| Python 3.6.5 Documentation > Command line and environment
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.
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
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.
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.
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
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
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.
-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.
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.
Options you shouldn’t use
-------------------------
-J
Reserved for use by Jython.
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.
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.
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.
PYTHONMALLOC
Set the Python memory allocators and/or install debug hooks.
Set the family of memory allocators used by Python:
* "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
allocator
* "malloc_debug": same as "malloc" but also install debug hooks
* "pymalloc_debug": same as "pymalloc" but also install debug
hooks
When Python is compiled in release mode, the default is "pymalloc".
When compiled in debug mode, the default is "pymalloc_debug" and
the debug hooks are used automatically.
If Python is configured without "pymalloc" support, "pymalloc" and
"pymalloc_debug" are not available, the default is "malloc" in
release mode and "malloc_debug" in debug mode.
See the "PyMem_SetupDebugHooks()" function for debug hooks on
Python memory allocators.
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.
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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