docs/source/dynamic_typing.rst

.. _dynamic_typing:


Dynamically typed code
======================

As mentioned earlier, bodies of functions that don't have 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``:

.. code-block:: python

   from typing import Any

   s = 1                 # Statically typed (type int)
   d = 1  # type: Any    # 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:

.. code-block:: python

    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)
        open(x).read()
        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``:

.. code-block:: python

    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:

.. code-block:: python

    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:

.. code-block:: python

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

You can use ``cast()`` (see chapter :ref:`casts`) or ``isinstance`` to
go from a general type such as ``object`` to a more specific
type (subtype) such as ``int``.  ``cast()`` is not needed with
dynamically typed values (values with type ``Any``).