# Implementing command pattern in Python

I did some study on the command pattern but most of its examples were in Java so, there must be some difference in implementation in Python. I implemented it in Python with some minor differences, please let me know if something is not correct.

from abc import ABCMeta
from abc import abstractmethod
import inspect
import os

class Command(object):
"""
Abstract / Interface base class for commands.
"""
__metaclass__ = ABCMeta

@abstractmethod
def execute(self):
pass

@abstractmethod
def undo(self):
pass

class CreateCommand(Command):
"""
Create command implementation.
"""
def __init__(self, name):
self.file_name = name

def execute(self, name):
open(self.file_name, 'w')
print str(self) + ':::Method:::' + inspect.stack()[0][3]

def undo(self):
os.remove(self.file_name)
print str(self) + ':::Method:::' + inspect.stack()[0][3]

class MoveCommand(Command):
"""
Move command implementation.
"""
def __init__(self, src, dest):
self.src = src
self.dest = dest

def execute(self, src, dest):
os.rename(self.src, self.dest)
print str(self) + ':::Method:::' + inspect.stack()[0][3]

def undo(self):
os.rename(self.dest, self.src)
print str(self) + ':::Method:::' + inspect.stack()[0][3]

class Invoker(object):

def __init__(self, command):
self.command = command

def do(self):
self.command.execute()

def undo(self):
self.command.undo()

# Client for the command pattern
if __name__ == '__main__':
create_cmd = CreateCommand('/tmp/foo.txt')
move_cmd = MoveCommand('/tmp/foo.txt', '/tmp/bar.txt')
create_invoker = Invoker(create_cmd)
move_invoker = Invoker(move_cmd)
create_invoker.do()
move_invoker.do()
move_invoker.undo()
create_invoker.undo()


O/P:

<__main__.CreateCommand object at 0xb705130c>:::Method:::execute
<__main__.MoveCommand object at 0xb705134c>:::Method:::execute
<__main__.MoveCommand object at 0xb705134c>:::Method:::undo
<__main__.CreateCommand object at 0xb705130c>:::Method:::undo


The important thing to know about commands in Python (or any language with first-class functions) is that they're usually trivial. Commands with only one operation (execute) are simply functions. There's no need to define classes. Even commands with two operations can be represented as pairs of functions: (do, undo). The whole, heavyweight pattern is almost never used.

• Command parameters such as filenames should be arguments to the constructor, not to execute. The code that calls execute doesn't know what these values should be, so they need to already be in the command.
• Invoker.execute manually dispatches by command name. Instead it should blindly call execute or undo, and let method dispatch find the appropriate method.
• The Invoker class does nothing useful.
• Why bother getting the method name from inspect.stack when the method already knows its own name?
• I might call execute do, for symmetry with undo.
• ABCMeta is unnecessary complexity. If you must use it, it's clearer to write class Command(metaclass=ABCMeta) rather than assigning to __metaclass__.
• "how it will find the respective command?": Commands should be represented as Commands, not as strings. Do something like MoveCommand('foo', 'bar').execute(), not invoke('move', 'foo', 'bar'), nor invoker.do(). The Invoker class does nothing; it should be removed. – Anonymous May 18 '14 at 4:49
• @vivekpoddar One common use for this pattern is to implement undo/redo. For redo to work, the UI that manages the queue must be able to call execute without passing any information. – David Harkness May 18 '14 at 7:00