List elements disappeared when I used a lambda expression or assignment

Question

I'm just trying to figure out what is happening in this python code. I was trying to use this answer here, and so I was tinkering with the console and my list elements just vanished.

What I was doing was loading the lines into a file into a list, then trying to remove the newline chars from each element by using the lambda thing from the other answer.

Can anyone help me explain why the list became empty?

>>> ================================ RESTART ================================
>>> x = ['a\n','b\n','c\n']
>>> x
['a\n', 'b\n', 'c\n']
>>> x = map(lambda s: s.strip(), x)
>>> x
<map object at 0x00000000035901D0>
>>> y = x
>>> y
<map object at 0x00000000035901D0>
>>> x
<map object at 0x00000000035901D0>
>>> list(x)
['a', 'b', 'c']
>>> x = list(x)
>>> x
[]
>>> y
<map object at 0x00000000035901D0>
>>> list(y)
[]
>>>

Wrap a `list` call around the `map` as Py3 returns a `map` object — Bhargav Rao, Jun 17 '15 at 03:25

score 1 · Accepted Answer · answered Jun 17 '15 at 03:25

1

You are using Python3, so map returns a mapobject. You can only iterate over a mapobject once. So convert it to a list if you need to iterate over the items more than once. (also if you need to look up by index etc.)

Use

x = list(map(lambda s: s.strip(), x))

or better - a list comprehension

x = [s.strip() for s in x]

answered Jun 17 '15 at 03:25

John La Rooy

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What's a `mapobject` for? – dfeuer Jun 17 '15 at 03:29
@dfeuer Lazy computation. It allows you to efficiently map over an iterable which is really big, or maybe even infinitely long. – jme Jun 17 '15 at 04:07
@jme Any idea why it returns a `mapobject` instead of a `generator`? It sounds just like the result of a generator comprehension, and for the same reason... – TessellatingHeckler Jun 17 '15 at 06:11

List elements disappeared when I used a lambda expression or assignment

1 Answers1