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command.py

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"""

Command pattern decouples the object invoking a job from the one who knows

how to do it. As mentioned in the GoF book, a good example is in menu items.

You have a menu that has lots of items. Each item is responsible for doing a

special thing and you want your menu item just call the execute method when

it is pressed. To achieve this you implement a command object with the execute

method for each menu item and pass to it.

*About the example

We have a menu containing two items. Each item accepts a file name, one hides the file

and the other deletes it. Both items have an undo option.

Each item is a MenuItem class that accepts the corresponding command as input and executes

it's execute method when it is pressed.

*TL;DR

Object oriented implementation of callback functions.

*Examples in Python ecosystem:

Django HttpRequest (without execute method):

https://docs.djangoproject.com/en/2.1/ref/request-response/#httprequest-objects

"""

class HideFileCommand:

"""

A command to hide a file given its name

"""

def __init__(self) -> None:

# an array of files hidden, to undo them as needed

self._hidden_files: list[str] = []

def execute(self, filename: str) -> None:

print(f"hiding {filename}")

self._hidden_files.append(filename)

def undo(self) -> None:

filename = self._hidden_files.pop()

print(f"un-hiding {filename}")

class DeleteFileCommand:

"""

A command to delete a file given its name

"""

def __init__(self) -> None:

# an array of deleted files, to undo them as needed

self._deleted_files: list[str] = []

def execute(self, filename: str) -> None:

print(f"deleting {filename}")

self._deleted_files.append(filename)

def undo(self) -> None:

filename = self._deleted_files.pop()

print(f"restoring {filename}")

class MenuItem:

"""

The invoker class. Here it is items in a menu.

"""

def __init__(self, command: HideFileCommand | DeleteFileCommand) -> None:

self._command = command

def on_do_press(self, filename: str) -> None:

self._command.execute(filename)

def on_undo_press(self) -> None:

self._command.undo()

def main():

"""

>>> item1 = MenuItem(DeleteFileCommand())

>>> item2 = MenuItem(HideFileCommand())

# create a file named `test-file` to work with

>>> test_file_name = 'test-file'

# deleting `test-file`

>>> item1.on_do_press(test_file_name)

deleting test-file

# restoring `test-file`

>>> item1.on_undo_press()

restoring test-file

# hiding `test-file`

>>> item2.on_do_press(test_file_name)

hiding test-file

# un-hiding `test-file`

>>> item2.on_undo_press()

un-hiding test-file

"""

if __name__ == "__main__":

import doctest

doctest.testmod()