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
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 + 'f') if x: x.z = x(2) open(x).read() return x
Values derived from an
Any value also often have the type
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
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#
object is another type that can have an instance of arbitrary
type as a value. Unlike
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(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!