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@chrisguitarguy
Last active January 6, 2022 23:56
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Command bus pattern in Python.
#! /usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Playing Around with the Command Bus Pattern in Python
"""
import inspect
import collections
class HandlerNotFound(Exception):
pass
class Resolver:
def handler_for(self, command):
"""
Retrieve the handler class for a command. If the command implements a
``handler`` method, it should return the class of the handler. Otherwise
it will search for a class with the name {CommandName}Handler.
"""
try:
return command.handler()
except AttributeError:
pass
try:
return getattr(self._getmodule(command), command.__class__.__name__+'Handler')
except AttributeError:
return None
def validator_for(self, command):
"""
Retrieve the validator class for a command. If the command implements a
``validator`` method, it should return the class of the handler. Otherwise
it will search for a class with the name {CommandName}Validator.
"""
try:
return command.validator()
except AttributeError:
pass
try:
return getattr(self._getmodule(command), command.__class__.__name__+'Validator')
except AttributeError:
return None
def _getmodule(self, command):
return inspect.getmodule(command)
class Bus:
"""
The actual command bus, when given a command, it finds an appropriate handler
and fires it.
"""
#: The command name resolver, used to figure out names for commands that
#: don't have a `handler` method.
resolver = None
def __init__(self, resolver=None):
self.resolver = resolver or Resolver()
def execute(self, command):
validator_cls = self.resolver.validator_for(command)
if validator_cls is not None:
validator_cls().validate(command)
handler_cls = self.resolver.handler_for(command)
if handler_cls is None:
raise HandlerNotFound('Unable to find handler for '+command.__class__.__name__)
return handler_cls().handle(command)
SayHelloCommand = collections.namedtuple('SayHelloCommand', 'name')
class SayHelloCommandHandler:
def handle(self, command):
print("Hello, "+command.name)
class SayHelloCommandValidator:
def validate(self, command):
print(command.__class__.__name__, 'seems normal')
return True
class SayGoodbyeCommand(collections.namedtuple('SayGoodbyeCommand', 'name')):
def handler(self):
return GoodbyeHandler
def validator(self):
return GoodbyeValidator
class GoodbyeHandler:
def handle(self, command):
print("Goodbye, "+command.name)
class GoodbyeValidator:
def validate(self, command):
print(command.__class__.__name__, 'seems okay')
SayGoodnightCommand = collections.namedtuple('SayGoodnightCommand', 'name')
class SayGoodnightCommandHandler:
def handle(self, command):
print('Goodnight, '+command.name)
NoHandlerCommand = collections.namedtuple('NoHandlerCommand', 'name')
if __name__ == '__main__':
bus = Bus()
bus.execute(SayHelloCommand('world'))
bus.execute(SayGoodbyeCommand('world'))
bus.execute(SayGoodnightCommand('moon'))
try:
bus.execute(NoHandlerCommand('nope'))
except HandlerNotFound:
print("No handler found!")
@eduardonunesp
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great example ! thanks

@gbrennon
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gbrennon commented Jun 4, 2021

this a nice example! but that approach using the classname + Handler to instantiate the CommandHandler doesnt feel right :~

@chrisguitarguy
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Author

this a nice example! but that approach using the classname + Handler to instantiate the CommandHandler doesnt feel righ

Then you'd implement your own Resolver to do what you like (like pull the handler from a service container type thing or map handler class name to a handler instance or callable that can create a handler instance, etc).

@gbrennon
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gbrennon commented Jun 9, 2021

Then you'd implement your own Resolver to do what you like (like pull the handler from a service container type thing or map handler class name to a handler instance or callable that can create a handler instance, etc).

i agree with u! but as this is an example do u mind if i push u a better approach for this?
i think we could improve people's learning by providing better examples of code principles!

@ambersariya
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ambersariya commented Jul 20, 2021

This is a pretty cool example of how to implement this pattern in Python. I'd love to see the better approach as mentioned by @gbrennon

Then you'd implement your own Resolver to do what you like (like pull the handler from a service container type thing or map handler class name to a handler instance or callable that can create a handler instance, etc).

i agree with u! but as this is an example do u mind if i push u a better approach for this?
i think we could improve people's learning by providing better examples of code principles!

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