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Section: Misc. Reference Manual Pages (1)



python - an interpreted, interactive, object-oriented programming language 








Python is an interpreted, interactive, object-oriented programminglanguage that combines remarkable power with very clear syntax.For an introduction to programming in Python, see the Python Tutorial.The Python Library Reference documents built-in and standard types,constants, functions and modules.Finally, the Python Reference Manual describes the syntax andsemantics of the core language in (perhaps too) much detail.(These documents may be located via theINTERNET RESOURCESbelow; they may be installed on your system as well.)

Python's basic power can be extended with your own modules written inC or C++.On most systems such modules may be dynamically loaded.Python is also adaptable as an extension language for existingapplications.See the internal documentation for hints.

Documentation for installed Python modules and packages can beviewed by running thepydocprogram. 


Don't write.pycfiles on import. See also PYTHONDONTWRITEBYTECODE.
Issue warnings about str(bytes_instance), str(bytearray_instance)and comparing bytes/bytearray with str. (-bb: issue errors)
-c command
Specify the command to execute (see next section).This terminates the option list (following options are passed asarguments to the command).
--check-hash-based-pycs mode
Configure how Python evaluates the up-to-dateness of hash-based .pyc files.
Turn on parser debugging output (for expert only, depending oncompilation options).
Ignore environment variables like PYTHONPATH and PYTHONHOME that modifythe behavior of the interpreter.
-h , -? , --help
Prints the usage for the interpreter executable and exits.
When a script is passed as first argument or the -c option isused, enter interactive mode after executing the script or thecommand. It does not read the $PYTHONSTARTUP file. This can beuseful to inspect global variables or a stack trace when a scriptraises an exception.
Run Python in isolated mode. This also implies -E and -s. Inisolated mode sys.path contains neither the script's directory nor the user'ssite-packages directory. All PYTHON* environment variables are ignored, too.Further restrictions may be imposed to prevent the user from injectingmalicious code.
-m module-name
Searchessys.pathfor the named module and runs the corresponding.pyfile as a script.
Remove assert statements and any code conditional on the value of__debug__; augment the filename for compiled (bytecode) files byadding .opt-1 before the .pyc extension.
Do -O and also discard docstrings; change the filename forcompiled (bytecode) files by adding .opt-2 before the .pyc extension.
Do not print the version and copyright messages. These messages arealso suppressed in non-interactive mode.
Don't add user site directory to sys.path.
Disable the import of the modulesiteand the site-dependent manipulations ofsys.paththat it entails. Also disable these manipulations ifsiteis explicitly imported later.
Force the stdout and stderr streams to be unbuffered.This option has no effect on the stdin stream.
Print a message each time a module is initialized, showing the place(filename or built-in module) from which it is loaded. When giventwice, print a message for each file that is checked for whensearching for a module. Also provides information on module cleanupat exit.
-V , --version
Prints the Python version number of the executable and exits. When giventwice, print more information about the build.
-W argument
Warning control. Python sometimes prints warning message tosys.stderr.A typical warning message has the following form:file:line: category: message.By default, each warning is printed once for each source line where itoccurs. This option controls how often warnings are printed.Multiple-Woptions may be given; when a warning matches more than oneoption, the action for the last matching option is performed.Invalid-Woptions are ignored (a warning message is printed about invalidoptions when the first warning is issued). Warnings can also becontrolled from within a Python program using thewarningsmodule.

The simplest form ofargumentis one of the followingactionstrings (or a unique abbreviation):ignoreto ignore all warnings;defaultto explicitly request the default behavior (printing each warning onceper source line);allto print a warning each time it occurs (this may generate manymessages if a warning is triggered repeatedly for the same sourceline, such as inside a loop);moduleto print each warning only the first time it occurs in eachmodule;onceto print each warning only the first time it occurs in the program; orerrorto raise an exception instead of printing a warning message.

The full form ofargumentisaction:message:category:module:line.Here,actionis as explained above but only applies to messages that match theremaining fields. Empty fields match all values; trailing emptyfields may be omitted. Themessagefield matches the start of the warning message printed; this match iscase-insensitive. Thecategoryfield matches the warning category. This must be a class name; thematch test whether the actual warning category of the message is asubclass of the specified warning category. The full class name mustbe given. Themodulefield matches the (fully-qualified) module name; this match iscase-sensitive. Thelinefield matches the line number, where zero matches all line numbers andis thus equivalent to an omitted line number.

-X option
Set implementation specific option. The following options are available:

    -X faulthandler: enable faulthandler

    -X showrefcount: output the total reference count and number of used
        memory blocks when the program finishes or after each statement in the
        interactive interpreter. This only works on debug builds

    -X tracemalloc: start tracing Python memory allocations using the
        tracemalloc module. By default, only the most recent frame is stored in a
        traceback of a trace. Use -X tracemalloc=NFRAME to start tracing with a
        traceback limit of NFRAME frames

    -X showalloccount: output the total count of allocated objects for each
        type when the program finishes. This only works when Python was built with
        COUNT_ALLOCS defined

    -X importtime: show how long each import takes. It shows module name,
        cumulative time (including nested imports) and self time (excluding
        nested imports). Note that its output may be broken in multi-threaded
        application. Typical usage is python3 -X importtime -c 'import asyncio'

    -X dev: enable CPython's "development mode", introducing additional runtime
        checks which are too expensive to be enabled by default. It will not be
        more verbose than the default if the code is correct: new warnings are
        only emitted when an issue is detected. Effect of the developer mode:
           * Add default warning filter, as -W default
           * Install debug hooks on memory allocators: see the PyMem_SetupDebugHooks() C function
           * Enable the faulthandler module to dump the Python traceback on a crash
           * Enable asyncio debug mode
           * Set the dev_mode attribute of sys.flags to True
           * io.IOBase destructor logs close() exceptions

    -X utf8: enable UTF-8 mode for operating system interfaces, overriding the default
        locale-aware mode. -X utf8=0 explicitly disables UTF-8 mode (even when it would
        otherwise activate automatically). See PYTHONUTF8 for more details

    -X pycache_prefix=PATH: enable writing .pyc files to a parallel tree rooted at the
         given directory instead of to the code tree.

Skip the first line of the source. This is intended for a DOSspecific hack only. Warning: the line numbers in error messages willbe off by one!


The interpreter interface resembles that of the UNIX shell: whencalled with standard input connected to a tty device, it prompts forcommands and executes them until an EOF is read; when called with afile name argument or with a file as standard input, it reads andexecutes ascriptfrom that file;when called with-ccommand,it executes the Python statement(s) given ascommand.Herecommandmay contain multiple statements separated by newlines.Leading whitespace is significant in Python statements!In non-interactive mode, the entire input is parsed before it isexecuted.

If available, the script name and additional arguments thereafter arepassed to the script in the Python variablesys.argv,which is a list of strings (you must firstimport systo be able to access it).If no script name is given,sys.argv[0]is an empty string; if-cis used,sys.argv[0]contains the string'-c'.Note that options interpreted by the Python interpreter itselfare not placed insys.argv.

In interactive mode, the primary prompt is `>>>'; the second prompt(which appears when a command is not complete) is `...'.The prompts can be changed by assignment tosys.ps1orsys.ps2.The interpreter quits when it reads an EOF at a prompt.When an unhandled exception occurs, a stack trace is printed andcontrol returns to the primary prompt; in non-interactive mode, theinterpreter exits after printing the stack trace.The interrupt signal raises theKeyboardInterruptexception; other UNIX signals are not caught (except that SIGPIPE issometimes ignored, in favor of theIOErrorexception). Error messages are written to stderr. 


These are subject to difference depending on local installationconventions; ${prefix} and ${exec_prefix} are installation-dependentand should be interpreted as for GNU software; they may be the same.The default for both is /usr/local.
Recommended location of the interpreter.


Recommended locations of the directories containing the standardmodules.


Recommended locations of the directories containing the include filesneeded for developing Python extensions and embedding theinterpreter.


Change the location of the standard Python libraries. By default, thelibraries are searched in ${prefix}/lib/python<version> and${exec_prefix}/lib/python<version>, where ${prefix} and ${exec_prefix}are installation-dependent directories, both defaulting to/usr/local. When $PYTHONHOME is set to a single directory, its valuereplaces both ${prefix} and ${exec_prefix}. To specify different valuesfor these, set $PYTHONHOME to ${prefix}:${exec_prefix}.
Augments the default search path for module files.The format is the same as the shell's $PATH: one or more directorypathnames separated by colons.Non-existent directories are silently ignored.The default search path is installation dependent, but generallybegins with ${prefix}/lib/python<version> (see PYTHONHOME above).The default search path is always appended to $PYTHONPATH.If a script argument is given, the directory containing the script isinserted in the path in front of $PYTHONPATH.The search path can be manipulated from within a Python program as thevariablesys.path.
If this is the name of a readable file, the Python commands in thatfile are executed before the first prompt is displayed in interactivemode.The file is executed in the same name space where interactive commandsare executed so that objects defined or imported in it can be usedwithout qualification in the interactive session.You can also change the promptssys.ps1andsys.ps2in this file.
If this is set to a non-empty string it is equivalent to specifyingthe -O option. If set to an integer, it is equivalent tospecifying -O multiple times.
If this is set to a non-empty string it is equivalent to specifyingthe -d option. If set to an integer, it is equivalent tospecifying -d multiple times.
If this is set to a non-empty string it is equivalent to specifyingthe -B option (don't try to write.pycfiles).
If this is set to a non-empty string it is equivalent to specifyingthe -i option.
If this is set before running the interpreter, it overrides the encoding usedfor stdin/stdout/stderr, in the syntaxencodingname:errorhandlerTheerrorhandlerpart is optional and has the same meaning as in str.encode. For stderr, theerrorhandler
 part is ignored; the handler will always be 'backslashreplace'.
If this is set to a non-empty string it is equivalent to specifying the-s option (Don't add the user site directory to sys.path).
If this is set to a non-empty string it is equivalent to specifyingthe -u option.
If this is set to a non-empty string it is equivalent to specifyingthe -v option. If set to an integer, it is equivalent tospecifying -v multiple times.
If this is set to a comma-separated string it is equivalent tospecifying the -W option for each separate value.
If this variable is set to "random", a random value is used to seed the hashesof str and bytes objects.

If PYTHONHASHSEED is set to an integer value, it is used as a fixed seed forgenerating the hash() of the types covered by the hash randomization. Itspurpose is to allow repeatable hashing, such as for selftests for theinterpreter itself, or to allow a cluster of python processes to share hashvalues.

The integer must be a decimal number in the range [0,4294967295]. Specifyingthe value 0 will disable hash randomization.

Set the Python memory allocators and/or install debug hooks. The availablememory allocators aremallocandpymalloc.The available debug hooks aredebug,malloc_debug,andpymalloc_debug.
When Python is compiled in debug mode, the default ispymalloc_debugand the debug hooks are automatically used. Otherwise, the default ispymalloc.
If set to a non-empty string, Python will print statistics of the pymallocmemory allocator every time a new pymalloc object arena is created, and onshutdown.
This variable is ignored if the$PYTHONMALLOCenvironment variable is used to force themalloc(3)allocator of the C library, or if Python is configured without pymalloc support.
If this environment variable is set to a non-empty string, enable the debugmode of the asyncio module.
If this environment variable is set to a non-empty string, start tracingPython memory allocations using the tracemalloc module.
The value of the variable is the maximum number of frames stored in atraceback of a trace. For example,PYTHONTRACEMALLOC=1stores only the most recent frame.
If this environment variable is set to a non-empty string,faulthandler.enable()is called at startup: install a handler for SIGSEGV, SIGFPE, SIGABRT, SIGBUSand SIGILL signals to dump the Python traceback.
This is equivalent to the -X faulthandler option.
If this environment variable is set,sys.argv[0]will be set to its value instead of the value got through the C runtime. Onlyworks on Mac OS X.
Defines the user base directory, which is used to compute the path of the usersite-packagesdirectory and Distutils installation paths forpython install --user.
If this environment variable is set to a non-empty string, Python willshow how long each import takes. This is exactly equivalent to setting-X importtime on the command line.
If this environment variable is set to 0, it disables the default debugger. Itcan be set to the callable of your debugger of choice.

Debug-mode variables

Setting these variables only has an effect in a debug build of Python, that is,if Python was configured with the--with-pydebug build option.
If this environment variable is set, Python will print threading debug info.
If this environment variable is set, Python will dump objects and referencecounts still alive after shutting down the interpreter.


The Python Software Foundation: 


Main website:
Developer resources:
Module repository:
Newsgroups: comp.lang.python, comp.lang.python.announce 


Python is distributed under an Open Source license. See the file"LICENSE" in the Python source distribution for information on terms &conditions for accessing and otherwise using Python and for aDISCLAIMER OF ALL WARRANTIES.



Debug-mode variables

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