def list_of ( cls ) : return lambda l : isinstance ( l , list ) and all ( isinstance ( x , cls ) for x in l )
declaring custom type list in python
Returns a function that checks that each element in a list is of a specific type .
def list_of(cls):
    """
    Returns a function that checks that each element in a
    list is of a specific type.
    """
    return lambda l: isinstance(l, list) and all(isinstance(x, cls) for x in l)