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 [-bBdEiOQsRStuUvVWxX3?] [-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 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 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. 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

   New in version 2.4.

   Changed in version 2.5: The named module can now be located inside
   a package.

   Changed in version 2.7: Supply the package name to run a "__main__"
   submodule. sys.argv[0] is now set to ""-m"" while searching for the
   module (it was previously incorrectly set to ""-c"")

-

   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".

   See also:

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

<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.

   Changed in version 2.5: Directories and zipfiles containing a
   "__main__.py" file at the top level are now considered valid Python
   scripts.

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".

See also: Invoking the Interpreter


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

-?
-h
--help

   Print a short description of all command line options.

   Changed in version 2.5: The "--help" variant.

-V
--version

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

      Python 2.5.1

   Changed in version 2.5: The "--version" variant.


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

-b

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

   Note that, unlike the corresponding Python 3.x flag, this will
   **not** emit warnings for comparisons between "str" and "unicode".
   Instead, the "str" instance will be implicitly decoded to "unicode"
   and Unicode comparison used.

   New in version 2.6.

-B

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

   New in version 2.6.

-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.

   New in version 2.2.

-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".

-O

   Turn on basic optimizations.  This changes the filename extension
   for compiled (*bytecode*) files from ".pyc" to ".pyo".  See also
   "PYTHONOPTIMIZE".

-OO

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

-Q <arg>

   Division control. The argument must be one of the following:

   "old"
      division of int/int and long/long return an int or long
      (*default*)

   "new"
      new division semantics, i.e. division of int/int and long/long
      returns a float

   "warn"
      old division semantics with a warning for int/int and long/long

   "warnall"
      old division semantics with a warning for all uses of the
      division operator

   See also:

     "Tools/scripts/fixdiv.py"
        for a use of "warnall"

     **PEP 238** – Changing the division operator

-R

   Turn on hash randomization, so that the "__hash__()" values of str,
   bytes and datetime 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.

   This 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.

   Changing hash values affects the order in which keys are retrieved
   from a dict.  Although Python has never made guarantees about this
   ordering (and it typically varies between 32-bit and 64-bit
   builds), enough real-world code implicitly relies on this non-
   guaranteed behavior that the randomization is disabled by default.

   See also "PYTHONHASHSEED".

   New in version 2.6.8.

-s

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

   New in version 2.6.

   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.

-t

   Issue a warning when a source file mixes tabs and spaces for
   indentation in a way that makes it depend on the worth of a tab
   expressed in spaces.  Issue an error when the option is given twice
   ("-tt").

-u

   Force stdin, stdout and stderr to be totally unbuffered.  On
   systems where it matters, also put stdin, stdout and stderr in
   binary mode.

   Note that there is internal buffering in "file.readlines()" and
   File Objects ("for line in sys.stdin") which is not influenced by
   this option.  To work around this, you will want to use
   "file.readline()" inside a "while 1:" loop.

   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).

   Starting from Python 2.7, "DeprecationWarning" and its descendants
   are ignored by default.  The "-Wd" option can be used to re-enable
   them.

   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) by themselves:

   "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.

-3

   Warn about Python 3.x possible incompatibilities by emitting a
   "DeprecationWarning" for features that are removed or significantly
   changed in Python 3 and can’t be detected using static code
   analysis.

   New in version 2.6.

   See Porting Python 2 Code to Python 3 for more details.


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

-J

   Reserved for use by Jython.

-U

   Turns all string literals into unicodes globally.  Do not be
   tempted to use this option as it will probably break your world.
   It also produces ".pyc" files with a different magic number than
   normal.  Instead, you can enable unicode literals on a per-module
   basis by using:

      from __future__ import unicode_literals

   at the top of the file.  See "__future__" for details.

-X

   Reserved for alternative implementations of Python to use for their
   own purposes.


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

These environment variables influence Python’s behavior, they are
processed before the command-line switches other than -E.  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"
   in this file.

PYTHONY2K

   Set this to a non-empty string to cause the "time" module to
   require dates specified as strings to include 4-digit years,
   otherwise 2-digit years are converted based on rules described in
   the "time" module documentation.

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, OS X, OS/2, and RiscOS.

PYTHONDONTWRITEBYTECODE

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

   New in version 2.6.

PYTHONHASHSEED

   If this variable is set to "random", the effect is the same as
   specifying the "-R" option: 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 lead to the same hash values as when
   hash randomization is disabled.

   New in version 2.6.8.

PYTHONIOENCODING

   Overrides the encoding used for stdin/stdout/stderr, in the syntax
   "encodingname:errorhandler".  The ":errorhandler" part is optional
   and has the same meaning as in "str.encode()".

   New in version 2.6.

PYTHONNOUSERSITE

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

   New in version 2.6.

   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".

   New in version 2.6.

   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.

PYTHONHTTPSVERIFY

   If this environment variable is set specifically to "0", then it is
   equivalent to implicitly calling "ssl._https_verify_certificates()"
   with "enable=False" when "ssl" is first imported.

   Refer to the documentation of "ssl._https_verify_certificates()"
   for details.

   New in version 2.7.12.


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.

   Changed in version 2.6: Previously, this variable was called
   "THREADDEBUG".

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.

PYTHONSHOWALLOCCOUNT

   If set and Python was compiled with "COUNT_ALLOCS" defined, Python
   will dump allocations counts into stderr on shutdown.

   New in version 2.7.15.

PYTHONSHOWREFCOUNT

   If set, Python will print the total reference count when the
   program finishes or after each statement in the interactive
   interpreter.

   New in version 2.7.15.
