Python assertions

You can include an optional error message in an assert statement:

>>> assert 1 + 1 == 2, "Uh oh"

There is a gotcha here! assert is a statement, not a function. Do not write it in parenthesized form! To do so is to evaluate a tuple, and since a tuple is truthy, it will always evaluate to true. Python will provide a warning in this case:

>>> assert(1 + 1 == 2, "Uh oh")
<stdin>:1: SyntaxWarning: assertion is always true, perhaps remove parentheses?

Sources

Dan Bader, Python Tricks: The Book, 2017.