Python type hints: Mypy doesn’t allow variables to change type
Mypy does not allow variables to change type. I found this a little bit of a shock at first, but after I adapted, I found my code was more readable.
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Mypy does not allow variables to change type. I found this a little bit of a shock at first, but after I adapted, I found my code was more readable.
A couple of recent PEPs have made writing type hints easier:
NoReturn?Sometimes functions never return, for example by always raising an exception. For such functions’ return types, we can “get away” with using None, but it’s best to use the special NoReturn type). This allows Mypy to better find unreachable code (as covered previously). It also shows future readers that the lack of return is intentional.
I recently discovered Mypy has a secondary function as an unreachable code detector. This feature is a great way to highlight places bugs may be hiding, as code paths that can’t possibly run normally show a logical error.
ProtocolDuck typing says “if it quacks like a duck, treat it like a duck.” Type checking seems at odds with duck typing: we restrict variables to named types (classes), allowing only that type or subtypes. This seems like it would stop us from passing in arbitrary objects that can “quack the right way”. But since PEP 0544, we can declare “duckish” types with typing.Protocol.
isinstance(), assert, and LiteralType narrowing is the ability for a type checker to infer that inside some branch, a variable has a more specific (narrower) type than its definition. This allows us to perform type-specific operations without any explicit casting.
In a type hint, if we specify a type (class), then we mark the variable as containing an instance of that type. To specify that a variable instead contains a type, we need to use type[Cls] (or the old syntax typing.Type).
__future__.annotationsPython 3.7 added a new “postponed evaluation” mode for type hints. In this post we’ll cover how it changes the behaviour of type hints, how to activate it, and the outstanding problems.
reveal_type()When working with type hints, it’s often useful to debug the types of variables. Type checkers allow you to do this with reveal_type() and reveal_locals().
Circular imports are always annoying when they arise in Python, and type hints make them more common. Thankfully, there’s a trick to add circular imports for type hints without causing ImportErrors.
Using TypedDict is a great way to type code that passes around dictionaries. The default way to create a TypedDict is with its class-based syntax:
*args and **kwargsWhen I started writing type hints, I was a little confused about what to do with Python’s variable argument operators, * and ** (often called *args and **kwargs). Here’s what I figured out.
Take this function:
When starting out with Python type hints, it’s common to overuse typing.Any. This is dangerous, since Any entirely disables type checking for a variable, allowing any operation.