Python syntax -> bool

Question

I have a lot of Matlab experience but trying to learn Python syntax from Leetcode challenges. What does the -> bool mean in the method below?

def isPalindrome(self, head: ListNode) -> bool:
    vals = []
    current_node = head
    while current_node is not None:
        vals.append(current_node.val)
        current_node = current_node.next
    return vals == vals[::-1]

Here's the entire code:

class ListNode(object):
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
class Solution(object):
    def isPalindrome(self, head: ListNode) -> bool:
        vals = []
        current_node = head
        while current_node is not None:
            vals.append(current_node.val)
            current_node = current_node.next
        return vals == vals[::-1]
isPalindrome(head)

It's a [type-hint](https://docs.python.org/3/library/typing.html). — Hampus Larsson, Nov 13 '21 at 19:34
It indicates (but does not enforce) that the function returns a `bool` value — rv.kvetch, Nov 13 '21 at 19:34
Thanks, so it's just a comment? And do you have any ideas why this is giving me a syntax error? Pasted the entire code in my original post — IAmTheWalrus, Nov 13 '21 at 19:35
It's a [function annotation](https://docs.python.org/3/glossary.html#term-function-annotation), intended to be interpreted as a type hint by 3rd-party tools like `mypy`. — chepner, Nov 13 '21 at 19:35
What is the exact error? Function annotations aren't part of Python 2, and your code has additional indentation problems. — chepner, Nov 13 '21 at 19:36
The last line `isPalindrome(head)` seems like it would result in a NameError for at least one reason. — rv.kvetch, Nov 13 '21 at 19:40

krmogi · Answer 1 · 2021-11-13T19:44:39.067

The -> bool just tells that isPalindrome() returns a boolean, but it doesn't force the function to return a boolean.

Look at PEP 3107: https://www.python.org/dev/peps/pep-3107/

And PEP 484: https://www.python.org/dev/peps/pep-0484/ It states:

Function annotations are nothing more than a way of associating arbitrary Python expressions with various parts of a function at compile-time.

This is called function annotation.

You could also use mypy to do type checking on your code.

Mypy is an optional static type checker for Python that aims to combine the benefits of dynamic (or “duck”) typing and static typing.

score 1 · Answer 2 · answered Nov 13 '21 at 19:38

1

It is called a type hint. Basically, it gives you hints as to what type the function will return.

In your code, -> bool hints that isPalindrome() returns a boolean value.

answered Nov 13 '21 at 19:38

Akash

17
6

You could also mention `Mypy` is a static type checker for Python for this post. – Daniel Hao Nov 13 '21 at 19:41

Python syntax -> bool

2 Answers2