+ def compile_expression(self, source, undefined_to_none=True):
+ """A handy helper method that returns a callable that accepts keyword
+ arguments that appear as variables in the expression. If called it
+ returns the result of the expression.
+
+ This is useful if applications want to use the same rules as Jinja
+ in template "configuration files" or similar situations.
+
+ Example usage:
+
+ >>> env = Environment()
+ >>> expr = env.compile_expression('foo == 42')
+ >>> expr(foo=23)
+ False
+ >>> expr(foo=42)
+ True
+
+ Per default the return value is converted to `None` if the
+ expression returns an undefined value. This can be changed
+ by setting `undefined_to_none` to `False`.
+
+ >>> env.compile_expression('var')() is None
+ True
+ >>> env.compile_expression('var', undefined_to_none=False)()
+ Undefined
+
+ **new in Jinja 2.1**
+ """
+ parser = Parser(self, source, state='variable')
+ try:
+ expr = parser.parse_expression()
+ if not parser.stream.eos:
+ raise TemplateSyntaxError('chunk after expression',
+ parser.stream.current.lineno,
+ None, None)
+ except TemplateSyntaxError, e:
+ e.source = source
+ raise e
+ body = [nodes.Assign(nodes.Name('result', 'store'), expr, lineno=1)]
+ template = self.from_string(nodes.Template(body, lineno=1))
+ return TemplateExpression(template, undefined_to_none)
+