example.py

# Stringify an integer
stringified_int = Schema(All(
    int,
    Coerce(str) # Same as `lambda x: str(x)
))


def AcceptSingleOrListOf(sub_schema):
    """
    Returns a wrapped schema that either accepts the original schema, or a list
    of values of the original schema. Transforms all values into a list of 
    the schema.
    :param sub_schema: Schema to use.
    :return: A wrapped schema.
    """""
    return Any(
        # List of sub_schema values
        [sub_schema],
        # 'All' is a pipeline. The original value is fed to the first filter,
        # and inputs/outputs are chained along the list.
        All(sub_schema, lambda x: [x])
    )
  
s = AcceptSingleOrListOf(stringified_int)
print(s(5))      # 5
print(s([1,2,3]) # [1,2,3]
print(s(None))   # Throws exception