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.
sys.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, sys.argv is set to the string
If no script name was passed to the Python interpreter,
sys.argv has zero length.
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 -- sys.modules.keys() only lists the imported
- 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
containing three 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 sys.exc_info() in a
function that does not itself handle an exception.
Use of these three variables is deprecated; they contain the same
values as returned by sys.exc_info() above. However, 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, sys.exc_type is set to None
and 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 also "/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
sys.exec_prefix+"/lib/pythonversion/config", and shared library
modules are installed in
where version is equal to sys.version[:3].
- exit (n)
Exit from Python with numeric exit status n. 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 catch the exit attempt at an outer
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 in any way (except when a
fatal error occurs: in that case the interpreter's internal state
cannot be trusted).
- 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().
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 sys.exc_info() above.
(Since there is only one interactive thread, thread-safety is not a
concern for these variables, unlike for sys.exc_type etc.)
Gives the list of modules that have already been loaded.
This can be manipulated to force reloading of modules and other tricks.
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, sys.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
standard input), sys.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. sunos5 or
linux1. This can be used to append platform-specific
components to sys.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
sys.prefix+"/lib/pythonversion" while the platform
independent header files (all except config.h) are stored in
where version is equal to sys.version[:3].
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 10, meaning
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.
- 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.
- 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
sys.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 None.
File objects corresponding to the interpreter's standard input,
output and error streams. sys.stdin is used for all
interpreter input except for scripts but including calls to
input() and raw_input(). sys.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
sys.stderr. sys.stdout and sys.stderr needn't
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 popen(), system() or the exec*()
family of functions in the os module.)
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.