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?

It looks like you're file is named `perfmon.py` but there's also a module `perfmon` on your search path. I'm guessing your import statement is picking the wrong one... — mgilson, Jun 13 '13 at 19:27
For example: if I want to say PerThreadSession. I say, perfmon.PerThreadSession(,,) — pistal, Jun 13 '13 at 19:28
ok I think I know answer `import session` and `test = session.USB()` — oleg, Jun 13 '13 at 19:29
I'm confused. In your session.py you have: class USB(Session): .... But you try to use USB from perfmon.USB? shouldn't you do this -> import session; session.USB() — jaime, Jun 13 '13 at 19:31
what do You think? where is your file `session.py` is situated? — oleg, Jun 13 '13 at 19:31
I'm trying to edit a library in python (`perfmon`) - the file is `session.py` <- that is why — oleg, Jun 13 '13 at 19:32
my session.py is in `/usr/lib64/python2.7/site-packages/perfmon/` — pistal, Jun 13 '13 at 19:32
that is not good but `from perfmon import session` and `test = session.USB()` I don't think it is good practice to modify library inplace — oleg, Jun 13 '13 at 19:34
AttributeError: USB instance has no attribute 'USB'. but that's the error I get. — pistal, Jun 13 '13 at 19:36

score 2 · Answer 1 · edited Jun 13 '13 at 19:42

2

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

otherwise one will get

TypeError: __init__() should return None

edited Jun 13 '13 at 19:42

oleg

4,082
16
16

answered Jun 13 '13 at 19:38

jaime

2,234
1
19
22

do you suspect that to be the reason I see no o/p on my screen? – pistal Jun 13 '13 at 19:39
Thanks. I should look into classes for python more to start with. – pistal Jun 13 '13 at 19:44

score 1 · Accepted Answer · answered Jun 13 '13 at 19:38

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

answered Jun 13 '13 at 19:38

oleg

4,082
16
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I could create a file in my own folder. that's one option. But, from what I see from `@jaime`'s answer is that, we can't return anything else other than none for a class constructor. – pistal Jun 13 '13 at 19:42
You have 3 errors in Your question 2 in the module and one in python session – oleg Jun 13 '13 at 19:45
I will re-write the module in the same folder. maybe that's simpler and better. – pistal Jun 13 '13 at 19:49
This allow You to save Your code. and move it to any other machines easier. – oleg Jun 13 '13 at 19:54

Python 2.7 - Classes

2 Answers2

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