Does the order of functions in a Python script matter?

Let's say I have two functions in my script: sum_numbers and print_sum. Their implementation is like this:

def sum_numbers(a, b):
    return a + b

def print_sum(a, b):
    print(sum_numbers(a, b))

So my question is: does the order in which the function are written matter? If I had written the print_sum function first and then the sum_numbers, would the code still work? If the answer is yes, does it always work?

The only thing that Python cares about is that the name is defined when it is actually looked up. That's all.

In your case, this is just fine, order doesn't really matter since you are just defining two functions. That is, you are just introducing two new names, no look-ups.

Now, if you called one of these (in effect, performed a look-up) and switched the order around:

def print_sum(a, b):
    print(sum_numbers(a, b))

print_sum(2, 4)

def sum_numbers(a, b):
    return a + b

you'd be in trouble (NameError) because it will try to find a name (sum_numbers) that just doesn't exist yet.

So in general, yes, the order does matter; there's no hoisting of names in Python like there is in other languages (e.g JavaScript).

It doesn't matter in which order the functions are created. It only matters when the call to the function is done:

def print_sum(a, b):
    print(sum_numbers(a, b))

def sum_numbers(a, b):
    return a + b

print_sum(1, 3)
# 4

that works because at the time print_sum is called both functions do exist. However if you call the function before defining sum_numbers it would fail because sum_numbers isn't defined yet:

def print_sum(a, b):
    print(sum_numbers(a, b))

print_sum(1, 3)

def sum_numbers(a, b):
    return a + b

throws:

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-34-37c0e3733861> in <module>()
      2     print(sum_numbers(a, b))
      3 
----> 4 print_sum(1, 3)
      5 
      6 def sum_numbers(a, b):

<ipython-input-34-37c0e3733861> in print_sum(a, b)
      1 def print_sum(a, b):
----> 2     print(sum_numbers(a, b))
      3 
      4 print_sum(1, 3)
      5 

NameError: name 'sum_numbers' is not defined