Gradual Typing in Python

Static types. Type is known before you run the program. Not necessarily before you compile.

• PEP3107: Function annotations.
• PEP484: Type hints. Python 3.5, and backported to all Python 3.x
• Type hints will never become mandatory.
• PyCharm. Can autogenerate type annotations.
• Pytype. Has some semantics that I dislike.
• mypy. The type checker with community support.
• Jedi - not quite a type checker but worth a mention.
• typeshed - type bindings.
• Type Variance.

Types in the real world

mypy-extensions - documentation.

• Still beta status.
• Problems with SQLAlchemy.
• Import cycles. Use typing.TYPE_CHECKING.