pycmd

Simple command pattern implementation in python.

Synopsys

The pycmd module consists of two classes:

• Command: The base class for commands, providing abstract do and undo methods
• Executor: The command executor, providing a means of doing a stack of commands, and undoing said stack if a failure occurs.

Usage

Define Commands

To define a new command, inherit from Command and override the do and undo methods. All exception handling is the responsibility of the Executor, so do not catch exceptions in the derived command. If the command is not undoable, implement undo as pass.

Choice 1 - Stack and Execute

There are two ways to use the Executor. The first way, is to first instantiate all commands, adding each to the executor via add_cmd, and then execute the stack of commands via execute.

Example

executor = Executor()
executor.add_cmd(cmd_a())
executor.add_cmd(cmd_b())
executor.add_cmd(cmd_c())
executor.execute()
if executor.failed:
    print "Failure", executor.failed_cmd

Choice 2 - Immediate Cmd execution

You can also execute commands as you go. You might want to use this if instantiating each command is expensive.

executor = Executor()
if not executor.exec_command(cmd_a()):
    return
...
if not executor.exec_command(cmd_b()):
    return