Python equivalent of golang's defer statement

Question

How would one implement something that works like the defer statement from go in python?

Defer pushes a function call to a stack. When the function containing the defer statement returns, the defered function calls are popped and executed one by one, in the scope that the defer statement was inside in the first place. Defer statements look like function calls, but are not executed until they are popped.

Go example of how it works:

func main() {     fmt.Println("counting")      var a *int     for i := 0; i < 10; i++ {         a = &i         defer fmt.Println(*a, i)     }      x := 42     a = &x      fmt.Println("done") } 

Outputs:

counting done 9 9 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 0 0 

Go example of a usecase:

var m sync.Mutex func someFunction() {     m.Lock()     defer m.Unlock()     // Whatever you want, with as many return statements as you want, wherever.     // Simply forget that you ever locked a mutex, or that you have to remember to release it again. } 

Answer 1

To emulate defer fmt.Println(*a, i) example, you could use contextlib.ExitStack:

#!/usr/bin/env python3 from contextlib import ExitStack from functools import partial  print("counting") with ExitStack() as stack:     for i in range(10):         a = i         stack.callback(partial(print, a, i))      x = 42     a = x     print("done") 

Output

counting done 9 9 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 0 0 

It is easy to emulate the mutex case:

def some_function(lock=Lock()):     with lock:         # whatever 

Answer 2

Python's with statement serves a similar purpose to Go's defer.

The similar code in Python is:

mutex = Lock()  def someFunction():     with mutex:         # Whatever you want, with as many return statements         # as you want, wherever. Simply forget that you ever         # locked a mutex, or that you have to remember to          # release it again.