This hook function is called by built-in breakpoint(). By default, it drops you into the pdb debugger, but it can be set to any other function so that you can choose which debugger gets used.

The signature of this function is dependent on what it calls. For example, the default binding (e.g. pdb.set_trace()) expects no arguments, but you might bind it to a function that expects additional arguments (positional and/or keyword). The built-in breakpoint() function passes its *args and **kws straight through. Whatever breakpointhooks() returns is returned from breakpoint().

The default implementation first consults the environment variable PYTHONBREAKPOINT. If that is set to "0" then this function returns immediately; i.e. it is a no-op. If the environment variable is not set, or is set to the empty string, pdb.set_trace() is called. Otherwise this variable should name a function to run, using Python’s dotted-import nomenclature, e.g. package.subpackage.module.function. In this case, package.subpackage.module would be imported and the resulting module must have a callable named function(). This is run, passing in *args and **kws, and whatever function() returns, sys.breakpointhook() returns to the built-in breakpoint() function.

Note that if anything goes wrong while importing the callable named by PYTHONBREAKPOINT, a RuntimeWarning is reported and the breakpoint is ignored.

Also note that if sys.breakpointhook() is overridden programmatically, PYTHONBREAKPOINT is not consulted.

3.7 版新加入.

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.

3.3 版新加入.

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

3.2 版更變: 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.__breakpointhook__¶
sys.__displayhook__¶
sys.__excepthook__¶: These objects contain the original values of breakpointhook, displayhook, and excepthook at the start of the program. They are saved so that breakpointhook, displayhook and excepthook can be restored in case they happen to get replaced with broken or alternative objects.

3.7 版新加入: __breakpointhook__

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.

備註

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.

3.6 版更變: 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.

3.7 版更變: Added utf8_mode attribute for the new -X utf8 flag.

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`
`dev_mode`	`-X` `dev`
`utf8_mode`	`-X` `utf8`

3.2 版更變: Added quiet attribute for the new -q flag.

3.2.3 版新加入: The hash_randomization attribute.

3.3 版更變: Removed obsolete division_warning attribute.

3.7 版更變: Added dev_mode attribute for the new -X dev flag and utf8_mode attribute for the new -X utf8 flag.

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')  

30.1. sys — System-specific parameters and functions¶

30.1. `sys` — System-specific parameters and functions¶