Determine if object is of type Foo without importing type Foo

Question

Let's say I have a class defined in a file:

import stuff
import more stuff
import stuff that takes a long time to import
class Foo(object):
    def __init__(self, arg1, arg2 etc.):
        self.arg1 = arg1 
        self.arg2 = arg2
        # Lots of other stuff
    # Lots of methods

And in another file I have this code:

from big_file import Foo
def do_stuff(obj):
    if isinstance(obj, Foo):
        do_stuff
    else:
        do_other_stuff

Let's say the file Foo takes a long time to import for reasons out of my control. How can I refactor this code to not import Foo but still reliably check the type? I don't think duck typing is appropriate for my particular case.

Should I e.g. check the string representations of the bases of the obj? Or is there another, more canonical way?

Answer 1

Typically, this would be a non-issue. If you have an instance of big_file.Foo, then its parent module has already been imported earlier on even if not explicitly referenced from your other file. Python modules are only loaded once on the first import (provided you're not doing any explicit reloading or messing with sys.modules). Since it's already been imported, doing an import big_file in your other file should run instantaneously.

However, if your other file will only encounter a big_file.Foo under certain circumstances and big_file is only imported elsewhere when it's actually need, then you could inspect the object's class as such (this does not support subclasses):

def do_stuff(obj):
    if (obj.__class__.__module__, obj.__class__.__name__) == ('big_file', 'Foo'):
        do_stuff
    else:
        do_other_stuff

Since you've indicated that big_file.Foo could be imported at any point in the application and you want to support subclasses, you could check to see if its module has been imported and conditionally check the type.

import sys

def is_Foo(obj):
    if 'big_file' in sys.modules:
        return isinstance(obj, sys.modules['big_file'].Foo)
    else:
        return False

def do_stuff(obj):
    if is_Foo(obj):
        do_stuff
    else:
        do_other_stuff