How to Create a String From Iterator in Python?

Question

How to create a string from an iterator over string in Python?

I am currently just trying to create a reversed copy of a string, I know I could just use slice:

s = 'abcde'
reversed_s = s[::-1]

I could also create a list from the iterator and join the list:

s = 'abcde'
reversed_s_it = reversed(s)
reversed_list = list(reversed_s_it)
reversed_s = ''.join(reversed_list)

But when I try to create a string from the iterator directly, it just gives a string representation of the iterator, if I do this:

s = 'abcde'
reversed_s_it = reversed(s)
reversed_s = str(reversed_s_it)

reversed_s will give a string representation of the iterator instead of iterating the iterator to create a string.

print(reversed_s)

Output

<reversed object at 0x10c3dedf0>

Furthermore

print(reversed_s == 'edcba')

Output

False

Therefore, I just want to know if there is a Pythonic way to create a string from an iterator?

@chepner, I just don't feel good about using an extra space for creating an extra list to construct a string, I'm not sure if there is a better way — SiAce
– SiAce, Commented Dec 25, 2020 at 22:33
Iterators can be materialized as lists or tuples by using the list() or tuple() constructor functions. So it is hard to imagine there’s another way to skip this step. — Daniel Hao
– Daniel Hao, Commented Dec 25, 2020 at 22:33
String is an iterable object in python. and type(reversed_s) shows it as a str object what do you mean when you say "reversed_s is still an iterator" — Sandeep Polamuri
– Sandeep Polamuri, Commented Dec 25, 2020 at 22:36
@SiAce because that's the string representation of that iterator, which is what you asked for when you did str(reversed_s_it), the same as any object, which just happens to be the default __str__ implementation inherited from object, so, for example, class Foo: pass then print(Foo()) will give you something similar. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Dec 25, 2020 at 22:47

chepner · Accepted Answer · 2020-12-25 22:35:26Z

4

''.join itself only needs an iterable; you don't have to create a list first.

>>> ''.join(reversed(s))
'edcba'

However, there's nothing wrong with s[::-1]; in fact, it's quite a bit faster than using ''.join to concatenated the elements from the reversed object.

edited Dec 25, 2020 at 22:35

answered Dec 25, 2020 at 22:32

chepner

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2 Comments

SiAce Over a year ago

Oh, thanks, didn't know about that. By the way, what's the time complexity of join()?

juanpa.arrivillaga Over a year ago

NOTE implementation detail, .join materializes a list underneath the hood anyway.

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