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).

-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".

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

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

   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".

<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.

   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 3.0


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".

-d

   Turn on parser debugging output (for wizards 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

   Turn on basic optimizations.  See also "PYTHONOPTIMIZE".

-OO

   Discard docstrings in addition to the "-O" optimizations.

-q

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

   New in version 3.2.

-R

   Kept for compatibility.  On Python 3.3 and greater, hash
   randomization is turned on by default.

   On previous versions of Python, this option turns on hash
   randomization, so that the "__hash__()" values of str, bytes and
   datetime 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.

   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 binary layer of the stdout and stderr streams (which is
   available as their "buffer" attribute) to be unbuffered. The text
   I/O layer will still be line-buffered if writing to the console, or
   block-buffered if redirected to a non-interactive file.

   See also "PYTHONUNBUFFERED".

-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 from within a Python program using
   the "warnings" module.

   The simplest form of argument is one of the following action
   strings (or a unique abbreviation):

   "ignore"
      Ignore all warnings.

   "default"
      Explicitly request the default behavior (printing each warning
      once per source line).

   "all"
      Print a warning each time it occurs (this may generate many
      messages if a warning is triggered repeatedly for the same
      source line, such as inside a loop).

   "module"
      Print each warning only the first time it occurs in each module.

   "once"
      Print each warning only the first time it occurs in the program.

   "error"
      Raise an exception instead of printing a warning message.

   The full form of argument is:

      action:message:category:module:line

   Here, *action* is as explained above but only applies to messages
   that match the remaining fields.  Empty fields match all values;
   trailing empty fields may be omitted.  The *message* field matches
   the start of the warning message printed; this match is case-
   insensitive.  The *category* field matches the warning category.
   This must be a class name; the match tests whether the actual
   warning category of the message is a subclass of the specified
   warning category.  The full class name must be given.  The *module*
   field matches the (fully-qualified) module name; this match is
   case-sensitive.  The *line* field matches the line number, where
   zero matches all line numbers and is thus equivalent to an omitted
   line number.

   See also: "warnings" – the warnings module

     **PEP 230** – Warning framework

     "PYTHONWARNINGS"

-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.

   Note: The line numbers in error messages will be off by one.

-X

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

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

   * "-X showrefcount" to enable the output of the total reference
     count and memory blocks (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.

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

   Changed in version 3.2: It is now allowed to pass "-X" with
   CPython.

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

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


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".

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.

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.

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.

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 OS X.

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.

PYTHONHASHSEED

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

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 Mac OS X.

PYTHONWARNINGS

   This is equivalent to the "-W" option. If set to a comma separated
   string, it is equivalent to specifying "-W" multiple times.

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.

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.


1.2.1. 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.

PYTHONTHREADDEBUG

   If set, Python will print threading debug info.

PYTHONDUMPREFS

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

PYTHONMALLOCSTATS

   If set, Python will print memory allocation statistics every time a
   new object arena is created, and on shutdown.
