very-cool-group

Imagine that you want to make all letters in a string upper case. Conveniently, strings have an .upper() method.

You might think that this would work:

>>> greeting = "hello"
>>> greeting.upper()
'HELLO'
>>> greeting
'hello'

greeting didn't change. Why is that so?

That's because strings in Python are immutable. You can't change them, you can only pass around existing strings or create new ones.

>>> greeting = "hello"
>>> greeting = greeting.upper()
>>> greeting
'HELLO'

greeting.upper() creates and returns a new string which is like the old one, but with all the letters turned to upper case.

int, float, complex, tuple, frozenset are other examples of immutable data types in Python.

Mutable data types like list, on the other hand, can be changed in-place:

>>> my_list = [1, 2, 3]
>>> my_list.append(4)
>>> my_list
[1, 2, 3, 4]

Other examples of mutable data types in Python are dict and set. Instances of user-defined classes are also mutable.

For an in-depth guide on mutability see Ned Batchelder's video on names and values.

A parameter is a variable defined in a function signature (the line with def in it), while arguments are objects passed to a function call.

def square(n): # n is the parameter
    return n*n

print(square(5)) # 5 is the argument

Note that 5 is the argument passed to square, but square(5) in its entirety is the argument passed to print