# 3.1 Built-in Module `sys`

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.

`argv`
  The list of command line arguments passed to a Python script. `sys.argv[0]` 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[0]` is set to the string `"-c"`. If no script name was passed to the Python interpreter, `sys.argv` has zero length.

`builtin_module_names`
  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 modules.)

`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.

`exc_type`
  ,exc_value ,exc_traceback 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.

`exec_prefix`
  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/pythonVER/config"`, and shared library modules are installed in `sys.exec_prefix+"/lib/pythonVER/lib-dynload"`, where VER 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 level.

`exitfunc`
  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).

`last_type`
  ,last_value ,last_traceback 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 information.)

  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.)

`modules`
  Gives the list of modules that have already been loaded. This can be manipulated to force reloading of modules and other tricks.

`path`
  A list of strings that specifies the search path for modules. Initialized from the environment variable `PYTHONPATH`, or an installation-dependent default.

  The first item of this list, `sys.path[0]`, 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[0]` 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`.

`platform`
  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.

`prefix`
  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/pythonVER"` while the platform independent header files (all except `config.h`) are stored in `sys.prefix+"/include/pythonVER"`, where VER is equal to `sys.version[:3]`.

`ps1`
  ,ps2 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`.

`stdin`
  ,stdout ,stderr 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.)

`tracebacklimit`
  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.

`version`
  A string containing the version number of the Python interpreter.