Preconditions are all of the guarantees a function or method expects to receive in order to work correctly. In general programming terms, this usually means to provide data that is properly formed, for example, objects that are initialized, non-null values, and many more. For Python, in particular, being dynamically typed, this also means that sometimes we need to check for the exact type of data that are provided. This is not exactly the same as type checking, the kind mypy would do this, but rather verify for exact values that are needed.
Part of these checks can be detected early on by using static analysis tools, such as mypy, which we already introduced in Chapter 1, Introduction, Code Formatting, and Tools, but these checks ...
