Dynamic module import of class list from module given its full path

Question

First, it seems proper to me to state that I've read over the following questions:

• Python imports by folder module
• Dynamic module import in Python

And, I don't feel like they address my use case. That said, here's my question:

How can I dynamically import from a configuration file like this:

[imports]
/var/imports/foo.py = baz monkey
/var/imports/bar.py = ape

So that I can call the known objects via RESTful interface like this.

# http://localhost/data?operation=get&class=baz&id=1
def handler(object):
  def handle(self):
    cls = instantiate(request.args["class"])
    meth = request.args["operation"]
    return getattr(cls,meth,request.args)

And it could respond with the result of get() on an instance of that class.

Answer 1

Here's a rough sketch of a class registry you could use like this:

class ClassRegistry(object):

    def __init__(self):
        self.classes = {}

    def add_file_classes(self, fname, class_list):
        globals = {}
        exec open(fname) in globals
        for class_name in class_list:
            self.classes[class_name] = getattr(globals, class_name)

    def get_class(self, class_name):
        return self.classes[class_name]

Read your config file and invoke add_file_classes like this:

reg = ClassRegistry()
reg.add_file_classes("/var/imports/foo.py", ["baz", "monkey"])
reg.add_file_classes("/var/imports/bar.py", ["ape"])

then later:

cls = reg.get_class("baz")

Note that we aren't actually importing the files, simply executing them and collecting their results.

Answer 2

Look at the 2nd link again. It already mentions imp.load_source:

import imp

# some sort of shared dict for everything imported
loaded = {}

def load(path, from_list):
    # load the path
    module = imp.load_source( "dummy", path)
    for name in from_list:
        # add the names
        loaded[name] = getattr(module, name)

#lets say you have this:
data = "/var/imports/foo.py", ["baz"]
load(*data)
print loaded['baz'] # whatever foo.baz was

Now you can do something like this:

def instantiate( clsname ):
    obj = loaded[clsname]
    return obj()

I hope this gets you started.