Detailed introduction to else in Python

We all know that the basic usage of else in Python is if...elif...else... in conditional control statements, but else also has two other uses. One is for the end of a loop, The other is used in error handling try. This was originally the standard syntax of Python, but because it is different from the habits of most other programming languages, people have ignored these usages intentionally or unintentionally. In addition, there is a lot of controversy over whether these usages are consistent with the principles of 0×00 The Zen of Python and whether they should be widely used. For example, in the two books I saw (Effective Python VS Write Idiomatic Python), the two authors held completely different attitudes towards it.

else in the loop

The else statement following the loop will only be executed when there is no break in the loop, that is, when the normal loop is completed. First, let's look at an example of insertion sorting:

from random import randrange
def insertion_sort(seq):
  if len(seq) 1:
    return seq
  _sorted = seq[:1]
  for i in seq[1:]:
    inserted = False
    for j in range(len(_sorted)):
      if i _sorted[j]:
        _sorted = [*_sorted[:j], i, *_sorted[j:]]
        inserted = True
        break
    if not inserted:
      _sorted.append(i)
  return _sorted
 
print(insertion_sort([randrange(1, 100) for i in range(10)]))

[8, 12, 12, 34, 38, 68, 72, 78, 84, 90]

In this example, the sorted _sorted elements are compared with i one by one. If i If it is larger than all the elements in the sorted list, it can only be placed at the end of the sorted list. At this time, we need an additional state variable inserted to mark whether the traversal loop is completed or broken halfway. In this case, we can use else to replace this state variable:

def insertion_sort(seq):
  if len(seq) 1:
    return seq
  _sorted = seq[:1]
  for i in seq[1:]:
    for j in range(len(_sorted)):
      if i _sorted[j]:
        _sorted = [*_sorted[:j], i, *_sorted[j:]]
        break
    else:
      _sorted.append(i)
  return _sorted
print(insertion_sort([randrange(1, 100) for i in range(10)]))

[1, 10, 27, 32, 32, 43, 50, 55, 80, 94]

I think this is a very cool approach! However, it should be noted that in addition to break can trigger the subsequent else statement, it will also trigger when there is no loop:

while False:
  print("Will never print!")
else:
  print("Loop failed!")


Loop failed!

else

# in error capture ##try...except...else...finally flow control syntax is used to catch possible exceptions and handle them accordingly. Except is used to catch errors that occur in try statements; else is used to handle errors that do not occur. In the event of an error; finally is responsible for the "aftercare" of the try statement and will be executed anyway. This can be demonstrated with a simple example:

def pide(x, y):
  try:
    result = x / y
  except ZeropisionError:
    print("pision by 0!")
  else:
    print("result = {}".format(result))
  finally:
    print("pide finished!")
pide(5,2)
print("*"*20)
pide(5,0)

result = 2.5
pide finished!
********************
pision by 0!
pide finished!

Of course, you can also use state variables instead of else:

def pide(x, y):
  result = None
  try:
    result = x / y
  except ZeropisionError:
    print("pision by 0!")
  if result is not None:
    print("result = {}".format(result))
  print("pide finished!")
 
pide(5,2)
print("*"*20)
pide(5,0)

##

result = 2.5
pide finished!
********************
pision by 0!
pide finished!

Summary

Some people think that these uses of else are counterintuitive or implicit rather than explicit, and are not worth promoting. But I think this "judgment" depends on the specific application scenario and our understanding of Python. It does not necessarily have to be newcomer-friendly syntax to be explicit. Of course, it is not recommended to use this syntax everywhere. The biggest disadvantage of for/while...else is that else needs to be aligned with for/file. It is very uncomfortable if there are multiple levels of nesting or the loop body is too long. It is suitable to use else (just recall the story of the vernier caliper :P). Only in some short loop control statements, we use else to get rid of some cumbersome state variables. This is the most Pythonic application scenario!

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