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6.2 Assert statements

Assert statements are a convenient way to insert debugging assertions into a program:

      assert_statement: "assert" expression ["," expression]

The simple form, ```assert expression`'', is equivalent to

      if __debug__:
         if not expression: raise AssertionError

The extended form, ```assert expression1, expression2`'', 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 1 under normal circumstances, 0 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.