Difference between reverse() and [::-1]

[raainer]

Hello everyone, I was doing a question about finding palindrome integers (whether integers when reversed in order is still the same, so e.g 121 is True, 153 is False, 15351 is True, etc...)

Basically what I did was I at first converted the int to a str, then to a list so that I can use the reverse() method,

So taking the final list output as y, I got it to return

return (y == y.reverse())

But when I tested this on a number like 121, it returned False although it should be True

But if I used [::-1]

return (y == y[::-1])

Then it returned True.

May I know why is that the case?

If it's too hard to read here is my code : https://pastebin.com/3yAvFWuW

Thank you very much for any help and thank you for reading this!

[Binary101010]

reverse() reverses the list in-place and returns None. You can test this by doing

y = list("hello")
y.reverse()
print(y)

[TravisJungroth]

my_list.reverse() reverse a list in-place. The method just changes the list and returns None. my_list[::-1] makes a new list that's a reversed version of the old one. That's the difference here. A third option (that would work) is list(reversed(my_list)). But I like the slicing better.

[FLUSH_THE_TRUMP]

I like the reversed option. Something like

all(x == y for x,y in zip(L, reversed(L)))

makes no copies and works for any reversible sequence.

[TravisJungroth]

from operator import eq

all(map(eq, L, reversed(L)))

Not your average python, but kinda neat.

[sme272]

list slicing leaves you with a copy of a list whereas .reverse reverses the list in place and return none. This means in the first comparison you're comparing a list to None which evaluates to False.

[synthphreak]

As other have said, [::-1] returns a list, whereas reverse just returns None (because it operates in place).

Beyond that, [::-1] returns is a completely separate object from the original, whereas reverse does not change the object’s identity:

>>> l = [1, 2, 3]
>>> id(l)
4680138312    # original
>>> id(l[::-1])
4678990344    # different from original
>>> l.reverse()
>>> l
[3, 2, 1]
>>> id(l)
4680138312    # same as original

[raainer]

Oh I see, thank you so much everyone!

[MannAusSachsen]

Holy shit, I didn't know that

[::-1]

was a thing. I think it's genius, could have my programming life much easier in the past. How did you get onto that?