Practical Programming, 3rd Edition

Type Annotations for Lists

When writing type contracts for functions, often we’ll want to specify that the values in a list parameter are all of a particular type. For example, we might write a function to calculate the average of a list of floats:

>>> def average(L: list) -> float:
...     """Return the average of the values in L.
...
...     >>> average([1.4, 1.6, 1.8, 2.0])
...     1.7
...     """

There is currently no indication that the function works only with lists of numbers, but it would be odd to call it with a list of strings, for example. To address this, Python includes module typing that allows us to specify the expected type of value contained in a list (and in other types that you’ll encounter in Chapter ...