# 6.2 Standard Modules

Python comes with a library of standard modules, described in a separate document (Python Library Reference). Some modules are built into the interpreter; these provide access to operations that are not part of the core of the language but are nevertheless built in, either for efficiency or to provide access to operating system primitives such as system calls. The set of such modules is a configuration option; e.g., the `amoeba` module is only provided on systems that somehow support Amoeba primitives. One particular module deserves some attention: `sys`, which is built into every Python interpreter. The variables `sys.ps1` and `sys.ps2` define the strings used as primary and secondary prompts:

    >>> import sys
    >>> sys.ps1
    '>>> '
    >>> sys.ps2
    '... '
    >>> sys.ps1 = 'C> '
    C> print 'Yuck!'
    Yuck!
    C>

These two variables are only defined if the interpreter is in interactive mode.

The variable `sys.path` is a list of strings that determine the interpreter's search path for modules. It is initialized to a default path taken from the environment variable `PYTHONPATH`, or from a built-in default if `PYTHONPATH` is not set. You can modify it using standard list operations, e.g.:

    >>> import sys
    >>> sys.path.append('/ufs/guido/lib/python')
    >>>