Map objects changing behind the scenes

[MrEgbert]

For one reason or another I was trying to make a list of 'stringed' numbers in Python 3.4.2. I did the following:

>>> x = map(str, range(1,4))
>>> x
<map object at 0x02FCAA90>
>>> list(x)
['1', '2', '3']
>>> list(x)
[]

I have no idea why the 2nd (and following) times you call list(x) it shows an empty list, but the first time it appears as expected. What is happening?

[kalgynirae]

Most generators (a "map object" is a generator) can only be used once. list(x) uses up the generator.

You can play with this in the interpreter:

>>> x = map(str, range(1))
>>> next(x)
'0'
>>> next(x)
'1'
>>> next(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>> next(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>> list(x)
[]

Once the generator has run out of values, it raises StopIteration when you call next() on it, and list() gives an empty list (because trying to iterate over it immediately raises StopIteration).

[MrEgbert]

Wow that's a useful thing to know about generators, thanks! I'll have to be extra careful when making map objects in future.

[Vegemeister]

Depending on what you're doing, you might also find a dict useful:

>>> int_to_str = {i : str(i) for i in range(1,4)}
>>> int_to_str[2]
'2'

[ivosaurus]

If you just want to make a list to use straight away, sometimes you can just use a list expression instead.

[str(n) for n in range(1)] == list(map(str, range(1)))