3.1. Type Annotation Primitives

• Also known as: "type annotations", "type hints", "gradual typing"

• Types are not required, and never will be

• Good IDE will give you hints

• Types are used extensively in system libraries

• More and more books and documentations use types

• 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)

• Since Python 3.9: PEP 585 -- Type Hinting Generics In Standard Collections

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

• To type check use: mypy, pyre-check, pytypes

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

Figure 3.3. Timeline of changes to type annotations from Python 3.0 to now 1

3.1.1. Int

• Used to inform static type checker that the variable should be int

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

3.1.2. Float

• Used to inform static type checker that the variable should be float

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

3.1.3. Str

• Used to inform static type checker that the variable should be str

>>> data: str = ''
>>> data: str = 'Mark Watney'

3.1.4. Bool

• Used to inform static type checker that the variable should be bool

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

3.1.5. None

• Used to inform static type checker that the variable should be None

>>> data: None = None

3.1.6. Union

• Used to inform static type checker that the variable should either X or Y

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

• int | str == str | int

>>> data: int | float = 1337
>>> data: int | float = 1.337

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

>>> isinstance(1337, int|float)
True

3.1.7. Optional

• Used to inform static type checker that the variable should be X or None

• int | None == None | int

>>> number: int | None = 1337
>>> number: int | None = None

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

>>> isinstance(1337, int|None)
True

3.1.8. Aliases

• Used to make types more readable

>>> number = int | float
>>>
>>> age: number = 10
>>> age: number = 10.5

3.1.9. Final

• Used to inform static type checker the value should not change

• Used to define constants

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

>>> from typing import Final

Could be used to define constants:

>>> SECOND: Final[int] = 1
>>> MINUTE: Final[int] = 60 * SECOND
>>> HOUR: Final[int] = 60 * MINUTE
>>> DAY: Final[int] = 24 * HOUR

Static type checker will ensure that the value should not change during the program.

3.1.10. Literal

• Since Python 3.8: PEP 586 -- Literal Types

• Literal de-duplicates parameters

• Equality comparisons of Literal objects are not order dependent

• https://docs.python.org/3/library/typing.html#typing.Literal

>>> from typing import Literal

Problem:

>>> agency: str
>>>
>>> agency = 'NASA'         # ok
>>> agency = 'ESA'          # ok
>>> agency = 'Not existing' # ok

Solution:

>>> agency: Literal['NASA', 'ESA', 'POLSA']
>>>
>>> agency = 'NASA'          # ok
>>> agency = 'ESA'           # ok
>>> agency = 'Not existing'  # error

3.1.11. Errors

• Types are not Enforced

• This code will run without any problems

• Types are not required, and never will be

• Although mypy, pyre-check or pytypes will throw error

>>> name: int = 'Mark Watney'

3.1.12. Use Case - 0x01

firstname: str = 'Mark' lastname: str = 'Watney' age: int = 40 agency: Literal['NASA', 'ESA', 'POLSA'] = 'NASA' height: int | float = 185.5 weight: int | float | None = None is_person: bool = True is_astronaut: bool | None = True

3.1.13. Further Reading

• More information in cicd-tools

• https://www.infoq.com/presentations/dynamic-static-typing/

• https://github.com/pandas-dev/pandas/blob/8fd2d0c1eea04d56ec0a63fae084a66dd482003e/pandas/core/frame.py#L505

3.1.14. References

1

Briggs, J. Type Annotations in Python. Year: 2021. Retrieved: 2022-04-08. URL: https://towardsdatascience.com/type-annotations-in-python-d90990b172dc