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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 [-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 ofsys.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
). Themodule-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 ofsys.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 ofsys.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
1.1.2. Generic options
1.1.3. Miscellaneous options
-b
-
Issue a warning when comparing
unicode
withbytearray
. 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
andunicode
. Instead, thestr
instance will be implicitly decoded tounicode
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 alsoPYTHONDONTWRITEBYTECODE
.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
andPYTHONHOME
, 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 whensys.stdin
does not appear to be a terminal. ThePYTHONSTARTUP
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 alsoPYTHONOPTIMIZE
.
-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
-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
tosys.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 ofsys.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 usefile.readline()
inside awhile 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 alsoPYTHONVERBOSE
.
-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.
-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.
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/pythonversion
andexec_prefix/lib/pythonversion
, whereprefix
andexec_prefix
are installation-dependent directories, both defaulting to/usr/local
.When
PYTHONHOME
is set to a single directory, its value replaces bothprefix
andexec_prefix
. To specify different values for these, setPYTHONHOME
toprefix: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 byos.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
(seePYTHONHOME
above). It is always appended toPYTHONPATH
.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 variablesys.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
andsys.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 thetime
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 instr.encode()
.New in version 2.6.
PYTHONNOUSERSITE
-
If this is set, Python won’t add the
user site-packages directory
tosys.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 theuser site-packages directory
and Distutils installation paths forpython 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 callingssl._https_verify_certificates()
withenable=False
whenssl
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.