在python中模拟增量（++）运算符

Question

My input is a list, say l

It can either contain 4 or 5 elements. I want to assign it to 5 variables , say a , b , c , d and e .

If the list has only 4 elements then the third variable ( c ) should be None .

If python had an increment (++) operator I could do something like this.

l = [4 or 5 string inputs]
i = -1
a = l[i++]
b = l[i++]
c = None
if len(l) > 4:
    c = l[i++]
d = l[i++]
e = l[i++]

I can't seem to find an elegant way to do this apart from writing i+=1 before each assignment. Is there a simpler pythonic way to do this?

Answer 1

You're trying to use a C solution because you're unfamiliar with Python's tools. Using unpacking is much cleaner than trying to emulate ++ :

a, b, *c, d, e = l
c = c[0] if c else None

The *c target receives a list of all elements of l that weren't unpacked into the other targets. If this list is nonempty, then c is considered true when coerced to boolean, so c[0] if c else None takes c[0] if there is a c[0] and None otherwise.

Answer 2

The the specific case of this question, list unpacking is the best solution .

For other users needing to emulate ++ for other purposes (typically the desire to increment without an explicit i += 1 statement), they can use itertools.count . This will return an iterator that will increment indefinitely each time it is passed to next() .

import itertools

i = itertools.count(0)  # start counting at 0
print(next(i))  # 0
print(next(i))  # 1
# ...and so on...

Answer 3

I can't see that you really need to be incrementing at all since you have fixed positions for each variable subject to your c condition.

l = [4 or 5 string inputs]

a = l[0]
b = l[1]

if len(l) > 4:
    c = l[2]
    d = l[3]
    e = l[4]
else:
    c = None
    d = l[2]
    e = l[3]

Answer 4

There is no ++ operator in Python. A similar question to this was answered here Behaviour of increment and decrement operators in Python