Reuven Lerner: Python function brain transplants

What happens when we define a function in Python?

The “def” keyword does two things: It creates a function object, and then assigns a variable (our function name) to that function object.  So when I say:

def foo():
      return "I'm foo!"

Python creates a new function object.  Inside of that object, we can see the bytecodes, the arity (i.e., number of parameters), and a bunch of other stuff having to do with our function.

Most of those things are located inside of the function object’s __code__ attribute.  Indeed, looking through __code__ is a great way to learn how Python functions work.  The arity of our function “foo”, for example, is available via foo.__code__.co_argcount.  And the byte codes are in foo.__code__.co_code.

The individual attributes of the __code__ object are read-only.  But the __code__ attribute itself isn’t!  We can thus be a bit mischievous, and perform a brain transplant on our function:

def foo():
      return "I'm foo!"

def bar():
      return "I'm bar!"

foo.__code__ = bar.__code__

Now, when we run foo(), we’re actually running the code that was defined for “bar”, and we get:

"I'm in bar!"

This is not likely to be something you want to put in your actual programs, but it does demonstrate some of the power of the __code__ object, and how much it can tell us about our functions.  Indeed, I’ve found over the last few years that exploring the __code__ object can be quite interesting!

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