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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:
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
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.
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:
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.
-
-B
If given, Python won’t try to write .pyc or .pyo 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.
-
-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:
olddivision of int/int and long/long return an int or long (default)
newnew division semantics, i.e. division of int/int and long/long returns a
float
warnold division semantics with a warning for int/int and long/long
warnallold division semantics with a warning for all uses of the division operator
See also
Tools/scripts/fixdiv.pyfor 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.
-
-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.
-
-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:
ignoreIgnore all warnings.
defaultExplicitly request the default behavior (printing each warning once per
source line).
allPrint 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).
modulePrint each warning only the first time it occurs in each module.
oncePrint each warning only the first time it occurs in the program.
errorRaise 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.
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/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 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.
-
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.
-
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().
-
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.
-
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.
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.
-
PYTHONSHOWREFCOUNT¶
If set, Python will print the total reference count when the program
finishes or after each statement in the interactive interpreter.
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