Python 中的 IF 条件是如何工作的？

Question

This is a Mergesort i found online and tried to implement. It worked but i am not sure how it works. Can someone please explain to me why/how this if-condition works? Where does the code continue when the first-if statement isnt true anymore. I know how if-condtions work in general but to the lack of indentions and/or 'else' confuses me here.

def mergeSort(alist):
   print("Splitting ",alist)
   if len(alist)>1:
       mid = len(alist)//2
       lefthalf = alist[:mid]
       righthalf = alist[mid:]
       #recursion
       mergeSort(lefthalf)
       mergeSort(righthalf)

       i=0
       j=0
       k=0

       while i < len(lefthalf) and j < len(righthalf):
           if lefthalf[i] < righthalf[j]:
               alist[k]=lefthalf[i]
               i=i+1
           else:
               alist[k]=righthalf[j]
               j=j+1
           k=k+1

       while i < len(lefthalf):
           alist[k]=lefthalf[i]
           i=i+1
           k=k+1

       while j < len(righthalf):
           alist[k]=righthalf[j]
           j=j+1
           k=k+1

alist = [54,26,93,17,77,31,44,55,20]
mergeSort(alist)
print(alist)```

Answer 1

This relies on alist effectively acting like a global variable, which tends to be a bit frowned upon (especially since it's not declared global). There are also some while loops that could be replaced by list comprehensions.

If the if condition is not true, then alist has only one element, and so sorting it is trivial: a sorted list of one element is just that list. A list of only element can't be out of order. So the function simply terminates without doing anything, because nothing is needed.

If the if condition is true, then function reassigns entries in the list, using recursive calls to itself. Each recursion calls the function on a smaller list, so eventually a list of length one is reached, terminating the recursion. So when the if condition is false, the function just returns to the next higher level of recursion.

Because of the way mutable data types such as lists work in Python, when a recursion level changes what's in a particular location of the alist that it was passed, it also alters what's in that location in the alist of the higher recursion levels. This results in the original alist ending up sorted, even though the function doesn't have the return keyword and thus doesn't explicitly return anything. This is known as a function operating on the input "in place".

Answer 2

In this code there is no else used - to find the matching else always look for else that starts at the same position(with same indentation) as if. Since there is no such code, it will exit the function when if condition is not satisfied.