Type Hints
==========
Static type hints -- together with a type checker like `Mypy `_ -- are an excellent way to make your code more robust, self-documenting, and maintainable in the long run.
And as of 20.2.0, ``structlog`` comes with type hints for all of its APIs.
Since ``structlog`` is highly configurable and tries to give a clean facade to its users, adding types without breaking compatibility, while remaining useful was a formidable task.
If you used ``structlog`` and Mypy before 20.2.0, you will probably find that Mypy is failing now.
As a quick fix, add the following lines into your ``mypy.ini`` that should be at the root of your project directory (and must start with a ``[mypy]`` section):
.. code:: ini
[mypy-structlog.*]
follow_imports = skip
It will ignore ``structlog``'s type stubs until you're ready to adapt your code base to them.
----
The main problem is that `structlog.get_logger()` returns whatever you've configured the bound logger to be.
The only commonality are the binding methods like ``bind()`` and we've extracted them into the `structlog.types.BindableLogger` :class:`~typing.Protocol`.
But using that as a return type is worse than useless, because you'd have to use `typing.cast` on every logger returned by `structlog.get_logger()`, if you wanted to actually call any logging methods.
The second problem is that said ``bind()`` and its cousins are inherited from a common base class (a `big `_ `mistake `_ in hindsight) and can't know what concrete class subclasses them and therefore what type they are returning.
The chosen solution is adding `structlog.stdlib.get_logger()` that just calls `structlog.get_logger()` but has the correct type hints and adding `structlog.stdlib.BoundLogger.bind` et al that also only delegate to the base class.
`structlog.get_logger()` is typed as returning `typing.Any` so you can use your own type annotation and stick to the old APIs, if that's what you prefer:
.. code::
import structlog
logger: structlog.stdlib.BoundLogger = structlog.get_logger()
logger.info("hi") # <- ok
logger.msg("hi") # <- Mypy: 'error: "BoundLogger" has no attribute "msg"'
----
Rather sooner than later, the concept of the base class will be replaced by proper delegation that will put the context-related methods into a proper class (with proxy stubs for backward compatibility).
In the end, we're already delegating anyway.