Question

In Python, assert is a statement, and not a function. Was this a deliberate decision? Are there any advantages to having assert be a statement (and reserved word) instead of a function?

According to the docs, assert expression1, expression2 is expanded to

if __debug__:
    if not expression1: raise AssertionError(expression2)

The docs also say that “The current code generator emits no code for an assert statement when optimization is requested at compile time.” Without knowing the details, it seems like a special case was required to make this possible. But then, a special case could also be used to optimize away calls to an assert() function.

If assert were a function, you could write:

assert(some_long_condition,
       "explanation")

But because assert is a statement, the tuple always evaluates to True, and
you get

SyntaxWarning: assertion is always true, perhaps remove parentheses?

The correct way to write it is

assert some_long_condition, \
       "explanation"

which is arguably less pretty.

Answer 1

Are there any advantages to having assert be a statement (and reserved word) instead of a function?

1. Cannot be reassigned to a user function, meaning it can be effectively disabled at compile time as @mgilson pointed out.
2. The evaluation of the second, optional parameter is deferred until if/when the assertion fails. Awkward to do that with functions and function arguments (would need to pass a lambda.) Not deferring the evaluation of the second parameter would introduce additional overhead.