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.
When invoking Python, you may specify any of these options:
python [-bBdEhiORsSuvVWx?] [-c command | -m module-name | script | - ] [args]
The most common use case is, of course, a simple invocation of a script:
The interpreter interface resembles that of the UNIX shell, but provides some additional methods of invocation:
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), is a string reflecting the program’s source.
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).
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.
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.
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
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.
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.
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.
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 3.1.5.
Don’t add user site directory to sys.path
Force the binary layer of the stdin, stdout and stderr streams (which is available as their buffer attribute) to be unbuffered. The text I/O layer will still be line-buffered.
See also PYTHONUNBUFFERED.
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):
The full form of argument is:
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 test 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.
Skip the first line of the source, allowing use of non-Unix forms of #!cmd. This is intended for a DOS specific hack only.
The line numbers in error messages will be off by one.
These environment variables influence Python’s behavior.
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.
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.
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.
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.
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 3.1.5.
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().
For stderr, the :errorhandler part is ignored; the handler will always be 'backslashreplace'.
If this is set, Python won’t add the user site directory to sys.path
Sets the base directory for the user site directory
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.