1. Command line and environment

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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

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 3.6.0b2+


When given twice, print more information about the build, like:
Python 3.6.0b2+ (3.6:84a3c5003510+, Oct 26 2016, 02:33:55)
[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

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

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, 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.

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 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 showalloccount to 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 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 CPython’s “development mode”, introducing additional
runtime checks which are too expensive to be enabled by default. It should
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


-X utf8 enables the UTF-8 mode, whereas -X utf8=0 disables the
UTF-8 mode.

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.




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/pythonversion and
exec_prefix/lib/pythonversion, 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/pythonversion (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.



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.



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.


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
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, 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 locale to a more capable UTF-8
based alternative.
If this variable is not set, or is set to a value other than 0, and
the current locale reported for the LC_CTYPE category is the default
C 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 the
updated setting is seen in subprocesses, 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) will automatically set the error handler for
sys.stdin and sys.stdout to surrogateescape. This
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.
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 the
CPython “development mode”. See the -X dev option.

New in version 3.7.




PYTHONUTF8
If set to 1, enable the UTF-8 mode. If set to 0, disable the UTF-8
mode. Any other non-empty string cause an error.

New in version 3.7.



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.



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