Enums in Python

Our enum type ServerState has an underlying int type.

from enum import Enum, auto

class ServerState(Enum):
    Idle = auto()
    Connected = auto()
    Error = auto()
    Retrying = auto()

The possible values for ServerState are defined as constants. The Enum class and auto function generate successive constant values automatically; in this case 1, 2, 3, and so on. By implementing the __str__ method, values of ServerState can be printed out or converted to strings.

state_name = {
    ServerState.Idle: "idle",
    ServerState.Connected: "connected",
    ServerState.Error: "error",
    ServerState.Retrying: "retrying"
}

def server_state_to_string(state):
    return state_name[state]

If we have a value of type int, we cannot pass it to transition - the compiler will complain about type mismatch. This provides some degree of compile-time type safety for enums.

def main():
    ns = transition(ServerState.Idle)
    print(ns)

    ns2 = transition(ns)
    print(ns2)

def transition(state):
    if state == ServerState.Idle:
        return ServerState.Connected
    elif state in [ServerState.Connected, ServerState.Retrying]:
        return ServerState.Idle
    elif state == ServerState.Error:
        return state
    else:
        raise ValueError(f"unknown state: {state}")

if __name__ == "__main__":
    main()

To run the program, save the code in a file (e.g., enums.py) and use Python to execute it.

$ python enums.py
ServerState.Connected
ServerState.Idle

The transition function emulates a state transition for a server; it takes the existing state and returns a new state.

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Enums in Python