Inject function calls into python function

Let's say I have some simple python function in a module

def f(x):
    print(x)

This function just prints its argument. However, I want to modify this function or class method, depending on what it actually is.

I have a second function:

def g(x):
    x.append(1)

which manipulates the input argument (probably a list) and appends some item. Now, I want to inject a call to g into f so that the latter would behave as

def f_patched(x):
    g(x)
    print(x)

Clearly, this can be done by monkey patching f. This however is not always practical, especially if f is some larger function as I would have to copy large portions of code (if this is even legal) just to change a single line. By injecting a call tog given some reference, for example "after line XXX" or "after instruction XXX" I could easily modify the behavior of f.

Is there a way to inject a function call into a different function? You can assume, that I know the source of f or at least that f is known well enough and unchanging for this to be possible.

Solution

Perhaps you could redefine the function early in the program (i.e. before it is first called) to replace it with your patched version:

import myModule

# before this f() is the original

old_f = myModule.f           # capture a reference to the original
def f_patched(x):
    g(x)                     # inject your code
    return old_f(x)          # use the original
myModule.f = f_patched       # replace the original

# from here onward, f() will be the patched version that calls the original