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 [-bBdEhiIOPqRsSuvVWx?] [-c command | -m module-name | script | - ] [args]
The most common use case is, of course, a simple invocation of a script:
python myscript.py
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. For more on interactive mode, see Interactive Mode.
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 using the standard
import mechanism 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.
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).
Raises an auditing event cpython.run_command with argument command.
Changed in version 3.14: command is automatically dedented before execution.
Locate the module using the standard import mechanism 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.
-I option can be used to run the script in isolated mode where
sys.path contains neither the current directory nor the user’s
site-packages directory. All PYTHON* environment variables are
ignored, too.
Many standard library modules contain code that is invoked on their execution
as a script. An example is the timeit module:
python -m timeit -s "setup here" "benchmarked code here"
python -m timeit -h # for details
Raises an auditing event cpython.run_module with argument module-name.
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.
Raises an auditing event cpython.run_stdin with no arguments.
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.
-I option can be used to run the script in isolated mode where
sys.path contains neither the script’s directory nor the user’s
site-packages directory. All PYTHON* environment variables are
ignored, too.
Raises an auditing event cpython.run_file with argument filename.
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
Changed in version 3.4: Automatic enabling of tab-completion and history editing.
Print a short description of all command line options and corresponding environment variables and exit.
Print a short description of Python-specific environment variables and exit.
Added in version 3.11.
Print a description of implementation-specific -X options
and exit.
Added in version 3.11.
Print complete usage information and exit.
Added in version 3.11.
Issue a warning when converting bytes or bytearray to
str without specifying encoding or comparing bytes or
bytearray with str or bytes with int.
Issue an error when the option is given twice (-bb).
If given, Python won’t try to write .pyc files on the
import of source modules. See also PYTHONDONTWRITEBYTECODE.
Control the validation behavior of hash-based .pyc files. See
Cached bytecode invalidation. When set to default, checked and unchecked
hash-based bytecode cache files are validated according to their default
semantics. When set to always, all hash-based .pyc files, whether
checked or unchecked, are validated against their corresponding source
file. When set to never, hash-based .pyc files are not validated
against their corresponding source files.
The semantics of timestamp-based .pyc files are unaffected by this
option.
Turn on parser debugging output (for expert only).
See also the PYTHONDEBUG environment variable.
This option requires a debug build of Python, otherwise it’s ignored.
Ignore all PYTHON* environment variables, e.g.
PYTHONPATH and PYTHONHOME, that might be set.
Enter interactive mode after execution.
Using the -i option will enter interactive mode in any of the following circumstances:
Interactive mode will start 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.
Run Python in isolated mode. This also implies -E, -P
and -s options.
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.
Added in version 3.4.
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.
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.
Don’t prepend a potentially unsafe path to sys.path:
python -m module command line: Don’t prepend the current working
directory.
python script.py command line: Don’t prepend the script’s directory.
If it’s a symbolic link, resolve symbolic links.
python -c code and python (REPL) command lines: Don’t prepend an
empty string, which means the current working directory.
See also the PYTHONSAFEPATH environment variable, and -E
and -I (isolated) options.
Added in version 3.11.
Don’t display the copyright and version messages even in interactive mode.
Added in version 3.2.
Turn on hash randomization. This option only has an effect if the
PYTHONHASHSEED environment variable is set to anything other
than random, since hash randomization is enabled by default.
On previous versions of Python, this option turns on hash randomization,
so that the __hash__() values of str and bytes 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.
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(n2) complexity. See https://ocert.org/advisories/ocert-2011-003.html for details.
PYTHONHASHSEED allows you to set a fixed value for the hash
seed secret.
Added in version 3.2.3.
Changed in version 3.7: The option is no longer ignored.
Don’t add the user site-packages directory to
sys.path.
See also PYTHONNOUSERSITE.
See also
PEP 370 – Per user site-packages directory
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).
Force the stdout and stderr streams to be unbuffered. This option has no effect on the stdin stream.
See also PYTHONUNBUFFERED.
Changed in version 3.7: The text layer of the stdout and stderr streams now is unbuffered.
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.
Changed in version 3.10: The site module reports the site-specific paths
and .pth files being processed.
See also PYTHONVERBOSE.
Warning control. Python’s warning machinery by default prints warning
messages to sys.stderr.
The simplest settings apply a particular action unconditionally to all warnings emitted by a process (even those that are otherwise ignored by default):
-Wdefault # Warn once per call location
-Werror # Convert to exceptions
-Walways # Warn every time
-Wall # Same as -Walways
-Wmodule # Warn once per calling module
-Wonce # Warn once per Python process
-Wignore # Never warn
The action names can be abbreviated as desired and the interpreter will
resolve them to the appropriate action name. For example, -Wi is the
same as -Wignore.
The full form of argument is:
action:message:category:module:lineno
Empty fields match all values; trailing empty fields may be omitted. For
example -W ignore::DeprecationWarning ignores all DeprecationWarning
warnings.
The action field is as explained above but only applies to warnings that match the remaining fields.
The message field must match the whole warning message; this match is case-insensitive.
The category field matches the warning category
(ex: DeprecationWarning). 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 module field matches the (fully qualified) module name; this match is case-sensitive.
The lineno field matches the line number, where zero matches all line numbers and is thus equivalent to an omitted line number.
Multiple -W options can 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 using the PYTHONWARNINGS
environment variable and from within a Python program using the
warnings module. For example, the warnings.filterwarnings()
function can be used to use a regular expression on the warning message.
See The Warnings Filter and Describing Warning Filters for more details.
Skip the first line of the source, allowing use of non-Unix forms of
#!cmd. This is intended for a DOS specific hack only.
Reserved for various implementation-specific options. CPython currently defines the following possible values:
-X faulthandler to enable faulthandler.
See also PYTHONFAULTHANDLER.
Added in version 3.3.
-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.
Added in version 3.4.
-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 tracemalloc.start() and PYTHONTRACEMALLOC
for more information.
Added in version 3.4.
-X int_max_str_digits configures the integer string conversion
length limitation. See also
PYTHONINTMAXSTRDIGITS.
Added in version 3.11.
-X importtime to show how long each import takes. It shows module
name, cumulative time (including nested imports) and self time (excluding
nested imports). Note that its output may be broken in multi-threaded
application. Typical usage is python -X importtime -c 'import asyncio'.
-X importtime=2 enables additional output that indicates when an
imported module has already been loaded. In such cases, the string
cached will be printed in both time columns.
See also PYTHONPROFILEIMPORTTIME.
Added in version 3.7.
Changed in version 3.14: Added -X importtime=2 to also trace imports of loaded modules,
and reserved values other than 1 and 2 for future use.
-X dev: enable Python Development Mode, introducing
additional runtime checks that are too expensive to be enabled by
default. See also PYTHONDEVMODE.
Added in version 3.7.
-X utf8 enables the Python UTF-8 Mode.
-X utf8=0 explicitly disables Python UTF-8 Mode
(even when it would otherwise activate automatically).
See also PYTHONUTF8.
Added in version 3.7.
-X pycache_prefix=PATH enables writing .pyc files to a parallel
tree rooted at the given directory instead of to the code tree. See also
PYTHONPYCACHEPREFIX.
Added in version 3.8.
-X warn_default_encoding issues a EncodingWarning when the
locale-specific default encoding is used for opening files.
See also PYTHONWARNDEFAULTENCODING.
Added in version 3.10.
-X no_debug_ranges disables the inclusion of the tables mapping extra
location information (end line, start column offset and end column offset)
to every instruction in code objects. This is useful when smaller code
objects and pyc files are desired as well as suppressing the extra visual
location indicators when the interpreter displays tracebacks. See also
PYTHONNODEBUGRANGES.
Added in version 3.11.
-X frozen_modules determines whether or not frozen modules are
ignored by the import machinery. A value of on means they get
imported and off means they are ignored. The default is on
if this is an installed Python (the normal case). If it’s under
development (running from the source tree) then the default is off.
Note that the importlib_bootstrap and
importlib_bootstrap_external frozen modules are always used, even
if this flag is set to off. See also PYTHON_FROZEN_MODULES.
Added in version 3.11.
-X perf enables support for the Linux perf profiler.
When this option is provided, the perf profiler will be able to
report Python calls. This option is only available on some platforms and
will do nothing if is not supported on the current system. The default value
is “off”. See also PYTHONPERFSUPPORT and Python support for the Linux perf profiler.
Added in version 3.12.
-X perf_jit enables support for the Linux perf profiler with DWARF
support. When this option is provided, the perf profiler will be able
to report Python calls using DWARF information. This option is only available on
some platforms and will do nothing if is not supported on the current
system. The default value is “off”. See also PYTHON_PERF_JIT_SUPPORT
and Python support for the Linux perf profiler.
Added in version 3.13.
-X disable_remote_debug disables the remote debugging support as described
in PEP 768. This includes both the functionality to schedule code for
execution in another process and the functionality to receive code for
execution in the current process.
This option is only available on some platforms and will do nothing
if is not supported on the current system. See also
PYTHON_DISABLE_REMOTE_DEBUG and PEP 768.
Added in version 3.14.
-X cpu_count=n overrides os.cpu_count(),
os.process_cpu_count(), and multiprocessing.cpu_count().
n must be greater than or equal to 1.
This option may be useful for users who need to limit CPU resources of a
container system. See also PYTHON_CPU_COUNT.
If n is default, nothing is overridden.
Added in version 3.13.
-X presite=package.module specifies a module that should be
imported before the site module is executed and before the
__main__ module exists. Therefore, the imported module isn’t
__main__. This can be used to execute code early during Python
initialization. Python needs to be built in debug mode
for this option to exist. See also PYTHON_PRESITE.
Added in version 3.13.
-X gil=0,1 forces the GIL to be disabled or enabled,
respectively. Setting to 0 is only available in builds configured with
--disable-gil. See also PYTHON_GIL and
Free-threaded CPython.
Added in version 3.13.
-X thread_inherit_context=0,1 causes Thread
to, by default, use a copy of context of the caller of
Thread.start() when starting. Otherwise, threads will start
with an empty context. If unset, the value of this option defaults
to 1 on free-threaded builds and to 0 otherwise. See also
PYTHON_THREAD_INHERIT_CONTEXT.
Added in version 3.14.
-X context_aware_warnings=0,1 causes the
warnings.catch_warnings context manager to use a
ContextVar to store warnings filter state. If
unset, the value of this option defaults to 1 on free-threaded builds
and to 0 otherwise. See also PYTHON_CONTEXT_AWARE_WARNINGS.
Added in version 3.14.
-X tlbc=0,1 enables (1, the default) or disables (0) thread-local
bytecode in builds configured with --disable-gil. When disabled,
this also disables the specializing interpreter. See also
PYTHON_TLBC.
Added in version 3.14.
It also allows passing arbitrary values and retrieving them through the
sys._xoptions dictionary.
Added in version 3.2.
Changed in version 3.9: Removed the -X showalloccount option.
Changed in version 3.10: Removed the -X oldparser option.
Removed in version 3.14: -J is no longer reserved for use by Jython,
and now has no special meaning.
The Python interpreter is configured by default to use colors to highlight output in certain situations such as when displaying tracebacks. This behavior can be controlled by setting different environment variables.
Setting the environment variable TERM to dumb will disable color.
If the FORCE_COLOR environment variable is set, then color will be
enabled regardless of the value of TERM. This is useful on CI systems which
aren’t terminals but can still display ANSI escape sequences.
If the NO_COLOR environment variable is set, Python will disable all color
in the output. This takes precedence over FORCE_COLOR.
All these environment variables are used also by other tools to control color
output. To control the color output only in the Python interpreter, the
PYTHON_COLORS environment variable can be used. This variable takes
precedence over NO_COLOR, which in turn takes precedence over
FORCE_COLOR.
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 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.
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.
If this is set to a non-empty string, don’t prepend a potentially unsafe
path to sys.path: see the -P option for details.
Added in version 3.11.
If this is set to a non-empty string, it overrides the sys.platlibdir
value.
Added in version 3.9.
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.
Raises an auditing event cpython.run_startup with
the filename as the argument when called on startup.
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.
If this is set, it names a callable using dotted-path notation. The module
containing the callable will be imported and then the callable will be run
by the default implementation of sys.breakpointhook() which itself is
called by built-in breakpoint(). If not set, or set to the empty
string, it is equivalent to the value “pdb.set_trace”. Setting this to the
string “0” causes the default implementation of sys.breakpointhook()
to do nothing but return immediately.
Added in version 3.7.
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.
This environment variable requires a debug build of Python, otherwise it’s ignored.
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.
Raises an auditing event cpython.run_stdin with no arguments.
Changed in version 3.12.5: (also 3.11.10, 3.10.15, 3.9.20, and 3.8.20) Emits audit events.
Changed in version 3.13: Uses PyREPL if possible, in which case PYTHONSTARTUP is
also executed. Emits audit events.
If this is set to a non-empty string it is equivalent to specifying the
-u option.
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.
If this is set, Python ignores case in import statements. This
only works on Windows and macOS.
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.
If this is set, Python will write .pyc files in a mirror directory tree
at this path, instead of in __pycache__ directories within the source
tree. This is equivalent to specifying the -X
pycache_prefix=PATH option.
Added in version 3.8.
If this variable is not set or set to random, a random value is used
to seed the hashes of str and bytes 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.
Added in version 3.2.3.
If this variable is set to an integer, it is used to configure the interpreter’s global integer string conversion length limitation.
Added in version 3.11.
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.
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
Defines the user base directory, which is used to
compute the path of the user site-packages directory
and installation paths for
python -m pip install --user.
See also
PEP 370 – Per user site-packages directory
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
macOS.
This is equivalent to the -W option. If set to a comma
separated string, it is equivalent to specifying -W multiple
times, with filters later in the list taking precedence over those earlier
in the list.
The simplest settings apply a particular action unconditionally to all warnings emitted by a process (even those that are otherwise ignored by default):
PYTHONWARNINGS=default # Warn once per call location
PYTHONWARNINGS=error # Convert to exceptions
PYTHONWARNINGS=always # Warn every time
PYTHONWARNINGS=all # Same as PYTHONWARNINGS=always
PYTHONWARNINGS=module # Warn once per calling module
PYTHONWARNINGS=once # Warn once per Python process
PYTHONWARNINGS=ignore # Never warn
See The Warnings Filter and Describing Warning Filters for more details.
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.
Added in version 3.3.
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() function for more information.
This is equivalent to setting the -X tracemalloc option.
Added in version 3.4.
If this environment variable is set to 1, Python will show
how long each import takes. If set to 2, Python will include output for
imported modules that have already been loaded.
This is equivalent to setting the -X importtime option.
Added in version 3.7.
Changed in version 3.14: Added PYTHONPROFILEIMPORTTIME=2 to also trace imports of loaded modules.
If this environment variable is set to a non-empty string, enable the
debug mode of the asyncio module.
Added in version 3.4.
Set the Python memory allocators and/or install debug hooks.
Set the family of memory allocators used by Python:
default: use the default memory allocators.
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.
mimalloc: use the mimalloc 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
allocators.
malloc_debug: same as malloc but also install debug hooks.
pymalloc_debug: same as pymalloc but also install debug hooks.
mimalloc_debug: same as mimalloc but also install debug hooks.
Note
In the free-threaded build, the malloc,
malloc_debug, pymalloc, and pymalloc_debug values are not
supported. Only default, debug, mimalloc, and
mimalloc_debug are accepted.
Added in version 3.6.
Changed in version 3.7: Added the "default" allocator.
If set to a non-empty string, Python will print statistics of the pymalloc memory allocator or the mimalloc memory allocator (whichever is in use) every time a new 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 both pymalloc and mimalloc 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.
If set to a non-empty string, the default filesystem encoding and error handler 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.
Added in version 3.6: See PEP 529 for more details.
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.
Added in version 3.6.
If set to the value 0, causes the main Python command line application
to skip coercing the legacy ASCII-based C and POSIX locales to a more
capable UTF-8 based alternative.
If this variable is not set (or is set to a value other than 0), the
LC_ALL locale override environment variable is also not set, and the
current locale reported for the LC_CTYPE category is either the default
C locale, or else the explicitly ASCII-based POSIX locale, then the
Python CLI will attempt to configure the following locales for the
LC_CTYPE category in the order listed before loading the interpreter
runtime:
C.UTF-8
C.utf8
UTF-8
If setting one of these locale categories succeeds, then the LC_CTYPE
environment variable will also be set accordingly in the current process
environment before the Python runtime is initialized. This ensures that in
addition to being seen by both the interpreter itself and other locale-aware
components running in the same process (such as the GNU readline
library), the updated setting is also seen in subprocesses (regardless of
whether or not those processes are running a Python interpreter), as well as
in operations that query the environment rather than the current C locale
(such as Python’s own locale.getdefaultlocale()).
Configuring one of these locales (either explicitly or via the above
implicit locale coercion) automatically enables the surrogateescape
error handler for sys.stdin and
sys.stdout (sys.stderr continues to use backslashreplace
as it does in any other locale). This stream handling behavior can be
overridden using PYTHONIOENCODING as usual.
For debugging purposes, setting PYTHONCOERCECLOCALE=warn will cause
Python to emit warning messages on stderr if either the locale coercion
activates, or else if a locale that would have triggered coercion is
still active when the Python runtime is initialized.
Also note that even when locale coercion is disabled, or when it fails to
find a suitable target locale, PYTHONUTF8 will still activate by
default in legacy ASCII-based locales. Both features must be disabled in
order to force the interpreter to use ASCII instead of UTF-8 for
system interfaces.
Availability: Unix.
Added in version 3.7: See PEP 538 for more details.
If this environment variable is set to a non-empty string, enable
Python Development Mode, introducing additional runtime
checks that are too expensive to be enabled by default.
This is equivalent to setting the -X dev option.
Added in version 3.7.
If set to 1, enable the Python UTF-8 Mode.
If set to 0, disable the Python UTF-8 Mode.
Setting any other non-empty string causes an error during interpreter initialisation.
Added in version 3.7.
If this environment variable is set to a non-empty string, issue a
EncodingWarning when the locale-specific default encoding is used.
See Opt-in EncodingWarning for details.
Added in version 3.10.
If this variable is set, it disables the inclusion of the tables mapping extra location information (end line, start column offset and end column offset) to every instruction in code objects. This is useful when smaller code objects and pyc files are desired as well as suppressing the extra visual location indicators when the interpreter displays tracebacks.
Added in version 3.11.
If this variable is set to a nonzero value, it enables support for
the Linux perf profiler so Python calls can be detected by it.
If set to 0, disable Linux perf profiler support.
See also the -X perf command-line option
and Python support for the Linux perf profiler.
Added in version 3.12.
If this variable is set to a nonzero value, it enables support for
the Linux perf profiler so Python calls can be detected by it
using DWARF information.
If set to 0, disable Linux perf profiler support.
See also the -X perf_jit command-line option
and Python support for the Linux perf profiler.
Added in version 3.13.
If this variable is set to a non-empty string, it disables the remote debugging feature described in PEP 768. This includes both the functionality to schedule code for execution in another process and the functionality to receive code for execution in the current process.
See also the -X disable_remote_debug command-line option.
Added in version 3.14.
If this variable is set to a positive integer, it overrides the return
values of os.cpu_count() and os.process_cpu_count().
See also the -X cpu_count command-line option.
Added in version 3.13.
If this variable is set to on or off, it determines whether or not
frozen modules are ignored by the import machinery. A value of on means
they get imported and off means they are ignored. The default is on
for non-debug builds (the normal case) and off for debug builds.
Note that the importlib_bootstrap and
importlib_bootstrap_external frozen modules are always used, even
if this flag is set to off.
See also the -X frozen_modules command-line option.
Added in version 3.13.
If this variable is set to 1, the interpreter will colorize various kinds
of output. Setting it to 0 deactivates this behavior.
See also Controlling color.
Added in version 3.13.
If this variable is set to any value, the interpreter will not attempt to
load the Python-based REPL that requires readline, and will
instead use the traditional parser-based REPL.
Added in version 3.13.
This environment variable can be used to set the location of a
.python_history file (by default, it is .python_history in the
user’s home directory).
Added in version 3.13.
If this variable is set to 1, the global interpreter lock (GIL) will be
forced on. Setting it to 0 forces the GIL off (needs Python configured with
the --disable-gil build option).
See also the -X gil command-line option, which takes
precedence over this variable, and Free-threaded CPython.
Added in version 3.13.
If this variable is set to 1 then Thread will,
by default, use a copy of context of the caller of Thread.start()
when starting. Otherwise, new threads will start with an empty context.
If unset, this variable defaults to 1 on free-threaded builds and to
0 otherwise. See also -X thread_inherit_context.
Added in version 3.14.
If set to 1 then the warnings.catch_warnings context
manager will use a ContextVar to store warnings
filter state. If unset, this variable defaults to 1 on
free-threaded builds and to 0 otherwise. See -X
context_aware_warnings.
Added in version 3.14.
On builds where experimental just-in-time compilation is available, this
variable can force the JIT to be disabled (0) or enabled (1) at
interpreter startup.
Added in version 3.13.
If set to 1 enables thread-local bytecode. If set to 0 thread-local
bytecode and the specializing interpreter are disabled. Only applies to
builds configured with --disable-gil.
See also the -X tlbc command-line option.
Added in version 3.14.
If set, Python will dump objects and reference counts still alive after shutting down the interpreter.
Needs Python configured with the --with-trace-refs build option.
If set, Python will dump objects and reference counts still alive after shutting down the interpreter into a file under the path given as the value to this environment variable.
Needs Python configured with the --with-trace-refs build option.
Added in version 3.11.
If this variable is set to a module, that module will be imported
early in the interpreter lifecycle, before the site module is
executed, and before the __main__ module is created.
Therefore, the imported module is not treated as __main__.
This can be used to execute code early during Python initialization.
To import a submodule, use package.module as the value, like in
an import statement.
See also the -X presite command-line option,
which takes precedence over this variable.
Needs Python configured with the --with-pydebug build option.
Added in version 3.13.