Command Classes vs Functions and functools.partial in Python

A function with its arguments bound is a command with execute() and nothing else.

In Python a command that only needs execute can be a function with its arguments already bound. functools.partial takes a function and some arguments and returns a new callable that, when called with nothing, calls the original with them. This card queues three such commands, then sets out what the class form gives that a callable cannot.

from functools import partial


def send_email(to: str, subject: str) -> None:
    print(f"email to {to}: {subject}")


def charge(card: str, pence: int) -> None:
    print(f"charged {pence} pence to card ending {card}")


queue = [
    partial(send_email, "[email protected]", "Order received"),
    partial(charge, "4242", 1600),
    lambda: print("stock reserved"),
]

print("queued:", len(queue))
for command in queue:
    command()

OutputRan with python:3.12-slim (Python 3.12.13)

queued: 3
email to [email protected]: Order received
charged 1600 pence to card ending 4242
stock reserved

Two partials and a lambda wait in a list, then run in order.

Each item in queue is a request that has not happened yet: the email, the charge and the stock reservation are values in a list, and nothing runs until the loop calls them. That is a command with an execute called by the name __call__, and no class was written.

The two forms compared
Concern	A command class	A function or partial
Run it later	`execute()`	Call it with `()`
Undo	A second method, `undo`, with the state it saved	No second method to call
State saved while running	Fields, such as the text a delete removed	Only through a closure, which is hard to read back
Inspect or log it	Fields and a `__repr__`	`partial` exposes `func`, `args` and `keywords`; a lambda shows only `<lambda>`
Serialise to a job queue	A class with plain fields	A partial of a module-level function can be pickled; a lambda cannot

The decision rule follows from the second row. Use a function or a partial when the request only has to run, later or elsewhere: callbacks, deferred jobs and button actions without undo. Use a class as soon as the request must be reversed, because undo needs a second method and the state the first one saved, and a callable has room for neither.

← → move through sections and cards