Dynamically typed code#

As mentioned earlier, bodies of functions that don’t have any explicit types in their function annotation are dynamically typed (operations are checked at runtime). Code outside functions is statically typed by default, and types of variables are inferred. This does usually the right thing, but you can also make any variable dynamically typed by defining it explicitly with the type Any:

from typing import Any

s = 1         # Statically typed (type int)
d: Any = 1    # Dynamically typed (type Any)
s = 'x'       # Type check error
d = 'x'       # OK

Operations on Any values#

You can do anything using a value with type Any, and type checker does not complain:

def f(x: Any) -> int:
    # All of these are valid!
    x.foobar(1, y=2)
    print(x[3] + 'f')
    if x:
        x.z = x(2)
    return x

Values derived from an Any value also often have the type Any implicitly, as mypy can’t infer a more precise result type. For example, if you get the attribute of an Any value or call a Any value the result is Any:

def f(x: Any) -> None:
    y = x.foo()  # y has type Any
    y.bar()      # Okay as well!

Any types may propagate through your program, making type checking less effective, unless you are careful.

Any vs. object#

The type object is another type that can have an instance of arbitrary type as a value. Unlike Any, object is an ordinary static type (it is similar to Object in Java), and only operations valid for all types are accepted for object values. These are all valid:

def f(o: object) -> None:
    if o:
    print(isinstance(o, int))
    o = 2
    o = 'foo'

These are, however, flagged as errors, since not all objects support these operations:

def f(o: object) -> None:
    o.foo()       # Error!
    o + 2         # Error!
    open(o)       # Error!
    n: int = 1
    n = o         # Error!

You can use different type narrowing techniques to narrow object to a more specific type (subtype) such as int. Type narrowing is not needed with dynamically typed values (values with type Any).