Python Cookbook

Dynamically Changing the Python Search Path

Credit: Robin Parmar

Problem

Modules must be on the Python search path before they can be imported, but you don’t want a huge permanent path, because that slows things down—you want to change the path dynamically.

Solution

We just conditionally add a directory to Python’s sys.path, carefully checking to avoid duplication:

def AddSysPath(new_path):
    """ AddSysPath(new_path): adds a directory to Python's sys.path

    Does not add the directory if it does not exist or if it's already on
    sys.path. Returns 1 if OK, -1 if new_path does not exist, 0 if it was
    already on sys.path.
    """
    import sys, os

    # Avoid adding nonexistent paths
    if not os.path.exists(new_path): return -1

    # Standardize the path. Windows is case-insensitive, so lowercase
    # for definiteness.
    new_path = os.path.abspath(new_path)
    if sys.platform == 'win32':
        new_path = new_path.lower(  )

    # Check against all currently available paths
    for x in sys.path:
        x = os.path.abspath(x)
        if sys.platform == 'win32':
            x = x.lower(  )
        if new_path in (x, x + os.sep):
            return 0
    sys.path.append(new_path)
    return 1

if _ _name_ _ == '_ _main_ _':
    # Test and show usage
    import sys

    print 'Before:'
    for x in sys.path: print x

    if sys.platform == 'win32':
          print AddSysPath('c:\\Temp')
          print AddSysPath('c:\\temp')
    else:
          print AddSysPath('usr/lib/my_modules')

    print 'After:'
    for x in sys.path: print x

Discussion

Modules must be on the Python search path before they can be imported, but we don’t want to have a huge permanent path, ...