.. getthecode:: annotations.py
    :language: python3
    :hidden:

============================
 Annotations and Type Hints
============================

This page contains a memo on annotations.

* https://docs.python.org/3/library/typing.html#module-typing
* `PEP 484 – Type Hints <https://www.python.org/dev/peps/pep-0484>`_
* `PEP 483 – The Theory of Type Hints <https://www.python.org/dev/peps/pep-0483>`_

Static type checker for Python:

* `Mypy <http://mypy-lang.org>`_  a static type checker for Python
* `PyAnnotate <https://github.com/dropbox/pyannotate>`_

Let a function with an a required parameter and a default one:


.. code-block:: py3

    def greeting(name, prefix='Mr.'):
       return 'Hello, {} {}'.format(name, prefix)
    
Same function with annotations:


.. code-block:: py3

    def greeting(name: str, prefix: str = 'Mr.') -> str:
       return 'Hello, {} {}'.format(name, prefix)
    
    print(greeting.__annotations__)

.. code-block:: none

    {'name': <class 'str'>, 'prefix': <class 'str'>, 'return': <class 'str'>}
    
A "void" function:


.. code-block:: py3

    def p() -> None:
        print('hello')
    
Advanced types:


.. code-block:: py3

    from typing import Iterator
    
    def fib(n: int) -> Iterator[int]:
        a, b = 0, 1
        while a < n:
            yield a
            a, b = b, a+b
    
    print(fib.__annotations__)

.. code-block:: none

    {'n': <class 'int'>, 'return': typing.Iterator[int]}