2.1. Type Annotation¶

Types are not required, and never will be. -- Guido van Rossum, Python initiator, core developer, former BDFL

It should be emphasized that Python will remain a dynamically typed language, and the authors have no desire to ever make type hints mandatory, even by convention. -- Python Software Foundation

2.1.1. Rationale¶

Introduced in Python 3.5
Since Python 3.5: PEP 484 -- Type Hints
Since Python 3.6: PEP 526 -- Syntax for Variable Annotations
Since Python 3.8: PEP 544 -- Protocols: Structural subtyping (static duck typing)
Also known as: "type annotations", "type hints", "gradual typing"
Good IDE will give you hints
Types are used extensively in system libraries
More and more books and documentations use types
To type check use: mypy, pyre-check, pytypes
More information in cicd-tools
https://www.infoq.com/presentations/dynamic-static-typing/
Example: https://github.com/pandas-dev/pandas/blob/8fd2d0c1eea04d56ec0a63fae084a66dd482003e/pandas/core/frame.py#L505

2.1.2. Int¶

>>> data: int = 0
>>> data: int = 1
>>> data: int = -1

2.1.3. Float¶

>>> data: float = 0.0
>>> data: float = 1.23
>>> data: float = -1.23

2.1.4. Str¶

>>> data: str = ''
>>> data: str = 'Jan Twardowski'

2.1.5. Bool¶

>>> data: bool = True
>>> data: bool = False

2.1.6. Union¶

>>> from typing import Union
>>>
>>> number: Union[int, float] = 1337
>>> number: Union[int, float] = 1.337

Since Python 3.10: PEP 604 -- Allow writing union types as X | Y

>>> number: int|float = 1337        # doctest: +SKIP
>>> number: int|float = 1.337       # doctest: +SKIP
>>> number: int|None = 1337         # doctest: +SKIP
>>> number: int|None = None         # doctest: +SKIP

Result of this expression would then be valid in isinstance() and issubclass()

>>> isinstance(1337, int|float)     # doctest: +SKIP
>>> isinstance(1337, int|None)      # doctest: +SKIP

2.1.7. Optional¶

>>> from typing import Optional
>>>
>>> data: Optional[int] = 1337
>>> data: Optional[int] = None

>>> from typing import Optional
>>>
>>> firstname: str = 'Melissa'
>>> lastname: str = 'Lewis'
>>> age: Optional[float] = None

Since Python 3.10: PEP 645 -- Allow writing optional types as x?

>>> age: int? = 1337                # doctest: +SKIP
>>> age: int? = None                # doctest: +SKIP

Result of this expression would then be valid in isinstance() and issubclass()

>>> isinstance(1337, int?)          # doctest: +SKIP

2.1.8. Aliases¶

>>> from typing import Union
>>>
>>> Number = Union[float, int]
>>> age: Number = 10
>>> age: Number = 10.5

Since Python 3.10 PEP 613 -- TypeAlias Annotation

PEP 484 introduced the concept of type aliases, only requiring them to be top-level unannotated assignments. This simplicity sometimes made it difficult for type checkers to distinguish between type aliases and ordinary assignments, especially when forward references or invalid types were involved. Compare:

>>> StrCache = 'Cache[str]'  # a type alias                 # doctest: +SKIP
>>> LOG_PREFIX = 'LOG[DEBUG]'  # a module constant          # doctest: +SKIP

Now the typing module has a special annotation TypeAlias to declare type aliases more explicitly:

>>> StrCache: TypeAlias = 'Cache[str]'  # a type alias      # doctest: +SKIP
>>> LOG_PREFIX = 'LOG[DEBUG]'  # a module constant          # doctest: +SKIP

2.1.9. Final¶

Since Python 3.8: PEP 591 -- Adding a final qualifier to typing

>>> from typing import Final
>>>
>>> m: Final[int] = 1
>>> km: Final[int] = 1000 * m

>>> from typing import Final
>>>
>>> second: Final[int] = 1
>>> minute: Final[int] = 60 * second
>>> hour: Final[int] = 60 * minute
>>> day: Final[int] = 24 * hour

2.1.10. Types are not Enforced¶

This code will run without any problems
Although mypy or pyre-check will throw error

>>> name: int = 'Jan Twardowski'
>>> age: float = 30
>>> is_adult: int = True