Python type hints

What is Python type hint?
Optional “type hints” (also called “type annotations”), special syntax that allow declaring the type of objects. By declaring types for your variables, editors and tools can give you better support and analyze your code. Type hints normally don’t affect the runtime behavior of your code.

Python has rich typing support, we can check related Typing PEPs.

In this note I collect some of the most important features, recipes related to type hints. Initial information is from FastAPI python types intro.

Basic examples

Initial code snippets, without any types:

def get_full_name(first_name, last_name):
    full_name = first_name.title() + " " + last_name.title()
    return full_name
 
 
print(get_full_name("john", "doe"))  # John Doe

Rewritten with type hints:

def get_full_name(first_name: str, last_name: str) -> str:
    full_name = first_name.title() + " " + last_name.title()
    return full_name
 
 
print(get_full_name("john", "doe"))  # John Doe

Type hints help us to analyze code better, in following example we get type hint warning, ’■ Operator ”+” not supported for types “str” and “int”‘:

def get_name_with_age(name: str, age: int) -> str:
 
    name_with_age = name + " is this old: " + age  # need to use str(age) here
    return name_with_age

Python types and type hints

We can use following basic types:

• int - integer
• float - floating point number
• str - string
• bool - boolean

def get_items(item_a: str, item_b: int, item_c: float, item_d: bool, item_e: bytes):
    return item_a, item_b, item_c, item_d, item_d, item_e
 
print(get_items("a", 1, 1.0, True, b"b"))

There are some data structures that can contain other values, like dict, list, set and tuple. And the internal values can have their own type too (list(int)). These types that have internal types are called "generic" types. And it’s possible to declare them, with optional internal types.

Python standard module typing provide runtime support for these type hints, but newer Python version can make this obsolete.

As Python advances, newer versions come with improved support for these type annotations and in many cases you won’t even need to import and use the typing module to declare the type annotations.

List type

We can declare variable type as list of integers with following syntax:
variable_name: list[int], in the brackets are type parameters. Usage in the function:

def process_items(items: list[str]):
    for item in items:
        print(item)