Python Forgotten in the loop else Options

Translated from https://s16h.medium.com/the-forgotten-optional-else-in-python-loops-90d9c465c830

In this paper, Python Of for-else and while-else grammar ,Python One of the least used and often misunderstood features in .

Grammar introduction

Python Medium for Circulation and while Each loop has an optional else Kit （ Be similar to if Statement and try sentence ）, If the cycle ends normally , Then the... Will be executed else Kit . let me put it another way , If we do not in any unnatural way （break、return Or throw an exception ） Exit loop ,else The suite will be executed . namely , No, break sentence , No, return sentence , Or not in the loop Trigger exception .

Take a simple example ：

In [1]: for i in range(5):
...: print(i)
...: else:
...: print("Iterated over everything :)")
...:
0
1
2
3
4
Iterated over everything :)

In the code above , We traverse range(5) And print each number . Because we let the loop complete normally , therefore else The suite will also be executed , also Iterated over everything :) Be printed out . contrary , If we stop the cycle , For example, use break sentence , that else The suite will not be executed :

In [2]: for i in range(5):
...: if i == 2:
...: break
...: print(i)
...: else:
...: print("Iterated over everything :)")
...:
0
1

Be careful , Even if the sequence iterated by the loop is empty ,else The suite will also be executed ; After all , The loop still completes normally :

In [3]: for i in []:
...: print(i)
...: else:
...: print("Iterated over everything :)")
...:
Iterated over everything :)

in addition , Let's not forget , These also apply to while-else:

In [4]: i = 0
In [5]: while i <= 5:
...: i += 1
...: print(i)
...: else:
...: print('Yep')
...:
1
2
3
4
5
6
Yep

Why? ？

else A common use of clause in a loop is to implement a search loop .
Suppose you are searching for an item that meets certain criteria , And additional processing needs to be performed , Or if no acceptable value is found , Causes an error :

for x in data:
if meet_condation(x):
break
else:
# raise error or do addition processing 

without else Clause , You need to set a flag , Then check the flag later , To see if any of the values meet the following conditions :

condition_is_met = False
for x in data:
if meets_condition(x):
condition_is_met = True
if not condition_is_met:
# raise error or do additional processing

It's no big deal , This is what you have to do in many other languages .
however , And Python Like many other features of ,else Clause can produce more elegant and Pythonic Code for . so to speak , In the example above , Using it can make the code more The Zen of Python-friendly.

There is no situation where you have no choice , Can only be used in a loop else Clause . You can always use flag Marking and other methods , but else Clauses usually make the code more elegant and readable . One might think that this is Pythonic Of , It makes the intention clearer , Others may think it is confusing and superfluous ！