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.
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-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
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.
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.