Executing .PYC Files in Python

I got this .PYC (compiled Python script) file that I used with command line Python, ala: “python.exe script.pyc”. And that would run the script.pyc file and let it do its job. The problem was that I wanted to run a few Python lines before running script.pyc itself. But apparently, it’s not really possible.

Suppose script.pyc does the familiar name check:
if __name__ == “__main__”:
main()

I guess everyone who wrote a module or two in Python knows this trick. This way you can tell in your script whether it was imported or executed and do whatever you are up to correspondingly. Thus, if it was executed, you will probably want to run a test case for the module, hence calling main() usually, and you could even pass command line arguments to it the normal way.

Since I wanted to do a few things before the script gets to run in Python, it means I had to open Python myself, do my stuff, then execute the script. Unfortunately, it is not possible to execfile() a .PYC file, beat me. Again, I couldn’t just import the file since then that if statement I presented above would fail and won’t call the main() function and nothing would happen, fail. execfile() also doesn’t work, simply because it runs only pure Python source code and not a compiled script.

What I eventually came up with was to import the file myself, but that required fooling a bit :)
You can __import__ file in the code in runtime. What I mean is that when you don’t have the filename to import in static time, you can use that function which receives a filename and dynamically loads the module in runtime.
I tried that on the script.pyc file and obviously it didn’t work as well, because the __name__ was wrong, so the main() didn’t get executed. That made me realize that I need to do the __import__’s internals on my own and only then I will be able to change the __name__ for the module (if that’s possible, but it has to be, since the distinction exists, right?)

Then after a bit of googling around I sumbled upon: imp.
Which shows how to import a file ourselves, then I changed it to:
import imp
fp, pathname, description = imp.find_module(“script”)
imp.load_module(“__main__”, fp, pathname, description)

Notice how I pass __main__ instead of the module’s real name, then the check for main() inside script.py would really work and execute the main() function and I’m all happy once again.

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2 Responses to “Executing .PYC Files in Python”

  1. ttbya says:

    Wouldn’t it be the far more obvious way to import the module and then call main() yourself?
    Like,
    import script
    script.main()

    Usually, after the __name__ == “__main__” check a single module-level function will be called, or at least, I’ve never seen anything else, so this should work, right?

  2. arkon says:

    You are right. However, sometimes we don’t have the source code…
    And even if I disassembled the file and saw the name of main function; I got a generic solution.

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