3.11. Typing Alias
Since Python 3.12 you can use
typesoft-keyword to define type aliasesSince Python 3.12 PEP 695 - Type Parameter Syntax
3.11.1. Union
>>> def add(a: int|float, b: int|float) -> int|float:
... return a + b
3.11.2. Type Keyword
Since Python 3.12 PEP 695 - Type Parameter Syntax
definition will stay until the end of the scope (module, class, function)
You can use type soft-keyword to define type aliases
>>> type number = int|float
>>>
>>> def add(a: number, b: number) -> number:
... return a + b
3.11.3. Type Parameter
Since Python 3.12 PEP 695 - Type Parameter Syntax
It is only available during function definition
But, also there is a new syntax for defining type aliases available during function definition
>>> def add[number: (int,float)](a: number, b: number) -> number:
... return a + b
3.11.4. Type Generic
class Array[T]- type genericclass Array[T: (int,float)]- type generic with constraints
>>> class List[T]:
... data: list[T]
...
... def __init__(self):
... self.data: list[T] = []
...
... def append(self, x: T):
... self.data.append(x)
...
... def pop(self) -> T:
... return self.data.pop()
3.11.5. Type Generic with Constraints
class Array[T: (int,float)]- type generic with constraints
>>> class Array[T: (int,float)]:
... data: list[T]
...
... def __init__(self):
... self.data: list[T] = []
...
... def append(self, x: T):
... self.data.append(x)
...
... def pop(self) -> T:
... return self.data.pop()
3.11.6. Use Case - 1
>>> type GeographicCoordinate = tuple[float, float]
>>>
>>> locations: list[GeographicCoordinate] = [
... (25.91375, -60.15503),
... (-11.01983, -166.48477),
... (-11.01983, -166.48477),
... ]
3.11.7. Use Case - 2
Without aliases:
>>> DATA: list[tuple[str, str, str, str, str] | tuple[float, float, float, float, str]] = [
... ('sepal_length', 'sepal_width', 'petal_length', 'petal_width', 'species'),
... (5.8, 2.7, 5.1, 1.9, 'virginica'),
... (5.1, 3.5, 1.4, 0.2, 'setosa'),
... (5.7, 2.8, 4.1, 1.3, 'versicolor'),
... (6.3, 2.9, 5.6, 1.8, 'virginica'),
... (6.4, 3.2, 4.5, 1.5, 'versicolor'),
... (4.7, 3.2, 1.3, 0.2, 'setosa'),
... ]
With aliases:
>>> type row = tuple[float, float, float, float, str]
>>>
>>> data: list[row] = [
... (4.7, 3.2, 1.3, 0.2, 'setosa'),
... (7.0, 3.2, 4.7, 1.4, 'versicolor'),
... (7.6, 3.0, 6.6, 2.1, 'virginica'),
... ]
With aliases and union:
>>> type header = tuple[str, str, str, str, str]
>>> type row = tuple[float, float, float, float, str]
>>>
>>> DATA: list[header|row] = [
... ('sepal_length', 'sepal_width', 'petal_length', 'petal_width', 'species'),
... (5.8, 2.7, 5.1, 1.9, 'virginica'),
... (5.1, 3.5, 1.4, 0.2, 'setosa'),
... (5.7, 2.8, 4.1, 1.3, 'versicolor'),
... (6.3, 2.9, 5.6, 1.8, 'virginica'),
... (6.4, 3.2, 4.5, 1.5, 'versicolor'),
... (4.7, 3.2, 1.3, 0.2, 'setosa'),
... ]
3.11.8. Assignments
# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author
# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -v myfile.py`
# %% About
# - Name: Typing Alias TypeKeyword
# - Difficulty: easy
# - Lines: 2
# - Minutes: 2
# %% English
# 1. Define type `T` to annotate variables
# 2. Use `type` keyword and union syntax
# 3. Run doctests - all must succeed
# %% Polish
# 1. Zdefiniuj typ `T` do anotacji zmiennych
# 2. Użyj słowa kluczowego `type` oraz notacji unii
# 3. Uruchom doctesty - wszystkie muszą się powieść
# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 12), \
'Python 3.12+ required'
>>> from typing import get_type_hints
>>> get_type_hints(add)
{'a': T, 'b': T, 'return': T}
"""
# Define type `T` to annotate variables
# Use `type` keyword and union syntax
def add(a: int | float, b: int | float) -> int | float:
return a + b
# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author
# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -v myfile.py`
# %% About
# - Name: Typing Alias TypeParameters
# - Difficulty: easy
# - Lines: 1
# - Minutes: 2
# %% English
# 1. Modify definition of a function `add`
# 2. Refactor type definition using Python 3.12 syntax,
# example: `def run[T: int](a: T, b: T) -> T: ...`
# 3. Run doctests - all must succeed
# %% Polish
# 1. Zmodyfikuj definicję funkcji `add`
# 2. Zrefaktoruj definiowanie typu używając składni Python 3.12,
# przykład: `def run[T: int](a: T, b: T) -> T: ...`
# 3. Uruchom doctesty - wszystkie muszą się powieść
# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 12), \
'Python 3.12+ required'
>>> from typing import get_type_hints
>>> get_type_hints(add)
{'a': T, 'b': T, 'return': T}
"""
# Modify definition of a function `add`
# Refactor type definition using Python 3.12 syntax,
# example: `def run[T: int](a: T, b: T) -> T: ...`
def add(a: int | float, b: int | float) -> int | float:
return a + b