Python 2.7 - Classes

Question

I'm trying to edit a library in python (perfmon) - the file is session.py

I want to add a module that can record some readings from a USB port.

This is also my first trial with classes in python

from perfmon import *
import os
import sys
import serial

# Common base class
class Session:
  def __init__(self, events):
    self.system = System()
    self.event_names = events
    self.events = []
    self.fds = []
    for e in events:
      err, encoding = pfm_get_perf_event_encoding(e, PFM_PLM0 | PFM_PLM3,
                                                  None, None)
      self.events.append(encoding)
    self.USB = serial.Serial("/dev/ttyUSB0",115200)

  def __del__(self):
    pass

  def read(self, fd):
    # TODO: determine counter width
    return os.read(fd, 8)

class PerThreadSession(Session):
  def __init__(self, pid, events):
    self.pid = pid
    Session.__init__(self, events)

  def __del__(self):
    Session.__del__(self)

  def start(self):
    for e in self.events:
      self.fds.append(perf_event_open(e, self.pid, -1, -1, 0))

  def read(self, i):
    return Session.read(self, self.fds[i])

class USB(Session):
  def __init__(self):   
    return self.USB

This is the following error I am getting:

Python 2.7.3 (default, Apr 14 2012, 08:58:41) [GCC] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import perfmon
>>> test = perfmon.USB()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'USB'

What is the mistake I am doing? I understood that self acts as a constructor in python, Can I return a value there?

Answer 1

as I understand in Your case answer is

>>> from perfmon import session
>>> test = session.USB()

But this way You move is wrong. You should create your own module in your workspace/project dir. Import session module and Create class there

from perfmon import session
class USB(Session):
    def __init__(self):   
        pass #I have changed this part as It has two errors in your example

Answer 2

You can not return anything (besides None) from a class constructor.

otherwise one will get

TypeError: __init__() should return None