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 list 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_type

,exc_value ,exc_traceback These three variables are not always defined; they are set when an exception handler (an except clause of a try statement) is invoked. Their meaning is: exc_type gets the exception type of the exception being handled; exc_value gets the exception parameter (its associated value or the second argument to raise); exc_traceback gets a traceback object (see the Reference Manual) which encapsulates the call stack at the point where the exception originally occurred.   

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 (which may be hard to reproduce). The meaning of the variables is the same as that of exc_type, exc_value and exc_tracaback, respectively.

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.

platform

This string contains a platform identifier. This can be used to append platform-specific components to sys.path, for instance.

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 '... '.

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 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.