3.2. Type Annotation Primitives
3.2.1. Alias
Used to make types more readable
Declaration:
>>> data = int | float
Example:
>>> number = int | float
>>>
>>> age: number = 10 # ok
>>> age: number = 10.5 # ok
>>> age: number = None # error
3.2.2. 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
Declaration:
>>> data: int | float
>>> data: int | float = 1337
>>> data: int | float = 1.337
Example:
>>> data: int | float
>>>
>>> data = 1337 # ok
>>> data = 1.337 # ok
>>> data = 'hello' # error
Result of this expression would then be valid in isinstance()
and issubclass():
>>> isinstance(1337, int|float)
True
3.2.3. Optional
Used to inform static type checker that the variable should be X or None
int | None == None | int
Declaration:
>>> data: int | None
>>> data: int | None = 1337
>>> data: int | None = None
Example:
>>> number: int | None
>>>
>>> number = 1337 # ok
>>> number = None # ok
>>> number = 1.0 # error
Result of this expression would then be valid in isinstance()
and issubclass():
>>> isinstance(1337, int|None)
True
3.2.4. 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
In Python there is not such thing as constants. All values can be changed
during the runtime. However using Final we can achieve similar effect.
Static type checker will ensure that the value should not change during
the program.
SetUp:
>>> from typing import Final
Declaration:
>>> data: Final
>>> data: Final[int]
>>> data: Final[float]
>>> data: Final[bool]
>>> data: Final[str]
Definition:
>>> pressure: Final[float] = 1013.25 # ok
>>> pressure = 1024.00 # error
3.2.5. 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
SetUp:
>>> from typing import Literal
Declaration:
>>> data: Literal['one', 'two', 'three']
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
3.2.6. Any
SetUp:
>>> from typing import Any
Declaration:
>>> x: Any = 1
>>> x: Any = 'one'
>>> x: Any = None
>>> x: Any = [1,2,3]
3.2.7. Use Case - 0x01
>>> firstname: str = 'Mark'
>>> lastname: str = 'Watney'
>>> age: int | float = 40
>>> adult: bool = True
>>> agency: Literal['NASA', 'ESA', 'POLSA'] = 'NASA'
>>> job: str | None = None
>>> height: int | float | None = 185
>>> weight: int | float | None = None
3.2.8. Use Case - 0x02
>>> SECOND: Final[int] = 1
>>> MINUTE: Final[int] = 60 * SECOND
>>> HOUR: Final[int] = 60 * MINUTE
>>> DAY: Final[int] = 24 * HOUR