Taming Mypy's Optional Chaining: A Quest for Concise Annotations
Mypy, the beloved static type checker for Python, can be a bit of a stickler when it comes to optional chaining. While it safeguards us from dreaded AttributeError exceptions, its insistence on explicit type checking can sometimes feel verbose.
The Problem:
Imagine you have a nested object structure where some attributes might be missing. You might want to access a deeply nested attribute using optional chaining (obj.a.b.c), but Mypy might complain if obj.a, obj.a.b, or obj.a.b.c could be None. It will throw an error like this:
# my_code.py
from typing import Optional
class Foo:
bar: Optional[int]
foo: Optional[Foo] = None
if foo is not None:
print(foo.bar.baz) # Mypy error: "Attribute 'baz' of 'Optional[int]' is not subscriptable"
The Solution:
While you can explicitly check each step of the chain for None, there's a more elegant solution using type narrowing. This involves telling Mypy that, in a specific context, a variable that was previously optional is guaranteed to have a value.
from typing import Optional
class Foo:
bar: Optional[int]
foo: Optional[Foo] = None
if foo is not None:
# Narrow the type of foo.bar to int, assuming foo.bar is not None
if foo.bar is not None:
print(foo.bar.baz) # No more Mypy error
Explanation:
The if foo.bar is not None statement explicitly asserts that foo.bar is not None within the if block. This allows Mypy to narrow the type of foo.bar to int, making the subsequent access of foo.bar.baz valid.
Benefits:
- Conciseness: Avoids redundant checks for
None, making your code more readable. - Clarity: Explicitly communicates the expected state of the variable within the specific code block.
- Improved Type Safety: Mypy can still enforce type safety, ensuring your code remains reliable.
Additional Notes:
- The
if foo is not Nonestatement outside the nestedifblock is necessary for Mypy to understand thatfooitself is notNone. - Type narrowing can be applied to other situations where you need to provide Mypy with extra context about the type of a variable.
In Conclusion:
By using type narrowing, you can effectively communicate your intentions to Mypy, silencing its concerns about optional chaining while maintaining strong type safety. This approach helps create more concise, readable, and reliable code.
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