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In programming, there are two types of operations: - "In-place" operations, which modify the original object - "Out-of-place" operations, which returns a new object and leaves the original object unchanged

For example, the .sort() method of lists is in-place, so it modifies the list you call .sort() on:

>>> my_list = [5, 2, 3, 1]
>>> my_list.sort()  # Returns None
>>> my_list
[1, 2, 3, 5]

On the other hand, the sorted() function is out-of-place, so it returns a new list and leaves the original list unchanged:

>>> my_list = [5, 2, 3, 1]
>>> sorted_list = sorted(my_list)
>>> sorted_list
[1, 2, 3, 5]
>>> my_list
[5, 2, 3, 1]

In general, methods of mutable objects tend to be in-place (since it can be expensive to create a new object), whereas operations on immutable objects are always out-of-place (since they cannot be modified).

A type hint indicates what type a variable is expected to be.

def add(a: int, b: int) -> int:
    return a + b

The type hints indicate that for our add function the parameters a and b should be integers, and the function should return an integer when called.

It's important to note these are just hints and are not enforced at runtime.

add("hello ", "world")

The above code won't error even though it doesn't follow the function's type hints; the two strings will be concatenated as normal.

Third party tools like mypy can validate your code to ensure it is type hinted correctly. This can help you identify potentially buggy code, for example it would error on the second example as our add function is not intended to concatenate strings.

mypy's documentation contains useful information on type hinting, and for more information check out this documentation page.