Why and when you need to use colon (:) in Python

In Python, the colon : symbol is used in many situations:

1. When you need to create an indented block
2. To slice a list or a string
3. Create a key:value pair in a dictionary object

This tutorial will show you when to use a colon in practice.

1. Creating an indented block

The colon symbol is commonly used in Python to create an indented block.

In Python, indents have a special meaning because they indicate that the code written in that line is a part of something.

That something can be a class, a function, a conditional, or a loop.

For example, you used the colon symbol when defining a class as follows:

class Car:
    pass
    # class body here

If you only indent the lines below the class Car line, Python will respond with an syntax error expecting a colon.

The colon here works as the indicator marking the start of the class body.

The same also applies for functions, conditionals, and loops:

# A function requires parentheses and a colon
def my_function():
    pass

# A conditonal requires a colon after the condition to evaluate
if True:
    pass

# A loop requires a colon after the loop
for i in range[5]:
    pass

The code above shows when you need to use a colon to start an indented block.

2. To slice a list or a string

Colons are also used to slice a list or a string in Python.

The slicing syntax is as follows:

string[start:end:step]

The three numbers marking the start, end, and step in a slice is separated using colons.

When you omit the other two parameters, you can use a double colon :: as follows:

my_str = "abcdefg"

print(my_str[::2])  # aceg
print(my_str[3::])  # defg
print(my_str[:5:])  # abcde

Here, the colons are used to slice a string. You can also do the same with a list.

3. Create a key:value pair in a dictionary object

Another use of the colon symbol is to create a key:value pair in a dictionary object.

You use the curly brackets to start a dictionary object, then create the key:value pair using colons like this:

my_dict = {
    'name': 'Nathan',
    'age': 29,
    'hobby': 'programming'
}

When you have more than one key:value pair, you need to separate them using a comma.

Conclusion

In other programming languages, you would see curly brackets used as the indicator of an indented block.

In Python, a colon is used to create an indented block, as well as for slicing a list or a string, and creating a key:value pair in a dictionary.

I hope this tutorial helps. See you in other tutorials!