from pymaltego import transforms, messages, entities


class EmailsToUsernamesTransform(transforms.BaseTransform):

    def transform(self):
        usernames = []

        for entity in self.message.entities:
            if len(usernames) >= self.message.soft_limit:
                break

            username = entity.value.split('@')[0]
            usernames.append(entities.Entity(name='Username', value=username))

        return usernames

xml = '''
<MaltegoMessage>
  <MaltegoTransformRequestMessage>
    <Entities>
      <Entity Type="EmailAddress">
        <Value>me@pyvim.com</Value>
      </Entity>
    </Entities>
    <Limits SoftLimit="12" HardLimit="12"/>
  </MaltegoTransformRequestMessage>
</MaltegoMessage>
'''
message = messages.TransformRequest(xml=xml)
transform = EmailsToUsernamesTransform(message)
print(transform.to_response().to_xml(pretty_print=True))

<MaltegoMessage>
  <MaltegoTransformResponseMessage>
    <Entities>
      <Entity Type="Username">
        <Value>me</Value>
      </Entity>
    </Entities>
  </MaltegoTransformResponseMessage>
</MaltegoMessage>