الدرس 24 · Idiomatic Python

Type hints

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A type hint is a note in your code that says what kind of value belongs where, such as n: int or -> int. Step through a five-line file, hints.py, to see what Python does with hints (it stores them and never checks them), why double("ha") quietly returns 'haha', and how a type checker, mypy, reads the same file without running it and flags the bug on line 5.

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01In def area(w: float, h: float) -> float: what does -> float say?

-> after the parameter list is the return hint: the function is meant to give back a float. Nothing converts or checks the result at run time.

02double is defined as def double(n: int) -> int: return n * 2. What does python run for print(double("ha"))?

Python does not enforce hints. n becomes "ha" and "ha" * 2 repeats the string, so it prints haha with no error.

03Where does Python keep the hints of a function named double?

Python stores them in the function's __annotations__ dictionary, here {'n': int, 'return': int}. Tools can read them; the interpreter does not act on them.

04What does a type checker such as mypy do with hints.py?

mypy is a separate tool. It analyses the code without running it and reports mismatches, such as passing a str where n: int asks for an int. It does not change or block your program.

05Which hint says a variable holds either an int or None (Python 3.10 and later)?

The | operator builds a union type: int | None means an int or None. Older code writes Optional[int] from the typing module.

المصطلحات الأساسية

المسرد

Type: The kind of a value, such as int (whole number), str (text) or list. It decides what you can do with the value.
Type hint (annotation): A note in the code that says which type a parameter, return value or variable should have, written n: int or -> int.
__annotations__: The dictionary where Python keeps a function's hints, for example {'n': int, 'return': int}.
Run time: While the program is running. Python does not check hints at run time.
Type checker: A separate tool, such as mypy, that reads code without running it and reports values that do not match their hints.
mypy: A widely used Python type checker, installed with pip and run as mypy file.py.
Union (int | None): A hint that allows any one of several types; int | None means an int or None.

تفاصيل إضافية

الملاحظات والمصادر

The typing documentation says it plainly: the Python runtime does not enforce function and variable type annotations. Tools such as type checkers, IDEs and linters use them.
The dictionary holds the class int itself, shown as <class 'int'>; the figure writes int. Since Python 3.14 annotations are evaluated lazily, when you first read them, but __annotations__ gives the same dictionary.
mypy prints each message on one long line; the terminal in the figure wraps it, and an indented row continues the row above.
mypy is one type checker among several (pyright and others work the same way). It is installed with pip, not shipped with Python.
list[int] works from Python 3.9 and int | None from Python 3.10; older code writes List[int] and Optional[int] from the typing module.

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