python从特定索引继续循环

Question

I wrote some code that uses enumerate to loop through a list.

for index, item in enumerate(list[start_idx:end_idx])
     print('index [%d] item [%s]'%(str(index), item))

the item in the list are just strings. Sometimes I do not want to enumerate for the whole list, sometimes I'll slice up the list do do different things.

The part that I am having trouble with is python's enumerate function.

The docs say you can do:

for index, item in enumerate(list, start_index):
    print(str(index))

the above code doesn't do what I expected. I though enumerate would start at the start position and stop at the end, but it doesn't.

I wrote a small program and it does indeed act wonky.

>>> for i, x in enumerate(range(0,20),start=4):
...      print(str(i)+'\t'+str(x))
... 
4   0
5   1
6   2
7   3
8   4
9   5
10  6
11  7
12  8
13  9
14  10
15  11
16  12
17  13
18  14
19  15
20  16
21  17
22  18
23  19

I would expect enumerate to start at index 4 and loop to the end. So it would get the range of 4-19 but it seems to just start the index but still iterates from 0-19..

Question, is there a way to do a iteration loop starting at a specific index in python?

My expected outcome would be

>>> for i, x in enumerate(range(0,20),start=4):
...      print(str(i)+'\t'+str(x))
... 
4   0 # skip
5   1 # until
6   2 # this
7   3 # item
8   4
9   5
10  6
11  7
12  8
13  9
14  10
15  11
16  12
17  13
18  14
19  15
20  16
21  17
22  18
23  19

instead of starting the index at the start position provide.

Answer 1

Actually if you got range object it's not a big deal to make a slice from it, because range(n)[:4] is still range object( as @MosesKoledoye mentioned it's Python 3 feature ). But if you got a list, for the sake of not creating new list you can choose itertools.islice , it will return iterator.

from itertools import islice
for index, item in enumerate(islice(range(20), 4, None)):
    print(index, item)

Output

0 4
1 5
2 6
3 7
4 8
...

Answer 2

The start parameter of enumerate doesn't have anything to do with what elements of the iterable get selected. It just tells enumerate what number to start with.

>>> list(enumerate(range(3)))
[(0, 0), (1, 1), (2, 2)]
>>> list(enumerate(range(3), 1))
[(1, 0), (2, 1), (3, 2)]

If you want to start at a specific index, you need to provide the start argument and a slice :

for i, v in enumerate(alist[4:], 4):
    ...

Answer 3

You can do:

for index, item in enumerate(list[4:]):
    print(str(index))