I don't understand this result

[Ponomarenko Pavlo]

>>> a = [1, 2, 4, 5]
>>> def az():
            x * 2

>>> q = map(az, [2, 3, 4])
>>> list(q)
[None, None, None]

[zivoni]

You forgot to return the value computed in your function:

>>> def az(x):
...   return x*2
... 
>>> q = map(az, [2, 3, 4])
>>> list(q)
[4, 6, 8]

[Larz60+]

This code is incomplete.
the definition of az is was missing it's body ok now

[buran]

just to mention that there is no need to explicitly convert the result to list

>>> def az(x):
return x*2

>>> q = map(az, [2, 3, 4])
>>> q
[4, 6, 8]

[snippsat]

just to mention that there is no need to explicitly convert the result to list

Python 3 has changed a lot buran 

# Python 3.6
>>> def az(x):
...     return x*2
... 
>>> q = map(az, [2, 3, 4])
>>> q
<map object at 0x049C8EB0>
>>> list(q)
[4, 6, 8]

# Python 2.7
>>> def az(x):
...     return x * 2
...     
>>> q = map(az, [2, 3, 4])
>>> q
[4, 6, 8]

Built-ins like range, map, zip, filter become iterables in 3.x to conserve space,
rather than producing a result list all at once in memory.

[buran]

Python 3 has changed a lot buran 

Built-ins like range, map, zip, filter become iterables in 3.x to conserve space,
rather than producing a result list all at once in memory.

Funny part is - I know it, but I tend to think python2 more often than python3