@classmethod<EOL><INDENT>def from_expr(cls, expr):<DEDENT>
return from_expr(cls, expr)<EOL>
Convenience constructor for enumerations.

::

    >>> import uqbar.enums
    >>> class MyEnum(uqbar.enums.IntEnumeration):
    ...     A_B = -1
    ...     B_C_D = 0
    ...     E = 1

::

    >>> MyEnum.from_expr("a b")
    MyEnum.A_B

::

    >>> MyEnum.from_expr(None)
    MyEnum.B_C_D

::

    >>> MyEnum.from_expr(1)
    MyEnum.E

Returns new enumeration item.