The CPython interpreter scans the command line and the environment for various settings.
CPython 实现细节： Other implementations’ command line schemes may differ. See Alternate Implementations for further resources.
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:
The interpreter interface resembles that of the UNIX shell, but provides some additional methods of invocation:
-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!
-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
– note that the first element, subscript zero (
), 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
and the current directory will be added to the start of
(allowing modules in that directory to be imported as top level modules).
Since the argument is a
name, you must not give a file extension (
). 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
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.
If this option is given, the first element of
will be the full path to the module file (while the module file is being located, the first element will be set to
). As with the
option, the current directory will be added to the start of
Many standard library modules contain code that is invoked on their execution as a script. An example is the
python -mtimeit -s 'setup here' 'benchmarked code here' python -mtimeit -h # for details
PEP 338 – Executing modules as scripts
Supply the package name to run a
3.4 版改变： namespace packages are also supported
Execute the Python code contained in
, which must be a filesystem path (absolute or relative) referring to either a Python file, a directory containing a
file, or a zipfile containing a
If this option is given, the first element of
will be the script name as given on the command line.
If no interface option is given,
is an empty string (
) and the current directory will be added to the start of
. Also, tab-completion and history editing is automatically enabled, if available on your platform (see
3.4 版改变： Automatic enabling of tab-completion and history editing.
If given, Python won’t try to write
files on the import of source modules. See also
Turn on parser debugging output (for wizards only, depending on compilation options). See also
When a script is passed as first argument or the
option is used, enter interactive mode after executing the script or the command, even when
does not appear to be a terminal. The
file is not read.
This can be useful to inspect global variables or a stack trace when a script raises an exception. See also
Run Python in isolated mode. This also implies -E and -s. In isolated mode
contains neither the script’s directory nor the user’s site-packages directory. All
environment variables are ignored, too. Further restrictions may be imposed to prevent the user from injecting malicious code.
Remove assert statements and any code conditional on the value of
. Augment the filename for compiled (
) files by adding
). See also
Don’t display the copyright and version messages even in interactive mode.
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
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 了解细节。
allows you to set a fixed value for the hash seed secret.
Disable the import of the module
and the site-dependent manipulations of
that it entails. Also disable these manipulations if
is explicitly imported later (call
if you want them to be triggered).
Force the binary layer of the stdout and stderr streams (which is available as their
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.
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 (
), 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
Warning control. Python’s warning machinery by default prints warning messages to
. 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.
options may be given; when a warning matches more than one option, the action for the last matching option is performed. Invalid
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
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 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 模块 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
. This is intended for a DOS specific hack only.
Reserved for various implementation-specific options. CPython currently defines the following possible values:
-X faulthandlerto enable
-X showrefcountto 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 tracemallocto start tracing Python memory allocations using the
tracemallocmodule. By default, only the most recent frame is stored in a traceback of a trace. Use
-X tracemalloc=NFRAMEto start tracing with a traceback limit of NFRAME frames. See the
-X showalloccountto output the total count of allocated objects for each type when the program finishes. This only works when Python was built with
It also allows passing arbitrary values and retrieving them through the
option was added.
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.
Change the location of the standard Python libraries. By default, the libraries are searched in
are installation-dependent directories, both defaulting to
Augment the default search path for module files. The format is the same as the shell’s
: one or more directory pathnames separated by
(e.g. colons on Unix or semicolons on Windows). Non-existent directories are silently ignored.
In addition to normal directories, individual
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
as described above under
. The search path can be manipulated from within a Python program as the variable
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
and the hook
in this file.
If this is set to a non-empty string it is equivalent to specifying the
This variable can also be modified by Python code using
to force inspect mode on program termination.
If this is set, Python ignores case in
statements. This only works on Windows and OS X.
If this is set to a non-empty string, Python won’t try to write
files on the import of source modules. This is equivalent to specifying the
If this variable is not set or set to
, a random value is used to seed the hashes of str, bytes and datetime objects.
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.
If this is set before running the interpreter, it overrides the encoding used for stdin/stdout/stderr, in the syntax
. Both the
parts are optional and have the same meaning as in
part is ignored; the handler will always be
On Windows, the encoding specified by this variable is ignored for interactive console buffers unless
is also specified. Files and pipes redirected through the standard streams are not affected.
If this environment variable is set,
will be set to its value instead of the value got through the C runtime. Only works on Mac OS X.
If this environment variable is set to a non-empty string,
is called at startup: install a handler for
signals to dump the Python traceback. This is equivalent to
If this environment variable is set to a non-empty string, start tracing Python memory allocations using the
module. The value of the variable is the maximum number of frames stored in a traceback of a trace. For example,
stores only the most recent frame. See the
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 (
pymalloc: use the pymalloc allocator for
PYMEM_DOMAIN_OBJdomains and use the
malloc()function for the
debug: install debug hooks on top of the default memory allocator
malloc_debug: same as
mallocbut also install debug hooks
pymalloc_debug: same as
pymallocbut also install debug hooks
When Python is compiled in release mode, the default is
. When compiled in debug mode, the default is
and the debug hooks are used automatically.
If Python is configured without
are not available, the default is
in release mode and
in debug mode.
function for debug hooks on Python memory allocators.
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
environment variable is used to force the
allocator of the C library, or if Python is configured without
3.6 版改变： This variable can now also be used on Python compiled in release mode. It now has no effect if set to an empty string.
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
3.6 版新增： 见 PEP 529 了解更多细节。
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.
Setting these variables only has an effect in a debug build of Python, that is, if Python was configured with the
If set, Python will print threading debug info.
If set, Python will dump objects and reference counts still alive after shutting down the interpreter.