Assert statements are a convenient way to insert debugging assertions into a program:
The first form is equivalent to
if __debug__: if not expression: raise AssertionError
The second form is equivalent to
if __debug__: if not expression1: raise AssertionError(expression2)
These equivalences assume that __debug__ and AssertionError refer to the built-in variables with those names. In the current implementation, the built-in variable __debug__ is True under normal circumstances, False when optimization is requested (command line option -O). The current code generator emits no code for an assert statement when optimization is requested at compile time.
Note that it is unnecessary to include the source code for the expression that failed in the error message; it will be displayed as part of the stack trace.