What is the equivalent of a Swift completion block in Python?

Question

In Swift we can write a function with a completion block like this:

func thisNeedsToFinishBeforeWeCanDoTheNextStep(completion: () -> ()) {
    print("The quick brown fox")
    completion()
}

And then when we call it, we can put something inside of that block to execute once it's finished it's process:

func thisFunctionNeedsToExecuteSecond() {
   print("jumped over the lazy dog")
}

What is the equivalent in Python?

All code executes in order. So even if it's an API call, it won't progress to the next line until the previous function has received a response? And what if I want a function to begin executing, but want the program to continue while it works, once that function has finished I want it to execute something else? — Max Phillips, Jul 03 '16 at 17:02
Unless you use mutlithreading, the functions will execute in order. — Eli Sadoff, Jul 03 '16 at 17:04
@spacemonkey "And what if I want a function to begin executing, but want the program to continue while it works (...)" *that* would make a great question :) — Philippe Aubertin, Jul 03 '16 at 17:06
Gotchya. Alright, well in case anyone was wondering how you do it with Swift, that's the code for it above :) — Max Phillips, Jul 03 '16 at 19:16

score 5 · Accepted Answer · answered Jul 03 '16 at 17:05

Python considers functions as objects, meaning you can pass them around (like in Swift, although I'm less familiar with the implementation details in Swift). You can't specify a type in the parameter, of course, since you can't do that with anything in Python, but that's okay. The implementation would look like:

def do_first(completion):
    print("The quick brown fox ")
    completion()

def do_second():
    print("jumped over the lazy dog.")

And then to use them:

do_first(do_second)

Unless you use asynchronous code ahead of the completion() call in the first function, the lines will execute sequentially as expected.

What is the equivalent of a Swift completion block in Python?

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