Summary - Type hints

Combine Optional, Callable and TypeVar to write a type-safe function.

Summary - Type hints

This group covered the core tools of type hints. Basic types (int, str) document signatures; Optional adds the None possibility, Union combines several types.

List and Dict declare container types; Callable declares function parameters, TypeVar declares generic functions that preserve types.

The final task combines Optional and List: you'll write a function that finds an item in a list and returns Optional[str].

Exercise pick function with List and Optional

Write pick(items: List[str], i: int) -> Optional[str]. If 0 <= i < len(items) return items[i]; otherwise return None. Import List and Optional from typing. Call print(pick(["a", "b"], 1)) and print(pick(["a", "b"], 5)).

Show example solution

from typing import List, Optional

def pick(items: List[str], i: int) -> Optional[str]:
    if 0 <= i < len(items):
        return items[i]
    return None

print(pick(["a", "b"], 1))
print(pick(["a", "b"], 5))

Test yourself

A function may return either a string or None; which type do you use?
- Union[int, str]
- List[str]
- Callable[[int], int]
- Optional[str]
Show answer

Optional[str] — Optional[str] accepts str or None; it fits this scenario exactly.

List and Optional combined

from typing import List, Optional

def pick(items: List[str], i: int) -> ____[str]:
    if 0 <= i < len(items):
        return items[i]
    return None

Show answer

from typing import List, Optional

def pick(items: List[str], i: int) -> Optional[str]:
    if 0 <= i < len(items):
        return items[i]
    return None

Match the group concepts

Optional[str]

str or None
List[int]

List of integers
Callable[[int], int]

Takes int, returns int
TypeVar("T")

Generic type variable

Next lesson: pytest basics

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