"""Key-Value store server.

The request takes "key=" and "value=" URL parameters. The key must be UUID
generated by token().

- When only the "key=" is specified, serves a 200 response whose body contains
  the stored value specified by the key. If the stored value doesn't exist,
  serves a 200 response with an empty body.
- When both the "key=" and "value=" are specified, stores the pair and serves
  a 200 response without body.
"""


def main(request, response):
    key = request.GET.get(b"key")
    value = request.GET.get(b"value")

    # Store the value.
    # We have two ways to check the truthiness of `value`:
    #   1. `if value`
    #   2. `if value != None`
    # While (1) is the most intuitive, we actually need (2), which is a looser
    # check. We need the looser check so that if the URL contains `&value=` to
    # set the value equal to the empty string (a case we need to support), this
    # condition still evaluates to true and we enter this branch.
    if value != None:
        # We opted for (2) above which is the looser of the truthiness tests
        # that lets empty strings into this branch. So you might think that when
        # the URL contains `&value=`, then the `value` variable here would be
        # equal `""`, but instead it equals the empty byte string. If you just
        # store that empty byte string into the stash and retrieve it later, you
        # actually get <Not set> because it doesn't recognize the empty byte
        # string as a real value, so we instead have to override it to the empty
        # normal string, and then we can store it for later use. This is
        # because we have to support storage and retrieval of empty strings.
        if type(value) is bytes and value == b'':
            value = ""

        request.server.stash.put(key, value)
        return (200, [], b"")

    # Get the value.
    data = request.server.stash.take(key)
    if not data and data != "":
        return (200, [], b"<Not set>")
    return (200, [], data)