A small thing nobody taught you in Python

Have you ever noticed that some stuffs like lower(), upper(), sort() etc. are called like this variable.lower() or variable.sort(). But most of the other function are called like this len(variable), str(variable).

What's the difference?

Actually, the first set of examples are actually called attributes or methods.

Let's take the example of lower(), which acts on the str datatype.

str datatype is actually a class and has a number of functions inside it. These functions are called attribute and can be accessed by using the . notation.

class Hello:
    def world(self):
        print("Hello World")

hello = Hello()        # Instantiate the class Hello
hello.world()          # Access the attribute world()

# Prints: Hello World

If you are familiar with OOP programming, this is all very familiar to you. If not, you can read more here.

So, back to our example:

string = "Hello World"
print(string.lower())   # Here, I have called string's (str class) 
                        # attribute lower() which changes case to 
                        # lowercase

l = [1, 3, 2]
l.sort()                # Here, sort() is an attribute of l 
                        # (list datatype) which sorts the list

# Thus, lower() and sort() are actually functions inside the 
# str and list class respectively

On the other hand, the latter examples were all functions, like your normal def func() thing. They perform a certain task on the argument that is passed to them. The task may or may not be unique to a particular datatype.

string = "Hello World"
l = [1, 2, 3]

print(len(string))      # Prints the no. of characters in string 
                        # (str datatype)
print(len(l))           # Notice, len() can be applied to list 
                        # datatype (and various other types) too