Today I learned about the type hint Any and its bidirectional compatibility with all other types.

Type hint Any

The special type Any provided in the module typing is a type that matches all Python objects. The documentation says that Any is compatible with all other types and all other types are compatible with Any.

I have a limited understanding of type hints in Python, but I don't think this sentence is redundant:

“A static type checker will treat every type as being compatible with Any and Any as being compatible with every type.”

This compatibility seems to be related to subclasses and the methods and attributes that objects provide.

For example, suppose we have this class hierarchy:

class Animal:
    pass

class Human(Animal):
    pass

All instances of Human are also instances of Animal (because Human is a subclass of Animal) but there may be instances of Animal that are not instances of Human. For example, dolphins are definitely animals and also definitely not humans.

So, the type Human is compatible with the type Animal. Why? Because all instances of Human can also be seen as instances of Animal. In fact, the function below would pass static type checking:

def foo(human: Human) -> Animal:
    return human

However, the reverse does not pass static type checking:

def bar(animal: Animal) -> Human:
    return animal

The static type checker complains with “Animal is incompatible with Human”. So, if Any is compatible with all types and if all types are compatible with Any, this means the functions f and g below should pass static type checking:

from typing import Any

def f(x: int) -> Any:
    return x

def g(x: Any) -> int:
    return x

And they do. If you run a static type checker against the functions f and g, they both pass static type checking.

What bothers me is that the function g seems to be typed improperly. After all, I can write this code:

from typing import Any

def g(x: Any) -> int:
    return x

print(g("Hello, world!"))

Running this code will print a string (and the function g has a type hint saying it returns an integer) but the code passes static type checking! After all, we have that:

• the function g is correctly typed because the return value x is of type Any, and Any is compatible with int; and
• the function call g("Hello, world!") is also ok because the argument is of type str and the function g expects an argument of type Any. The type str is compatible with Any, so this function call is ok.

What is Any used for?

So, why is Any useful if it gives you total freedom and doesn't seem to do anything for you? Any is useful for gradually adding types to an untyped codebase. So, if you have some code that you'd like to typecheck, you can start by inserting Any everywhere, and the code will typecheck. Then, you can gradually start replacing Any with more specific types. (Thanks to the reader who pointed this out in the comments section below!)

If you are a practical type of person, instead of a purist, there is another situation where Any might come in handy. If you have a piece of code that is very dynamic you might say that a given variable is typed as Any, either because the actual type is completely arbitrary during runtime or because the actual type is unwieldy to write.

That's it for now! Stay tuned and I'll see you around!

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References

• Python 3 Documentation, typing, “The Any type”, https://docs.python.org/3/library/typing.html#the-any-type [last accessed 4-11-2022];