The map function of python

Question

>>> dataset
[[1, 3, 4], [2, 3, 5], [1, 2, 3, 5], [2, 5]]
>>> D=map(set,dataset)
>>> D
<map object at 0x0000000002ABF5F8>

When I input D into interactive window of python3.3,I supposed that it should appear:

[set([1, 3, 4]), set([2, 3, 5]), set([1, 2, 3, 5]), set([2, 5])]

Why a map object?

Read Getting a map() to return a list in python 3.1 I didn't read anywhere But I observe the difference that in new Python version most in-built functions return generator instead of sequences (for efficiency purpose I suppose) — Grijesh Chauhan
– Grijesh Chauhan, Commented Oct 27, 2013 at 9:13
When noticing strange behaviour if using Py3, it is always worth to take a look at the function's documentation. There are many subtledifferences, making your life hard if you are used to Py2. — glglgl
– glglgl, Commented Oct 27, 2013 at 9:23

Salvador Dali · Accepted Answer · 2013-10-27 09:14:59Z

2

Why the result of map is an object? Because in documentation for python 3.3 it is written that it yields the result, thus it is a generator.

You can read it by using

for i in D:
  print i

or as it was suggested by Grijesh

list(D) will convert it to list

answered Oct 27, 2013 at 9:14

Salvador Dali

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Community · Accepted Answer · 2017-05-23 11:52:00Z

2

In Python 3, map doesn't return a list, but a map object, an iterator. There's a similar question about this. If you need a list, you can easily convert it to a list:

D = list(map(set,dataset))

See the docs about this.

By the way, it's a good practice to use lower case letters for variable names. Hope this helps!

edited May 23, 2017 at 11:52

CommunityBot

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answered Oct 27, 2013 at 9:19

aIKid

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1 Comment

aIKid Over a year ago

Haven't seen yours when i wrote it. Sorry mate.

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