Can Python parameter annotations use strings?

Flexible application of Python parameter annotations

Python's parameter annotation function is powerful, which not only improves code readability, but also assists in static type checking. However, its flexible nature allows for the use of multiple annotations, including some non-traditional usages, such as using strings as annotations.

Let's look at an example:

 import time
from multiprocessing import queue, process


def produce(q: "queue[int]", length: int) -> None:
    for _ in range(length):
        q.put(3)

Here, the annotation "queue[int]" of the parameter q is a string. This is not a standard type hint, but the Python interpreter allows this approach. It indicates that q is expected to be a queue object containing integers. While static type checking tools such as mypy may not recognize string annotations, they have no effect on Python runtime.

The advantage of using string annotations is that it can describe the type and purpose of the parameter more clearly, especially when dealing with complex types or custom classes. For example:

 def my_function(param: "MyCustomClass with specific attributes") -> None:
    pass

Although not all tools support this annotation method, it can significantly enhance the readability and documentability of the code.

In short, Python's parameter annotation has a high degree of flexibility, allowing developers to choose appropriate annotation methods based on actual conditions to maximize the comprehensibility and maintenance of the code. Even using string annotations will not affect the running of the code, but static type checking may not work.

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