Class Undefined

object --+
         |
        Undefined

Represents a reference to an undefined variable.

Unlike the Python runtime, template expressions can refer to an undefined variable without causing a NameError to be raised. The result will be an instance of the Undefined class, which is treated the same as False in conditions, but raise an exception on any other operation:

>>> foo = Undefined('foo')
>>> bool(foo)
False
>>> list(foo)
[]
>>> print(foo)
undefined

However, calling an undefined variable, or trying to access an attribute of that variable, will raise an exception that includes the name used to reference that undefined variable.

>>> foo('bar')
Traceback (most recent call last):
    ...
UndefinedError: "foo" not defined

>>> foo.bar
Traceback (most recent call last):
    ...
UndefinedError: "foo" not defined