What is the TypeVar bound parameter?

Question

Accepted Answer

**TypeVar bound** restricts the type to **subclasses of a specific type**.

```python
from typing import TypeVar

# Bounded TypeVar
Number = TypeVar("Number", bound=int)

def double(value: Number) -> Number:
    return value * 2

double(5)        # OK — int is bound
double(3.14)     # OK — float is subclass of Number
# double("hello")  # Error — str is not a Number

# Without bound — any type
T = TypeVar("T")

def identity(value: T) -> T:
    return value

identity(5)       # OK
identity("hello") # OK

# With constraints (only these types)
StrOrInt = TypeVar("StrOrInt", str, int)

def process(value: StrOrInt) -> StrOrInt:
    return value

process(5)       # OK
process("hello") # OK
# process(3.14)   # Error — float not allowed

# Complex bound
from abc import ABC

class Shape(ABC):
    def area(self) -> float: ...

ShapeType = TypeVar("ShapeType", bound=Shape)

def calculate_area(shape: ShapeType) -> float:
    return shape.area()
```

**When to use:**
- Generic functions with type constraints.
- Inheritance-aware generics.
- Method chaining patterns.

Python: What is the TypeVar bound parameter?