TypeVar

Learn generic functions with TypeVar.

TypeVar

Sometimes a function must return the same type it received. TypeVar declares such a type variable.

from typing import TypeVar, List

T = TypeVar("T")

def first(items: List[T]) -> T:
    return items[0]

Here T stands for any type. Calling first([1, 2, 3]) returns int; calling first(["a", "b"]) returns str. The type is preserved with the argument.

You can bound a TypeVar: TypeVar("T", bound=int) accepts only int subtypes.

Safe first item with TypeVar

Without TypeVar, first looks like it returns Any; the type is lost. With TypeVar, first([1, 2, 3]) clearly returns int.

This lets the IDE offer correct completion. When you write first(["a", "b"]).upper(), the IDE suggests .upper() because it knows the return is str.

Multiple TypeVars are allowed: K = TypeVar("K"), V = TypeVar("V"). This is useful for generic dict functions.

Exercise first function with TypeVar

Define T = TypeVar("T"). Write first(items: List[T]) -> T; the body contains return items[0]. Import TypeVar and List from typing. Call print(first([1, 2, 3])).

Show example solution

from typing import TypeVar, List

T = TypeVar("T")

def first(items: List[T]) -> T:
    return items[0]

print(first([1, 2, 3]))

Test yourself

Define a TypeVar

from typing import TypeVar

T = ____("T")

def first(items):
    return items[0]

Show answer

from typing import TypeVar

T = TypeVar("T")

def first(items):
    return items[0]

Match the TypeVar concepts

T = TypeVar("T")

Type variable declaration
List[T]

List of T
-> T

Returns T
bound=int

Bound: int subtypes
What is the main benefit of TypeVar?
- Faster runtime
- Lower memory use
- Type preservation and correct IDE suggestions
- Automatic error fixing
Show answer

Type preservation and correct IDE suggestions — TypeVar preserves type info; mypy and the IDE become more accurate.

Next lesson: Summary - Type hints

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