3.9. Sequence Type Annotation¶

3.9.1. Rationale¶

Python 3.9 introduced PEP 585 -- Type Hinting Generics In Standard Collections
Before Python 3.9 you need from typing import List, Set, Tuple, Dict

DATA = [
    (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'),
]

for row in DATA:
    species = row[-1]
    species  # IDE don't know what type is species
             # and cannot give hints for autocompletion

for row in DATA:
    species: str = row[-1]
    species  # IDE knows exactly what type is species
             # and what methods hint for autocompletion

3.9.2. Tuple¶

Listing 3.43. Generic type annotation¶

data: tuple = ()
data: tuple = tuple()
data: tuple = 'a', 2, 3.3
data: tuple = ('a', 2, 3.3)

data: tuple[int, int, int] = (1, 2, 3)
data: tuple[str, str, str] = ('setosa', 'virginica', 'versicolor')
data: tuple[str, int, float] = ('a', 2, 3.3)

data: tuple[int, ...] = (1, 2, 3)
data: tuple[str, ...] = ('setosa', 'virginica', 'versicolor')

Listing 3.44. Before Python 3.9¶

from typing import Tuple

data: tuple = ()
data: tuple = tuple()
data: tuple = 'a', 2, 3.3
data: tuple = ('a', 2, 3.3)

data: Tuple[int, int, int] = (1, 2, 3)
data: Tuple[str, str, str] = ('setosa', 'virginica', 'versicolor')
data: Tuple[str, int, float] = ('a', 2, 3.3)

data: Tuple[int, ...] = (1, 2, 3)
data: Tuple[str, ...] = ('setosa', 'virginica', 'versicolor')

3.9.3. List¶

data: list = list()
data: list = []
data: list = ['a', 1, 2.2]

data: list[int] = [1, 2, 3, 4]
data: list[float] = [5.8, 2.7, 5.1, 1.9]
data: list[str] = ['a', 'b', 'c', 'd']

Listing 3.45. Before Python 3.9¶

from typing import List

data: list = list()
data: list = []
data: list = ['a', 1, 2.2]

data: List[int] = [1, 2, 3, 4]
data: List[float] = [5.8, 2.7, 5.1, 1.9]
data: List[str] = ['a', 'b', 'c', 'd']

3.9.4. Set¶

data: set = set()
data: set = {'a', 1, 2.2}

data: set[int] = {1, 2, 3}
data: set[float] = {0.0, 1.1, 2.2}
data: set[str] = {'a', 'b', 'c'}

Listing 3.46. Before Python 3.9¶

from typing import Set

data: set = set()
data: set = {'a', 1, 2.2}

data: Set[int] = {1, 2, 3}
data: Set[float] = {0.0, 1.1, 2.2}
data: Set[str] = {'a', 'b', 'c'}

3.9.5. Frozenset¶

data: frozenset = set()
data: frozenset = {'a', 1, 2.2}

data: frozenset[int] = {1, 2, 3}
data: frozenset[float] = {0.0, 1.1, 2.2}
data: frozenset[str] = {'a', 'b', 'c'}

Listing 3.47. Before Python 3.9¶

from typing import FrozenSet

data: frozenset = set()
data: frozenset = {'a', 1, 2.2}

data: FrozenSet[int] = {1, 2, 3}
data: FrozenSet[float] = {0.0, 1.1, 2.2}
data: FrozenSet[str] = {'a', 'b', 'c'}

3.9.6. List of Tuples¶

data: list[tuple] = [
    (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'),
]

data: list[tuple[float, float, float, float, str]] = [
    (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'),
]

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'),
]

Listing 3.48. Before Python 3.9¶

from typing import List, Tuple

data: List[tuple] = [
    (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'),
]

data: List[Tuple[float, float, float, float, str]] = [
    (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'),
]

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'),
]

3.9.7. List of Lists¶

data: list[list] = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9],
]

data: list[list[int]] = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9],
]

Listing 3.49. Before Python 3.9¶

from typing import List

data: List[list] = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9],
]

data: List[List[int]] = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9],
]

3.9.8. Aliases¶

Point = tuple[int, int]
locations: list[Point] = [
    (0, 1),
    (5, -3),
    (-10, 20)
]

GeographicCoordinate = tuple[float, float]
locations: list[GeographicCoordinate] = [
    (25.91375, -60.15503),
    (-11.01983, -166.48477),
    (-11.01983, -166.48477)
]

Iris = tuple[float, float, float, float, str]
data: list[Iris] = [
    (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'),
]

Listing 3.50. Before Python 3.9¶

from typing import List, Tuple


GeographicCoordinate = Tuple[float, float]
locations: List[GeographicCoordinate] = [
    (25.91375, -60.15503),
    (-11.01983, -166.48477),
    (-11.01983, -166.48477)
]

Iris = Tuple[float, float, float, float, str]
data: List[Iris] = [
    (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'),
]

3.9.9. Unions¶

from typing import Union


data: list[Union[list, tuple, set]] = [
    [1, 2, 3],
    (4, 5, 6),
    {7, 8, 9},
]

data: list[Union[list[int], tuple[int, ...], set[int]]] = [
    [1, 2, 3],
    (4, 5, 6),
    {7, 8, 9},
]

row = Union[list[int], tuple[int, ...], set[int]]
data: list[row] = [
    [1, 2, 3],
    (4, 5, 6),
    {7, 8, 9},
]

Listing 3.51. Before Python 3.9¶

from typing import Union, List, Tuple, Set


data: List[Union[list, tuple, set]] = [
    [1, 2, 3],
    (4, 5, 6),
    {7, 8, 9},
]


data: List[Union[List[int], Tuple[int, int, int], Set[int]]] = [
    [1, 2, 3],
    (4, 5, 6),
    {7, 8, 9},
]


Row = Union[List[int],
            Tuple[int, int, int],
            Set[int]]

data: List[Row] = [
    [1, 2, 3],
    (4, 5, 6),
    {7, 8, 9},
]

3.9.10. More Information¶

Note

More information in Type Annotations and Type Checking