Python Cookbook

Reloading All Loaded Modules

Credit: Sébastien Keim

Problem

When you repeatedly run a test script during an interactive session, it always uses the first version of the modules you are developing, even if you made changes to the code. You need to ensure that modules are reloaded.

Solution

There are a few ways to accomplish this goal. Here’s a solution that is simple and drastic, but it may not work with integrated development environments (IDEs):

import sys
sys.modules.clear(  )

And here is a solution that is a bit more careful and is compatible with IDEs:

import sys
if globals(  ).has_key('init_modules'):
    # second or subsequent run: remove all but initially loaded modules
    for m in sys.modules.keys(  ):
        if m not in init_modules:
            del(sys.modules[m])
else:
    # first run: find out which modules were initially loaded
    init_modules = sys.modules.keys(  )

Discussion

When you create a Python module, you can use a test script that imports your module. But you have probably noticed that when you repeatedly run the test script inside a given interactive session, it always uses the first version of your module, even if you made changes in the code. This is because the import statement checks if the module is already in memory and does the actual importing only if this is the first time the module is used. This is an important optimization that lets you use the import statement freely, but it does get in the way in such development situations.

You can use the reload function, but this is difficult if you ...