A Place for John on the Web | Reloading dynamically loaded python modules

A Place for John on the Web

Just another WordPress weblog

Search

// you’re reading...

Development

Reloading dynamically loaded python modules

By John ⋅ October 30, 2008 ⋅ Post a comment

Hello there! If you are new here, you might want to subscribe to the RSS feed for updates on this topic.

Powered by WP Greet Box WordPress Plugin

In working on a python project I came across the need to dynamically load a module function. Think of it as a plugin mechanism where you have a package space that looks like:

/main.py /drivers/ /fakedriver.py /anotherdriver.py /moredriver.py

main.py spins up a loop and on each iteration is told which driver to use via a String that it retrieves. So, one call could require fakedriver.py and the next could require anotherdriver.py.

Easy enough, use dynamic imports:

def importName(modulename, name): """ Import a named object from a module in the context of this function, which means you should use fully qualified module paths.


        Return None on failure.

    """

    try:

        module = __import__(modulename, globals(), locals(), [name])

    except ImportError:

        return None

return vars(module)[name]

Works fine but main.py is a long living process and one of the things it knows how to do is update its drivers. Normally you would just call ‘reload(module)’ but in this case the module has no handle, its just a string so reload() fails. The fix for me was to check and delete it from sys.modules.

ef importName(modulename, name): """ Import a named object from a module in the context of this function, which means you should use fully qualified module paths.


        Return None on failure.

    """

    try:

        # Delete the module from cache on every load to keep it fresh

        # We have to wrap this in a try, on first load the key does not exist and throws a KeyError

        try:

            del sys.modules[modulename]

        except KeyError:

                print "Error"

        module = __import__(modulename, globals(), locals(), [name])

    except ImportError:

        return None

return vars(module)[name]

Python people probably see this as obvious but it took me 30 minutes to solve it so I post it here for posterity.

Discussion

Comments are disallowed for this post.

Comments are closed.

Search

Development

Reloading dynamically loaded python modules

Discussion

Comments are disallowed for this post.

Categories