is Python list comprehension with access to the in

问题:

consider the following Python code with which I add in a new list2 all the items with indices from 1 to 3 of list1:

for ind, obj in enumerate(list1):
    if 4> ind > 0: list2.append(obj)

how would you write this using python list comprehension, if I have no access to the indices through enumerate?

something like:

list2 = [x for x in list1 if 4>ind>0]

but since I have no 'ind' number, would this work? :

list2 = [x for x in enumerate(list1) if 4>ind>0]

回答1:

list2 = [x for ind, x in enumerate(list1) if 4 > ind > 0]

回答2:

If you use enumerate, you do have access to the index:

list2 = [x for ind, x in enumerate(list1) if 4>ind>0]

回答3:

Unless your real use case is more complicated, you should just use a list slice as suggested by @wim

>>> list1 = ['zero', 'one', 'two', 'three', 'four', 'five', 'six']
>>> [x for ind, x in enumerate(list1) if 4 > ind > 0]
['one', 'two', 'three']
>>> list1[1:4]
['one', 'two', 'three']

For more complicated cases - if you don't actually need the index - it's clearer to iterate over a slice or an islice

list2 = [x*2 for x in list1[1:4]]

or

from itertools import islice
list2 = [x*2 for x in islice(list1, 1, 4)]

For small slices, the simple list1[1:4]. If the slices can get quite large it may be better to use an islice to avoid copying the memory