This module provides access to some variables used or maintained by the
interpreter and to functions that interact strongly with the interpreter.
It is always available.
The list of command line arguments passed to a Python script.
argv is the script name (it is operating system
dependent whether this is a full pathname or not).
If the command was executed using the -c command line
option to the interpreter,
argv is set to the string
If no script name was passed to the Python interpreter,
argv has zero length.
An indicator of the native byte order. This will have the value
'big' on big-endian (most-signigicant byte first) platforms,
'little' on little-endian (least-significant byte first)
New in version 2.0.
A tuple of strings giving the names of all modules that are compiled
into this Python interpreter. (This information is not available in
any other way --
modules.keys() only lists the imported
A string containing the copyright pertaining to the Python interpreter.
Integer specifying the handle of the Python DLL.
- exc_info ()
This function returns a tuple of three values that give information
about the exception that is currently being handled. The information
returned is specific both to the current thread and to the current
stack frame. If the current stack frame is not handling an exception,
the information is taken from the calling stack frame, or its caller,
and so on until a stack frame is found that is handling an exception.
Here, ``handling an exception'' is defined as ``executing or having
executed an except clause.'' For any stack frame, only
information about the most recently handled exception is accessible.
If no exception is being handled anywhere on the stack, a tuple
None values is returned. Otherwise, the
values returned are
(type, value, traceback).
Their meaning is: type gets the exception type of the exception
being handled (a string or class object); value gets the
exception parameter (its associated value or the second argument
to raise, which is always a class instance if the exception
type is a class object); traceback gets a traceback object (see
the Reference Manual) which encapsulates the call stack at the point
where the exception originally occurred.
Warning: assigning the traceback return value to a
local variable in a function that is handling an exception will cause
a circular reference. This will prevent anything referenced by a local
variable in the same function or by the traceback from being garbage
collected. Since most functions don't need access to the traceback,
the best solution is to use something like
type, value = sys.exc_info()[:2]
to extract only the exception type and value. If you do need the
traceback, make sure to delete it after use (best done with a
try ... finally statement) or to call
exc_info() in a function that does not itself handle an
Deprecated since release 1.5.
Use exc_info() instead.
Since they are global variables, they are not specific to the current
thread, so their use is not safe in a multi-threaded program. When no
exception is being handled,
exc_type is set to
the other two are undefined.
A string giving the site-specific directory prefix where the
platform-dependent Python files are installed; by default, this is
'/usr/local'. This can be set at build time with the
--exec-prefix argument to the
configure script. Specifically, all configuration files
(e.g. the config.h header file) are installed in the directory
exec_prefix + '/lib/pythonversion/config', and shared
library modules are installed in
'/lib/pythonversion/lib-dynload', where version is equal
A string giving the name of the executable binary for the Python
interpreter, on systems where this makes sense.
- exit ([arg])
Exit from Python. This is implemented by raising the
SystemExit exception, so cleanup actions specified by
finally clauses of try statements are honored, and it is
possible to intercept the exit attempt at an outer level. The
optional argument arg can be an integer giving the exit status
(defaulting to zero), or another type of object. If it is an integer,
zero is considered ``successful termination'' and any nonzero value is
considered ``abnormal termination'' by shells and the like. Most
systems require it to be in the range 0-127, and produce undefined
results otherwise. Some systems have a convention for assigning
specific meanings to specific exit codes, but these are generally
underdeveloped; Unix programs generally use 2 for command line syntax
errors and 1 for all other kind of errors. If another type of object
None is equivalent to passing zero, and any other
object is printed to
sys.stderr and results in an exit code of
1. In particular,
sys.exit("some error message") is a quick
way to exit a program when an error occurs.
This value is not actually defined by the module, but can be set by
the user (or by a program) to specify a clean-up action at program
exit. When set, it should be a parameterless function. This function
will be called when the interpreter exits. Only one function may be
installed in this way; to allow multiple functions which will be called
at termination, use the atexit module. Note: the exit function
is not called when the program is killed by a signal, when a Python
fatal internal error is detected, or when
os._exit() is called.
- getrefcount (object)
Return the reference count of the object. The count returned is
generally one higher than you might expect, because it includes the
(temporary) reference as an argument to getrefcount().
- getrecursionlimit ()
Return the current value of the recursion limit, the maximum depth of
the Python interpreter stack. This limit prevents infinite recursion
from causing an overflow of the C stack and crashing Python. It can
be set by setrecursionlimit().
The version number encoded as a single integer. This is guaranteed to
increase with each version, including proper support for
non-production releases. For example, to test that the Python
interpreter is at least version 1.5.2, use:
if sys.hexversion >= 0x010502F0:
# use some advanced feature
# use an alternative implementation or warn the user
This is called "hexversion" since it only really looks meaningful
when viewed as the result of passing it to the built-in
hex() function. The
version_info value may be used
for a more human-friendly encoding of the same information.
New in version 1.5.2.
These three variables are not always defined; they are set when an
exception is not handled and the interpreter prints an error message
and a stack traceback. Their intended use is to allow an interactive
user to import a debugger module and engage in post-mortem debugging
without having to re-execute the command that caused the error.
(Typical use is "import pdb; pdb.pm()" to enter the post-mortem
debugger; see the chapter ``The Python Debugger'' for more
The meaning of the variables is the same
as that of the return values from exc_info() above.
(Since there is only one interactive thread, thread-safety is not a
concern for these variables, unlike for
The largest positive integer supported by Python's regular integer
type. This is at least 2**31-1. The largest negative integer is
-maxint-1 - the asymmetry results from the use of 2's
complement binary arithmetic.
This is a dictionary that maps module names to modules which have
already been loaded. This can be manipulated to force reloading of
modules and other tricks. Note that removing a module from this
dictionary is not the same as calling
reload() on the corresponding module
A list of strings that specifies the search path for modules.
Initialized from the environment variable $PYTHONPATH, or an
The first item of this list,
path, is the
directory containing the script that was used to invoke the Python
interpreter. If the script directory is not available (e.g. if the
interpreter is invoked interactively or if the script is read from
path is the empty string, which directs
Python to search modules in the current directory first. Notice that
the script directory is inserted before the entries inserted as
a result of $PYTHONPATH.
This string contains a platform identifier, e.g.
'linux1'. This can be used to append platform-specific
path, for instance.
A string giving the site-specific directory prefix where the platform
independent Python files are installed; by default, this is the string
'/usr/local'. This can be set at build time with the
--prefix argument to the
configure script. The main collection of Python library
modules is installed in the directory
'/lib/pythonversion' while the platform independent header
files (all except config.h) are stored in
'/include/pythonversion', where version is equal to
Strings specifying the primary and secondary prompt of the
interpreter. These are only defined if the interpreter is in
interactive mode. Their initial values in this case are
'»> ' and
'... '. If a non-string object is assigned
to either variable, its str() is re-evaluated each time
the interpreter prepares to read a new interactive command; this can
be used to implement a dynamic prompt.
- setcheckinterval (interval)
Set the interpreter's ``check interval''. This integer value
determines how often the interpreter checks for periodic things such
as thread switches and signal handlers. The default is
the check is performed every 10 Python virtual instructions. Setting
it to a larger value may increase performance for programs using
threads. Setting it to a value
<= 0 checks every virtual instruction,
maximizing responsiveness as well as overhead.
- setprofile (profilefunc)
Set the system's profile function, which allows you to implement a
Python source code profiler in Python. See the chapter on the
Python Profiler. The system's profile function
is called similarly to the system's trace function (see
settrace()), but it isn't called for each executed line of
code (only on call and return and when an exception occurs). Also,
its return value is not used, so it can just return
- setrecursionlimit (limit)
Set the maximum depth of the Python interpreter stack to limit.
This limit prevents infinite recursion from causing an overflow of the
C stack and crashing Python.
The highest possible limit is platform-dependent. A user may need to
set the limit higher when she has a program that requires deep
recursion and a platform that supports a higher limit. This should be
done with care, because a too-high limit can lead to a crash.
- settrace (tracefunc)
Set the system's trace function, which allows you to implement a
Python source code debugger in Python. See section ``How It Works''
in the chapter on the Python Debugger.
File objects corresponding to the interpreter's standard input,
output and error streams.
stdin is used for all
interpreter input except for scripts but including calls to
stdout is used
for the output of print and expression statements and for the
prompts of input() and raw_input(). The interpreter's
own prompts and (almost all of) its error messages go to
be built-in file objects: any object is acceptable as long as it has
a write() method that takes a string argument. (Changing these
objects doesn't affect the standard I/O streams of processes
executed by os.popen(), os.system() or the
exec*() family of functions in the os module.)
These objects contain the original values of
stdout at the start of the program. They are
used during finalization, and could be useful to restore the actual
files to known working file objects in case they have been overwritten
with a broken object.
When this variable is set to an integer value, it determines the
maximum number of levels of traceback information printed when an
unhandled exception occurs. The default is
1000. When set to
0 or less, all traceback information is suppressed and only the
exception type and value are printed.
A string containing the version number of the Python interpreter plus
additional information on the build number and compiler used. It has
a value of the form
build_date, build_time) [compiler]'. The first
three characters are used to identify the version in the installation
directories (where appropriate on each platform). An example:
>>> import sys
'1.5.2 (#0 Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)]'
A tuple containing the five components of the version number:
major, minor, micro, releaselevel, and
serial. All values except releaselevel are integers; the
release level is
corresponding to the Python version 2.0 is
(2, 0, 0, 'final', 0).
New in version 2.0.
The version number used to form registry keys on Windows platforms.
This is stored as string resource 1000 in the Python DLL. The value
is normally the first three characters of version. It is
provided in the sys module for informational purposes;
modifying this value has no effect on the registry keys used by
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