Python 中的 for 循环条件检查是如何工作的？

Question

Consider this script in Python :

>>> import math
>>> raw_data = [56.2, float('NaN'), 51.7, 55.3, 52.5, float('NaN'), 47.8]
>>> for i in range(len(raw_data)):
...     print(i, raw_data[i])
...     if math.isnan(raw_data[i]):
...         del raw_data[i]

As you can see we will get index out of range error, so I conclude that Python does not check the condition after each iteration because otherwise if it does, when i becomes 5 the length of list is 5 too so it should not enter the loop body at the first place since 5 is not lesser than 5 .

So I conclude that Python saves len(raw_data) = 7 at first iteration and it checks whether i < 7 after each iteration and does not call the len() function each time. Am I right ?

Answer 1

range() is a generator function that is evaluated once at the beginning of the for loop giving an iterable: [0, 1, 2, 3, 4, 5, 6] The for loop then runs once for each item in the iterable. To achieve what you are trying to do it would be better to use enumerate() ie.

for index, datum in enumerate(raw_data):
    if math.isnan(datum):
        del raw_data[index]

This will work (in python 2 and 3), however, it is generally considered to be bad practice to edit a list that you are iterating over - you might get some strange behaviour if you try to do other operations by index in the same loop so you might also consider storing the indexes of items you wish to delete and then doing the deletions all at once afterwards to make your code more robust. ie.

indexes_to_delete = []
for index, datum in enumerate(raw_data):
    if math.isnan(datum):
        indexes_to_delete.append(index)
# go through the indexes in reverse, to make sure the deletions don't affect earlier indices
for index in sorted(indexes_to_delete, reverse=True):
    del raw_data[index]