遍历python中的列表理解

Question

Why does when I run :

a = ['a','n','g']
b = [range(0,4)]

print [(x,y) for x in a for y in b]

returns :

[('a', [0, 1, 2, 3]), ('n', [0, 1, 2, 3]), ('g', [0, 1, 2, 3])]

but when I run this piece of code:

a = ['a','n','g']
b = [0,1,2,3,4]

print [(x,y) for x in a for y in b]

It returns:

[('a', 0), ('a', 1), ('a', 2), ('a', 3), ('a', 4), ('n', 0), ('n', 1), ('n', 2), ('n', 3), ('n', 4), ('g', 0), ('g', 1), ('g', 2), ('g', 3), ('g', 4)]

So as you may have noticed, if I use the range function, I get a wrong output but If I manually key in ['a','n','g'] I get the desired output

b = [range(0,4)]
and
b = [0,1,2,3,4]

Both are obviously the same right? therefore why do there results vary (if they are the same) ?

I'm basically trying to build a program which has A in a range of 999 reversed and B with a range of 999 and believe me hard coding those 1000 digits is not efficient and I might probably be considered insane

Answer 1

[range(0,4)] is the same as [[0,1,2,3]] , so it is very much not the same as [0,1,2,3,4] .

I think you just want range(0,5) (note that there are no square brackets around this, and to include 4 in the result you need to have 5 for the stop argument of range() ).

And for your specific problem: "I'm basically trying to build a program which has A in a range of 999 reversed and B with a range of 999":

A = range(999, -1, -1)
B = range(1000)

This assumes that you want ranges like [999, 998, ..., 1, 0] and [0, 1, ..., 998, 999] .

Answer 2

Both are obviously the same right?

>>> [1, 2, 3, 4]
[1, 2, 3, 4]
>>> [range(0,4)]
[[0, 1, 2, 3]]

The two are not the same; you probably meant b = range(0,4) without the extra set of brackets.

Answer 3

No, these two are not the same. You can think of

[range(0, 4)]

as this

[[0, 1, 2, 3]]

Note it is a list of lists.

So when you are doing a list comprehension over this list, it will simply iterate over the single element [0, 1, 2, 3] over and over again.

Answer 4

Obviously no, thery are not the same, range(0,4) is [0,1,2,3]

Note that is different than: [range(0,4)] and [[0,1,2,3] ]

Check any python source.

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