Return the AST statements to replace the ast.Assert instance.

        This re-writes the test of an assertion to provide
        intermediate values and replace it with an if statement which
        raises an assertion error with a detailed explanation in case
        the expression is false.

def visit_Assert(self, assert_):
        """Return the AST statements to replace the ast.Assert instance.

        This re-writes the test of an assertion to provide
        intermediate values and replace it with an if statement which
        raises an assertion error with a detailed explanation in case
        the expression is false.

        """
        if isinstance(assert_.test, ast.Tuple) and self.config is not None:
            fslocation = (self.module_path, assert_.lineno)
            self.config.warn('R1', 'assertion is always true, perhaps '
                              'remove parentheses?', fslocation=fslocation)
        self.statements = []
        self.variables = []
        self.variable_counter = itertools.count()
        self.stack = []
        self.on_failure = []
        self.push_format_context()
        # Rewrite assert into a bunch of statements.
        top_condition, explanation = self.visit(assert_.test)
        # Create failure message.
        body = self.on_failure
        negation = ast.UnaryOp(ast.Not(), top_condition)
        self.statements.append(ast.If(negation, body, []))
        if assert_.msg:
            assertmsg = self.helper('format_assertmsg', assert_.msg)
            explanation = "\n>assert " + explanation
        else:
            assertmsg = ast.Str("")
            explanation = "assert " + explanation

        if _MARK_ASSERTION_INTROSPECTION:
            explanation = 'dessert* ' + explanation

        template = ast.BinOp(assertmsg, ast.Add(), ast.Str(explanation))
        msg = self.pop_format_context(template)
        fmt = self.helper("format_explanation", msg, assertmsg)
        err_name = ast.Name("AssertionError", ast.Load())
        exc = ast_Call(err_name, [fmt], [])
        if sys.version_info[0] >= 3:
            raise_ = ast.Raise(exc, None)
        else:
            raise_ = ast.Raise(exc, None, None)
        body.append(raise_)
        # Clear temporary variables by setting them to None.
        if self.variables:
            variables = [ast.Name(name, ast.Store())
                         for name in self.variables]
            clear = ast.Assign(variables, _NameConstant(None))
            self.statements.append(clear)
        # Fix line numbers.
        for stmt in self.statements:
            set_location(stmt, assert_.lineno, assert_.col_offset)
        return self.statements