"If you want a vegetable and I give you a carrot, you'll be happy. If you want a function which works on all vegetables and I give you a function which only works on carrots, you'll be disappointed."
"The reason we can't assign Callable[[Carrot], None] to Callable[[Vegetable], None] is that Callable is contravariant in its parameters."
"Callable[[B], ...] is a subtype of Callable[[A], ...], meaning we cannot derive functions in a way that makes them less general."
"By the end, you'll no longer fear type checker error messages related to variance."
Contravariance refers to the relationship between types in Python, particularly with type checkers like mypy or Pyright. An example illustrates how assigning a more specific function type to a more general function type can lead to errors. Specifically, assigning Callable[[Carrot], None] to Callable[[Vegetable], None] fails due to contravariance in parameters. This means that if a function is specific to a subtype, it cannot replace a function that operates on a supertype. Understanding contravariance is crucial for resolving type checker errors and avoiding confusion.
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