How to limit the permitted values that can be passed to a method parameter (Using Type Hinting to allow Static Code Analysis)

Question

In Python 3, I want to limit the permitted values that are passed to this method:

my_request(protocol_type, url)

Using type hinting I can write:

my_request(protocol_type: str, url: str)

so the protocol and url are limited to strings, but how can I validate that protocol_type accepts only limited set of values, e.g. 'http' and 'https'?

Answer 1

One way is to write code in the method to validate that the value passed in is 'http' or 'https', something in the lines of:

if (protocol_type == 'http') or (protocol_type == 'https'):
  Do Something
else:
  Throw an exception

Which will work fine during runtime, but doesn't provide an indication of a problem while writing the code.

This is why I prefer using Enum and the type-hinting mechanism that Pycharm and mypy implement.

For the code example below you will get a warning in Pycharm from its code-inspection, see attached screenshot. The screenshot shows that if you enter a value that is not enum you will get the "Expected Type:..." warning.

Code:

"""Test of ENUM"""

from enum import Enum


class ProtocolEnum(Enum):
    """
    ENUM to hold the allowed values for protocol
    """
    HTTP: str = 'http'
    HTTPS: str = 'https'


def try_protocol_enum(protocol: ProtocolEnum) -> None:
    """
    Test of ProtocolEnum
    :rtype: None
    :param protocol: a ProtocolEnum value allows for HTTP or HTTPS only
    :return:
    """
    print(type(protocol))
    print(protocol.value)
    print(protocol.name)


try_protocol_enum(ProtocolEnum.HTTP)

try_protocol_enum('https')

Output:

<enum 'ProtocolEnum'>
http
HTTP