1. Command line and environment
*******************************

The CPython interpreter scans the command line and the environment for
various settings.

**CPython implementation detail:** Other implementations’ command line
schemes may differ.  See Alternate Implementations for further
resources.


1.1. Command line
=================

When invoking Python, you may specify any of these options:

   python [-bBdEhiIOqsSuvVWx?] [-c command | -m module-name | script | - ] [args]

The most common use case is, of course, a simple invocation of a
script:

   python myscript.py


1.1.1. Interface options
------------------------

The interpreter interface resembles that of the UNIX shell, but
provides some additional methods of invocation:

* When called with standard input connected to a tty device, it
  prompts for commands and executes them until an EOF (an end-of-file
  character, you can produce that with "Ctrl-D" on UNIX or "Ctrl-Z,
  Enter" on Windows) is read.

* When called with a file name argument or with a file as standard
  input, it reads and executes a script from that file.

* When called with a directory name argument, it reads and executes an
  appropriately named script from that directory.

* When called with "-c command", it executes the Python statement(s)
  given as *command*.  Here *command* may contain multiple statements
  separated by newlines. Leading whitespace is significant in Python
  statements!

* When called with "-m module-name", the given module is located on
  the Python module path and executed as a script.

In non-interactive mode, the entire input is parsed before it is
executed.

An interface option terminates the list of options consumed by the
interpreter, all consecutive arguments will end up in "sys.argv" –
note that the first element, subscript zero ("sys.argv[0]"), is a
string reflecting the program’s source.

-c <command>

   Execute the Python code in *command*.  *command* can be one or more
   statements separated by newlines, with significant leading
   whitespace as in normal module code.

   If this option is given, the first element of "sys.argv" will be
   ""-c"" and the current directory will be added to the start of
   "sys.path" (allowing modules in that directory to be imported as
   top level modules).

   Raises an auditing event "cpython.run_command" with argument
   "command".

-m <module-name>

   Search "sys.path" for the named module and execute its contents as
   the "__main__" module.

   Since the argument is a *module* name, you must not give a file
   extension (".py").  The module name should be a valid absolute
   Python module name, but the implementation may not always enforce
   this (e.g. it may allow you to use a name that includes a hyphen).

   Package names (including namespace packages) are also permitted.
   When a package name is supplied instead of a normal module, the
   interpreter will execute "<pkg>.__main__" as the main module. This
   behaviour is deliberately similar to the handling of directories
   and zipfiles that are passed to the interpreter as the script
   argument.

   Note:

     This option cannot be used with built-in modules and extension
     modules written in C, since they do not have Python module files.
     However, it can still be used for precompiled modules, even if
     the original source file is not available.

   If this option is given, the first element of "sys.argv" will be
   the full path to the module file (while the module file is being
   located, the first element will be set to ""-m""). As with the "-c"
   option, the current directory will be added to the start of
   "sys.path".

   "-I" option can  be used to run the script in isolated mode where
   "sys.path" contains neither the current directory nor the user’s
   site-packages directory. All "PYTHON*" environment variables are
   ignored, too.

   Many standard library modules contain code that is invoked on their
   execution as a script.  An example is the "timeit" module:

      python -m timeit -s 'setup here' 'benchmarked code here'
      python -m timeit -h # for details

   Raises an auditing event "cpython.run_module" with argument
   "module-name".

   See also:

     "runpy.run_module()"
        Equivalent functionality directly available to Python code

     **PEP 338** – Executing modules as scripts

   Changed in version 3.1: Supply the package name to run a "__main__"
   submodule.

   Changed in version 3.4: namespace packages are also supported

-

   Read commands from standard input ("sys.stdin").  If standard input
   is a terminal, "-i" is implied.

   If this option is given, the first element of "sys.argv" will be
   ""-"" and the current directory will be added to the start of
   "sys.path".

   Raises an auditing event "cpython.run_stdin" with no arguments.

<script>

   Execute the Python code contained in *script*, which must be a
   filesystem path (absolute or relative) referring to either a Python
   file, a directory containing a "__main__.py" file, or a zipfile
   containing a "__main__.py" file.

   If this option is given, the first element of "sys.argv" will be
   the script name as given on the command line.

   If the script name refers directly to a Python file, the directory
   containing that file is added to the start of "sys.path", and the
   file is executed as the "__main__" module.

   If the script name refers to a directory or zipfile, the script
   name is added to the start of "sys.path" and the "__main__.py" file
   in that location is executed as the "__main__" module.

   "-I" option can  be used to run the script in isolated mode where
   "sys.path" contains neither the script’s directory nor the user’s
   site-packages directory. All "PYTHON*" environment variables are
   ignored, too.

   Raises an auditing event "cpython.run_file" with argument
   "filename".

   See also:

     "runpy.run_path()"
        Equivalent functionality directly available to Python code

If no interface option is given, "-i" is implied, "sys.argv[0]" is an
empty string ("""") and the current directory will be added to the
start of "sys.path".  Also, tab-completion and history editing is
automatically enabled, if available on your platform (see Readline
configuration).

See also: Invoking the Interpreter

Changed in version 3.4: Automatic enabling of tab-completion and
history editing.


1.1.2. Generic options
----------------------

-?
-h
--help

   Print a short description of all command line options.

-V
--version

   Print the Python version number and exit.  Example output could be:

      Python 3.8.0b2+

   When given twice, print more information about the build, like:

      Python 3.8.0b2+ (3.8:0c076caaa8, Apr 20 2019, 21:55:00)
      [GCC 6.2.0 20161005]

   New in version 3.6: The "-VV" option.


1.1.3. Miscellaneous options
----------------------------

-b

   Issue a warning when comparing "bytes" or "bytearray" with "str" or
   "bytes" with "int".  Issue an error when the option is given twice
   ("-bb").

   Changed in version 3.5: Affects comparisons of "bytes" with "int".

-B

   If given, Python won’t try to write ".pyc" files on the import of
   source modules.  See also "PYTHONDONTWRITEBYTECODE".

--check-hash-based-pycs default|always|never

   Control the validation behavior of hash-based ".pyc" files. See
   Cached bytecode invalidation. When set to "default", checked and
   unchecked hash-based bytecode cache files are validated according
   to their default semantics. When set to "always", all hash-based
   ".pyc" files, whether checked or unchecked, are validated against
   their corresponding source file. When set to "never", hash-based
   ".pyc" files are not validated against their corresponding source
   files.

   The semantics of timestamp-based ".pyc" files are unaffected by
   this option.

-d

   Turn on parser debugging output (for expert only, depending on
   compilation options).  See also "PYTHONDEBUG".

-E

   Ignore all "PYTHON*" environment variables, e.g. "PYTHONPATH" and
   "PYTHONHOME", that might be set.

-i

   When a script is passed as first argument or the "-c" option is
   used, enter interactive mode after executing the script or the
   command, even when "sys.stdin" does not appear to be a terminal.
   The "PYTHONSTARTUP" file is not read.

   This can be useful to inspect global variables or a stack trace
   when a script raises an exception.  See also "PYTHONINSPECT".

-I

   Run Python in isolated mode. This also implies -E and -s. In
   isolated mode "sys.path" contains neither the script’s directory
   nor the user’s site-packages directory. All "PYTHON*" environment
   variables are ignored, too. Further restrictions may be imposed to
   prevent the user from injecting malicious code.

   New in version 3.4.

-O

   Remove assert statements and any code conditional on the value of
   "__debug__".  Augment the filename for compiled (*bytecode*) files
   by adding ".opt-1" before the ".pyc" extension (see **PEP 488**).
   See also "PYTHONOPTIMIZE".

   Changed in version 3.5: Modify ".pyc" filenames according to **PEP
   488**.

-OO

   Do "-O" and also discard docstrings.  Augment the filename for
   compiled (*bytecode*) files by adding ".opt-2" before the ".pyc"
   extension (see **PEP 488**).

   Changed in version 3.5: Modify ".pyc" filenames according to **PEP
   488**.

-q

   Don’t display the copyright and version messages even in
   interactive mode.

   New in version 3.2.

-R

   Turn on hash randomization. This option only has an effect if the
   "PYTHONHASHSEED" environment variable is set to "0", since hash
   randomization is enabled by default.

   On previous versions of Python, this option turns on hash
   randomization, so that the "__hash__()" values of str and bytes
   objects are “salted” with an unpredictable random value.  Although
   they remain constant within an individual Python process, they are
   not predictable between repeated invocations of Python.

   Hash randomization is intended to provide protection against a
   denial-of-service caused by carefully-chosen inputs that exploit
   the worst case performance of a dict construction, O(n^2)
   complexity.  See
   http://www.ocert.org/advisories/ocert-2011-003.html for details.

   "PYTHONHASHSEED" allows you to set a fixed value for the hash seed
   secret.

   Changed in version 3.7: The option is no longer ignored.

   New in version 3.2.3.

-s

   Don’t add the "user site-packages directory" to "sys.path".

   See also: **PEP 370** – Per user site-packages directory

-S

   Disable the import of the module "site" and the site-dependent
   manipulations of "sys.path" that it entails.  Also disable these
   manipulations if "site" is explicitly imported later (call
   "site.main()" if you want them to be triggered).

-u

   Force the stdout and stderr streams to be unbuffered.  This option
   has no effect on the stdin stream.

   See also "PYTHONUNBUFFERED".

   Changed in version 3.7: The text layer of the stdout and stderr
   streams now is unbuffered.

-v

   Print a message each time a module is initialized, showing the
   place (filename or built-in module) from which it is loaded.  When
   given twice ("-vv"), print a message for each file that is checked
   for when searching for a module.  Also provides information on
   module cleanup at exit. See also "PYTHONVERBOSE".

-W arg

   Warning control.  Python’s warning machinery by default prints
   warning messages to "sys.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
   it occurs.  This option controls how often warnings are printed.

   Multiple "-W" options may be given; when a warning matches more
   than one option, the action for the last matching option is
   performed.  Invalid "-W" options are ignored (though, a warning
   message is printed about invalid options when the first warning is
   issued).

   Warnings can also be controlled using the "PYTHONWARNINGS"
   environment variable and from within a Python program using the
   "warnings" module.

   The simplest settings apply a particular action unconditionally to
   all warnings emitted by a process (even those that are otherwise
   ignored by default):

      -Wdefault  # Warn once per call location
      -Werror    # Convert to exceptions
      -Walways   # Warn every time
      -Wmodule   # Warn once per calling module
      -Wonce     # Warn once per Python process
      -Wignore   # Never warn

   The action names can be abbreviated as desired (e.g. "-Wi", "-Wd",
   "-Wa", "-We") and the interpreter will resolve them to the
   appropriate action name.

   See The Warnings Filter and Describing Warning Filters for more
   details.

-x

   Skip the first line of the source, allowing use of non-Unix forms
   of "#!cmd".  This is intended for a DOS specific hack only.

-X

   Reserved for various implementation-specific options.  CPython
   currently defines the following possible values:

   * "-X faulthandler" to enable "faulthandler";

   * "-X oldparser": enable the traditional LL(1) parser.  See also
     "PYTHONOLDPARSER" and **PEP 617**.

   * "-X showrefcount" to 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" to 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. See
     the "tracemalloc.start()" for more information.

   * "-X importtime" to 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'".  See also
     "PYTHONPROFILEIMPORTTIME".

   * "-X dev": enable Python Development Mode, introducing additional
     runtime checks that are too expensive to be enabled by default.

   * "-X utf8" enables 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" enables writing ".pyc" files to a
     parallel tree rooted at the given directory instead of to the
     code tree. See also "PYTHONPYCACHEPREFIX".

   It also allows passing arbitrary values and retrieving them through
   the "sys._xoptions" dictionary.

   Changed in version 3.2: The "-X" option was added.

   New in version 3.3: The "-X faulthandler" option.

   New in version 3.4: The "-X showrefcount" and "-X tracemalloc"
   options.

   New in version 3.6: The "-X showalloccount" option.

   New in version 3.7: The "-X importtime", "-X dev" and "-X utf8"
   options.

   New in version 3.8: The "-X pycache_prefix" option. The "-X dev"
   option now logs "close()" exceptions in "io.IOBase" destructor.

   Changed in version 3.9: Using "-X dev" option, check *encoding* and
   *errors* arguments on string encoding and decoding operations.The
   "-X showalloccount" option has been removed.

   Deprecated since version 3.9, will be removed in version 3.10: The
   "-X oldparser" option.


1.1.4. Options you shouldn’t use
--------------------------------

-J

   Reserved for use by Jython.


1.2. Environment variables
==========================

These environment variables influence Python’s behavior, they are
processed before the command-line switches other than -E or -I.  It is
customary that command-line switches override environmental variables
where there is a conflict.

PYTHONHOME

   Change the location of the standard Python libraries.  By default,
   the libraries 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 value replaces
   both "*prefix*" and "*exec_prefix*".  To specify different values
   for these, set "PYTHONHOME" to "*prefix*:*exec_prefix*".

PYTHONPATH

   Augment the default search path for module files.  The format is
   the same as the shell’s "PATH": one or more directory pathnames
   separated by "os.pathsep" (e.g. colons on Unix or semicolons on
   Windows). Non-existent directories are silently ignored.

   In addition to normal directories, individual "PYTHONPATH" entries
   may refer to zipfiles containing pure Python modules (in either
   source or compiled form). Extension modules cannot be imported from
   zipfiles.

   The default search path is installation dependent, but generally
   begins with "*prefix*/lib/python*version*" (see "PYTHONHOME"
   above).  It is *always* appended to "PYTHONPATH".

   An additional directory will be inserted in the search path in
   front of "PYTHONPATH" as described above under Interface options.
   The search path can be manipulated from within a Python program as
   the variable "sys.path".

PYTHONPLATLIBDIR

   If this is set to a non-empty string, it overrides the
   "sys.platlibdir" value.

   New in version 3.9.

PYTHONSTARTUP

   If this is the name of a readable file, the Python commands in that
   file are executed before the first prompt is displayed in
   interactive mode.  The file is executed in the same namespace where
   interactive commands are executed so that objects defined or
   imported in it can be used without qualification in the interactive
   session.  You can also change the prompts "sys.ps1" and "sys.ps2"
   and the hook "sys.__interactivehook__" in this file.

   Raises an auditing event "cpython.run_startup" with the filename as
   the argument when called on startup.

PYTHONOPTIMIZE

   If this is set to a non-empty string it is equivalent to specifying
   the "-O" option.  If set to an integer, it is equivalent to
   specifying "-O" multiple times.

PYTHONBREAKPOINT

   If this is set, it names a callable using dotted-path notation.
   The module containing the callable will be imported and then the
   callable will be run by the default implementation of
   "sys.breakpointhook()" which itself is called by built-in
   "breakpoint()".  If not set, or set to the empty string, it is
   equivalent to the value “pdb.set_trace”.  Setting this to the
   string “0” causes the default implementation of
   "sys.breakpointhook()" to do nothing but return immediately.

   New in version 3.7.

PYTHONDEBUG

   If this is set to a non-empty string it is equivalent to specifying
   the "-d" option.  If set to an integer, it is equivalent to
   specifying "-d" multiple times.

PYTHONOLDPARSER

   If this is set to a non-empty string, enable the traditional LL(1)
   parser.

   See also the "-X" "oldparser" option and **PEP 617**.

   Deprecated since version 3.9, will be removed in version 3.10.

PYTHONINSPECT

   If this is set to a non-empty string it is equivalent to specifying
   the "-i" option.

   This variable can also be modified by Python code using
   "os.environ" to force inspect mode on program termination.

PYTHONUNBUFFERED

   If this is set to a non-empty string it is equivalent to specifying
   the "-u" option.

PYTHONVERBOSE

   If this is set to a non-empty string it is equivalent to specifying
   the "-v" option.  If set to an integer, it is equivalent to
   specifying "-v" multiple times.

PYTHONCASEOK

   If this is set, Python ignores case in "import" statements.  This
   only works on Windows and macOS.

PYTHONDONTWRITEBYTECODE

   If this is set to a non-empty string, Python won’t try to write
   ".pyc" files on the import of source modules.  This is equivalent
   to specifying the "-B" option.

PYTHONPYCACHEPREFIX

   If this is set, Python will write ".pyc" files in a mirror
   directory tree at this path, instead of in "__pycache__"
   directories within the source tree. This is equivalent to
   specifying the "-X" "pycache_prefix=PATH" option.

   New in version 3.8.

PYTHONHASHSEED

   If this variable is not set or set to "random", a random value is
   used to seed the hashes of str and bytes objects.

   If "PYTHONHASHSEED" is set to an integer value, it is used as a
   fixed seed for generating the hash() of the types covered by the
   hash randomization.

   Its purpose is to allow repeatable hashing, such as for selftests
   for the interpreter itself, or to allow a cluster of python
   processes to share hash values.

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

   New in version 3.2.3.

PYTHONIOENCODING

   If this is set before running the interpreter, it overrides the
   encoding used for stdin/stdout/stderr, in the syntax
   "encodingname:errorhandler".  Both the "encodingname" and the
   ":errorhandler" parts are optional and have the same meaning as in
   "str.encode()".

   For stderr, the ":errorhandler" part is ignored; the handler will
   always be "'backslashreplace'".

   Changed in version 3.4: The "encodingname" part is now optional.

   Changed in version 3.6: On Windows, the encoding specified by this
   variable is ignored for interactive console buffers unless
   "PYTHONLEGACYWINDOWSSTDIO" is also specified. Files and pipes
   redirected through the standard streams are not affected.

PYTHONNOUSERSITE

   If this is set, Python won’t add the "user site-packages directory"
   to "sys.path".

   See also: **PEP 370** – Per user site-packages directory

PYTHONUSERBASE

   Defines the "user base directory", which is used to compute the
   path of the "user site-packages directory" and Distutils
   installation paths for "python setup.py install --user".

   See also: **PEP 370** – Per user site-packages directory

PYTHONEXECUTABLE

   If this environment variable is set, "sys.argv[0]" will be set to
   its value instead of the value got through the C runtime.  Only
   works on macOS.

PYTHONWARNINGS

   This is equivalent to the "-W" option. If set to a comma separated
   string, it is equivalent to specifying "-W" multiple times, with
   filters later in the list taking precedence over those earlier in
   the list.

   The simplest settings apply a particular action unconditionally to
   all warnings emitted by a process (even those that are otherwise
   ignored by default):

      PYTHONWARNINGS=default  # Warn once per call location
      PYTHONWARNINGS=error    # Convert to exceptions
      PYTHONWARNINGS=always   # Warn every time
      PYTHONWARNINGS=module   # Warn once per calling module
      PYTHONWARNINGS=once     # Warn once per Python process
      PYTHONWARNINGS=ignore   # Never warn

   See The Warnings Filter and Describing Warning Filters for more
   details.

PYTHONFAULTHANDLER

   If this environment variable is set to a non-empty string,
   "faulthandler.enable()" is called at startup: install a handler for
   "SIGSEGV", "SIGFPE", "SIGABRT", "SIGBUS" and "SIGILL" signals to
   dump the Python traceback.  This is equivalent to "-X"
   "faulthandler" option.

   New in version 3.3.

PYTHONTRACEMALLOC

   If this environment variable is set to a non-empty string, start
   tracing Python memory allocations using the "tracemalloc" module.
   The value of the variable is the maximum number of frames stored in
   a traceback of a trace. For example, "PYTHONTRACEMALLOC=1" stores
   only the most recent frame. See the "tracemalloc.start()" for more
   information.

   New in version 3.4.

PYTHONPROFILEIMPORTTIME

   If this environment variable is set to a non-empty string, Python
   will show how long each import takes.  This is exactly equivalent
   to setting "-X importtime" on the command line.

   New in version 3.7.

PYTHONASYNCIODEBUG

   If this environment variable is set to a non-empty string, enable
   the debug mode of the "asyncio" module.

   New in version 3.4.

PYTHONMALLOC

   Set the Python memory allocators and/or install debug hooks.

   Set the family of memory allocators used by Python:

   * "default": use the default memory allocators.

   * "malloc": use the "malloc()" function of the C library for all
     domains ("PYMEM_DOMAIN_RAW", "PYMEM_DOMAIN_MEM",
     "PYMEM_DOMAIN_OBJ").

   * "pymalloc": use the pymalloc allocator for "PYMEM_DOMAIN_MEM" and
     "PYMEM_DOMAIN_OBJ" domains and use the "malloc()" function for
     the "PYMEM_DOMAIN_RAW" domain.

   Install debug hooks:

   * "debug": install debug hooks on top of the default memory
     allocators.

   * "malloc_debug": same as "malloc" but also install debug hooks.

   * "pymalloc_debug": same as "pymalloc" but also install debug
     hooks.

   See the default memory allocators and the "PyMem_SetupDebugHooks()"
   function (install debug hooks on Python memory allocators).

   Changed in version 3.7: Added the ""default"" allocator.

   New in version 3.6.

PYTHONMALLOCSTATS

   If set to a non-empty string, Python will print statistics of the
   pymalloc memory allocator every time a new pymalloc object arena is
   created, and on shutdown.

   This variable is ignored if the "PYTHONMALLOC" environment variable
   is used to force the "malloc()" allocator of the C library, or if
   Python is configured without "pymalloc" support.

   Changed in version 3.6: This variable can now also be used on
   Python compiled in release mode. It now has no effect if set to an
   empty string.

PYTHONLEGACYWINDOWSFSENCODING

   If set to a non-empty string, the default filesystem encoding and
   errors mode will revert to their pre-3.6 values of ‘mbcs’ and
   ‘replace’, respectively. Otherwise, the new defaults ‘utf-8’ and
   ‘surrogatepass’ are used.

   This may also be enabled at runtime with
   "sys._enablelegacywindowsfsencoding()".

   Availability: Windows.

   New in version 3.6: See **PEP 529** for more details.

PYTHONLEGACYWINDOWSSTDIO

   If set to a non-empty string, does not use the new console reader
   and writer. This means that Unicode characters will be encoded
   according to the active console code page, rather than using utf-8.

   This variable is ignored if the standard streams are redirected (to
   files or pipes) rather than referring to console buffers.

   Availability: Windows.

   New in version 3.6.

PYTHONCOERCECLOCALE

   If set to the value "0", causes the main Python command line
   application to skip coercing the legacy ASCII-based C and POSIX
   locales to a more capable UTF-8 based alternative.

   If this variable is *not* set (or is set to a value other than
   "0"), the "LC_ALL" locale override environment variable is also not
   set, and the current locale reported for the "LC_CTYPE" category is
   either the default "C" locale, or else the explicitly ASCII-based
   "POSIX" locale, then the Python CLI will attempt to configure the
   following locales for the "LC_CTYPE" category in the order listed
   before loading the interpreter runtime:

   * "C.UTF-8"

   * "C.utf8"

   * "UTF-8"

   If setting one of these locale categories succeeds, then the
   "LC_CTYPE" environment variable will also be set accordingly in the
   current process environment before the Python runtime is
   initialized. This ensures that in addition to being seen by both
   the interpreter itself and other locale-aware components running in
   the same process (such as the GNU "readline" library), the updated
   setting is also seen in subprocesses (regardless of whether or not
   those processes are running a Python interpreter), as well as in
   operations that query the environment rather than the current C
   locale (such as Python’s own "locale.getdefaultlocale()").

   Configuring one of these locales (either explicitly or via the
   above implicit locale coercion) automatically enables the
   "surrogateescape" error handler for "sys.stdin" and "sys.stdout"
   ("sys.stderr" continues to use "backslashreplace" as it does in any
   other locale). This stream handling behavior can be overridden
   using "PYTHONIOENCODING" as usual.

   For debugging purposes, setting "PYTHONCOERCECLOCALE=warn" will
   cause Python to emit warning messages on "stderr" if either the
   locale coercion activates, or else if a locale that *would* have
   triggered coercion is still active when the Python runtime is
   initialized.

   Also note that even when locale coercion is disabled, or when it
   fails to find a suitable target locale, "PYTHONUTF8" will still
   activate by default in legacy ASCII-based locales. Both features
   must be disabled in order to force the interpreter to use "ASCII"
   instead of "UTF-8" for system interfaces.

   Availability: *nix.

   New in version 3.7: See **PEP 538** for more details.

PYTHONDEVMODE

   If this environment variable is set to a non-empty string, enable
   Python Development Mode, introducing additional runtime checks that
   are too expensive to be enabled by default.

   New in version 3.7.

PYTHONUTF8

   If set to "1", enables the interpreter’s UTF-8 mode, where "UTF-8"
   is used as the text encoding for system interfaces, regardless of
   the current locale setting.

   This means that:

      * "sys.getfilesystemencoding()" returns "'UTF-8'" (the locale
        encoding is ignored).

      * "locale.getpreferredencoding()" returns "'UTF-8'" (the locale
        encoding is ignored, and the function’s "do_setlocale"
        parameter has no effect).

      * "sys.stdin", "sys.stdout", and "sys.stderr" all use UTF-8 as
        their text encoding, with the "surrogateescape" error handler
        being enabled for "sys.stdin" and "sys.stdout" ("sys.stderr"
        continues to use "backslashreplace" as it does in the default
        locale-aware mode)

   As a consequence of the changes in those lower level APIs, other
   higher level APIs also exhibit different default behaviours:

      * Command line arguments, environment variables and filenames
        are decoded to text using the UTF-8 encoding.

      * "os.fsdecode()" and "os.fsencode()" use the UTF-8 encoding.

      * "open()", "io.open()", and "codecs.open()" use the UTF-8
        encoding by default. However, they still use the strict error
        handler by default so that attempting to open a binary file in
        text mode is likely to raise an exception rather than
        producing nonsense data.

   Note that the standard stream settings in UTF-8 mode can be
   overridden by "PYTHONIOENCODING" (just as they can be in the
   default locale-aware mode).

   If set to "0", the interpreter runs in its default locale-aware
   mode.

   Setting any other non-empty string causes an error during
   interpreter initialisation.

   If this environment variable is not set at all, then the
   interpreter defaults to using the current locale settings, *unless*
   the current locale is identified as a legacy ASCII-based locale (as
   described for "PYTHONCOERCECLOCALE"), and locale coercion is either
   disabled or fails. In such legacy locales, the interpreter will
   default to enabling UTF-8 mode unless explicitly instructed not to
   do so.

   Also available as the "-X" "utf8" option.

   New in version 3.7: See **PEP 540** for more details.


1.2.1. Debug-mode variables
---------------------------

Setting these variables only has an effect in a debug build of Python.

PYTHONTHREADDEBUG

   If set, Python will print threading debug info.

   Need Python configured with the "--with-pydebug" build option.

PYTHONDUMPREFS

   If set, Python will dump objects and reference counts still alive
   after shutting down the interpreter.

   Need Python configured with the "--with-trace-refs" build option.
