Python While Loop Condition Evaluation

Question

Say I have the following loop:

i = 0
l = [0, 1, 2, 3]
while i < len(l):
    if something_happens:
         l.append(something)
    i += 1

Will the len(i) condition being evaluated in the while loop be updated when something is appended to l ?

Answer 1

是的，它会的。

Answer 2

Your code will work, but using a loop counter is often not considered very "pythonic". Using for works just as well and eliminates the counter:

>>> foo = [0, 1, 2]
>>> for bar in foo:
    if bar % 2: # append to foo for every odd number
        foo.append(len(foo))
    print bar

0
1
2
3
4

If you need to know how "far" into the list you are, you can use enumerate :

>>> foo = ["wibble", "wobble", "wubble"]
>>> for i, bar in enumerate(foo):
    if i % 2: # append to foo for every odd number
        foo.append("appended")
    print bar

wibble
wobble
wubble
appended
appended