How do I override a Python import?

Question

I'm working on pypreprocessor which is a preprocessor that takes c-style directives and I've been able to make it work like a traditional preprocessor (it's self-consuming and executes postprocessed code on-the-fly) except that it breaks library imports.

The problem is: The preprocessor runs through the file, processes it, outputs to a temporary file, and exec() the temporary file. Libraries that are imported need to be handled a little different, because they aren't executed, but rather they are loaded and made accessible to the caller module.

What I need to be able to do is: Interrupt the import (since the preprocessor is being run in the middle of the import), load the postprocessed code as a tempModule, and replace the original import with the tempModule to trick the calling script with the import into believing that the tempModule is the original module.

I have searched everywhere and so far and have no solution.

This Stack Overflow question is the closest I've seen so far to providing an answer: Override namespace in Python

Here's what I have.

# Remove the bytecode file created by the first import
os.remove(moduleName + '.pyc')

# Remove the first import
del sys.modules[moduleName]

# Import the postprocessed module
tmpModule = __import__(tmpModuleName)

# Set first module's reference to point to the preprocessed module
sys.modules[moduleName] = tmpModule

moduleName is the name of the original module, and tmpModuleName is the name of the postprocessed code file.

The strange part is this solution still runs completely normal as if the first module completed loaded normally; unless you remove the last line, then you get a module not found error.

Hopefully someone on Stack Overflow know a lot more about imports than I do, because this one has me stumped.

Note: I will only award a solution, or, if this is not possible in Python; the best, most detailed explanation of why this is not impossible.

Update: For anybody who is interested, here is the working code.

if imp.lock_held() is True:
    del sys.modules[moduleName]
    sys.modules[tmpModuleName] = __import__(tmpModuleName)
    sys.modules[moduleName] = __import__(tmpModuleName)

The 'imp.lock_held' part detects whether the module is being loaded as a library. The following lines do the rest.

Answer 1

Does this answer your question? The second import does the trick.

Mod_1.py

def test_function():
    print "Test Function -- Mod 1"

Mod_2.py

def test_function():
    print "Test Function -- Mod 2"

Test.py

#!/usr/bin/python

import sys

import Mod_1

Mod_1.test_function()

del sys.modules['Mod_1']

sys.modules['Mod_1'] = __import__('Mod_2')

import Mod_1

Mod_1.test_function()