Python | Avoid repeating chunk of code

Question

I have couple of methods inside a class that control a virtual machine life-cycle. Operations like start, stop, terminate, retire .. etc

The code for these methods is almost identical, for example:

def stop(self, instances):
    """
    Stop instance or a group of instances
    :param instances: List of instances
    :return:
    """
    try:
        response = self.ec2_client.stop_instances(InstanceIds=instances, DryRun=False)
        print(response)
    except ClientError as e:
        print(e)

    return response

def start(self, instances):
    """
    Start instance or a group of instances
    :param instances: List of instances
    :return:
    """
    try:
        response = self.ec2_client.start_instances(InstanceIds=instances, DryRun=False)
        print(response)
    except ClientError as e:
        print(e)

    return response

As you can see, the two methods are almost identical except for the API call to perform the required action (start_instances and stop_instance).

Is there is a way to write such methods or functions in general and prevent repeating code?

Thanks in advance.

P.S. I was thinking of decorators, instance functions, closures -- but just don't know how!

Below answers inspired me to the following solution:

    @staticmethod
def _request_action_method(action, instances):
    instance_ids = FleetManager._instance_to_str(instances)

    def _action_method():
        try:
            response = action(InstanceIds=instance_ids, DryRun=False)
            print(response)
        except ClientError as e:
            print(e)

    return _action_method

I could replace +50 lines of code with those few lines and it works :)

user459872 · Accepted Answer · 2017-10-08 14:43:33Z

3

Make a function let's say

@staticmethod
def _request(method, instances):
 try:
    response = method(InstanceIds=instances, DryRun=False)
    print(response)
 except ClientError as e:
    print(e)
 return response

and call them

def stop(self, instances):
  self._request(self.ec2_client.stop_instances, instances)

def start(self, instances):
  self._request(self.ec2_client.start_instances, instances)

edited Oct 8, 2017 at 14:43

user459872

25.9k4 gold badges51 silver badges74 bronze badges

answered Oct 8, 2017 at 13:45

Vaibhav Singh

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Comments

Eli Bendersky · Accepted Answer · 2017-10-08 13:47:43Z

You can store a map of of stop: {stop_instances, start: start_instances} and call a single function that does the rest. Use getattr to obtain the member from self.ec2_client by name, or just the whole method.

Pseudo code:

In __init__:

self.methodmap = {'start': self.ec2_client.start_instances,
                  'stop': self.ec2_client.stop_instances}

Then, for example:

def start(self, instances):
  return self.run('start', instances)

def run(self, command, instances):
  method = self.methodmap[command]
  try:
    response = method(InstanceIds=instances, DryRun=False)
    print(response)
  except ClientError as e:
    print (e)
  return response

Depending on how much flexibility you want, you don't have to define self.methodmap but can also pass the method direction in the invocation of self.run.

For extra magic (be careful!) you can auto-generate the start, stop etc. methods too, because they all follow the same pattern.

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