What's the difference between a constrained TypeVar and a Union in python?

What's the difference between a constrained TypeVar and a Union in python?

Both TypeVar with constraints and Union are used to annotate type hints in Python, but they serve slightly different purposes.

Constrained TypeVar: A constrained TypeVar is used when you want to specify that a variable or function parameter should accept any type that is a subtype of a particular class or protocol. It's particularly useful when you want to ensure that the input type adheres to a specific interface or contract.
For example:
```
from typing import TypeVar, List T = TypeVar('T', int, float) def process_numeric(value: T) -> T: return value * 2 result_int = process_numeric(5) # Accepts int, returns int result_float = process_numeric(3.14) # Accepts float, returns float 
```
In this example, T is constrained to accept only int and float types.
Union: Union is used to specify that a variable or function parameter can accept multiple types. It allows you to indicate that the input can be of any type from the provided options.
For example:
```
from typing import Union def process_value(value: Union[int, float]) -> Union[int, float]: return value * 2 result_int = process_value(5) # Accepts int, returns int result_float = process_value(3.14) # Accepts float, returns float 
```
In this example, Union[int, float] indicates that the parameter can accept either an int or a float.

Key differences:

Constrained TypeVar is more specific and enforces a subtype relationship with a particular class or protocol.
Union allows you to specify multiple acceptable types without enforcing a strict subtype relationship.

When to use which:

Use constrained TypeVar when you want to enforce a more specific type relationship.
Use Union when you want to allow multiple types without enforcing a strict relationship.

In some cases, you might even combine these two approaches. For instance, you could use a constrained TypeVar within a Union to indicate that you expect a type to be both a subtype of a specific class and one of the types listed in the Union.

Examples

What's a Constrained TypeVar in Python?

Description: This query explores what a constrained TypeVar is in Python's typing system.

Code:

from typing import TypeVar T = TypeVar('T', int, float) # Constrained TypeVar: can be int or float def add(a: T, b: T) -> T: return a + b # Works with both int and float

What's a Union in Python Typing?

Description: This query discusses what a Union is in Python's typing system.

Code:

from typing import Union def add(a: Union[int, float], b: Union[int, float]) -> Union[int, float]: return a + b # Accepts both int and float

What's the Difference Between a Constrained TypeVar and a Union in Python?

Description: This query explains the key differences between using a constrained TypeVar and a Union.

Code:

from typing import TypeVar, Union T = TypeVar('T', int, float) # Constrained TypeVar # TypeVar implies the same type must be used for both arguments def constrained_add(a: T, b: T) -> T: return a + b # Both must be the same type (int or float) # Union allows mixing types def union_add(a: Union[int, float], b: Union[int, float]) -> Union[int, float]: return a + b # Can mix int and float

When to Use a Constrained TypeVar in Python?

Description: This query discusses when to use a constrained TypeVar in Python.

Code:

from typing import TypeVar T = TypeVar('T', int, float) # Use constrained TypeVar when you want to enforce consistent type for arguments def multiply(a: T, b: T) -> T: return a * b # Both must be int or float

When to Use a Union in Python?

Description: This query explains when to use a Union in Python.

Code:

from typing import Union # Use Union when you want to allow multiple types def concatenate(value: Union[str, bytes]) -> Union[str, bytes]: if isinstance(value, str): return value + " added text" elif isinstance(value, bytes): return value + b" added bytes"

How to Constrain a TypeVar to Custom Classes in Python?

Description: This query discusses constraining a TypeVar to specific custom classes.

Code:

from typing import TypeVar class Animal: pass class Dog(Animal): pass class Cat(Animal): pass T = TypeVar('T', Dog, Cat) # Constrained TypeVar to Dog or Cat def get_name(animal: T) -> str: if isinstance(animal, Dog): return "Dog" elif isinstance(animal, Cat): return "Cat"

How to Use Union with Custom Classes in Python?

Description: This query explores using Union with custom classes in Python.

Code:

from typing import Union class Animal: pass class Dog(Animal): pass class Cat(Animal): pass # Use Union to allow either Dog or Cat def describe(animal: Union[Dog, Cat]) -> str: if isinstance(animal, Dog): return "This is a Dog" elif isinstance(animal, Cat): return "This is a Cat"

How to Use a Constrained TypeVar with Functions in Python?
- Description: This query discusses using a constrained TypeVar with functions in Python.
- Code:
```
from typing import TypeVar T = TypeVar('T', int, float) def divide(a: T, b: T) -> T: return a / b # Constrained to int or float 
```

How to Use Union with Functions in Python?

Description: This query explores using Union with functions in Python.

Code:

from typing import Union def sum_values(values: Union[list[int], list[float]]) -> Union[int, float]: return sum(values) # Allows list of int or list of float

How to Use a Constrained TypeVar with Classes in Python?

Description: This query discusses using a constrained TypeVar with classes in Python.

Code:

from typing import TypeVar, Generic T = TypeVar('T', int, float) class NumberHolder(Generic[T]): def __init__(self, number: T): self.number = number def get_number(self) -> T: return self.number int_holder = NumberHolder(10) # Works with int float_holder = NumberHolder(10.5) # Works with float

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