Re: object.enable() anti-pattern

Date	2013-05-13 08:15 -0500
From	Wayne Werner <wayne@waynewerner.com>
Subject	Re: object.enable() anti-pattern
References	<518a123c$0$11094$c3e8da3@news.astraweb.com> <mailman.1490.1368098298.3114.python-list@python.org> <av2qleF58afU1@mid.individual.net> <alpine.DEB.2.02.1305121214180.3322@gilgamesh> <51902332.8090301@canterbury.ac.nz>
Newsgroups	comp.lang.python
Message-ID	<mailman.1625.1368450946.3114.python-list@python.org> (permalink)

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On Mon, 13 May 2013, Greg Ewing wrote:

> Wayne Werner wrote:
>> On Fri, 10 May 2013, Gregory Ewing wrote:
>>
>>>   f = open("myfile.dat")
>>>   f.close()
>>>   data = f.read()
>> 
>> To clarify - you don't want a class that has functions that need to be 
>> called in a certain order with *valid input* in order to not crash.
>> 
>> Exactly what does happen - a ValueError is raised because you're(*) passing 
>> self into the file.read() function, and that input is invalid
>
> The same argument can be applied to:
>
>   foo = Foo()
>   foo.do_something()
>   foo.enable() # should have done this first
>
> You're passing an invalid input to Foo.do_something,
> namely a Foo that hasn't been enabled yet.

That is the crux of the argument - as designer of the class *you* need to 
ensure that when your constructor is done, your class is in a stable 
state. And that every other state transition (with valid input) results in 
your class then being in a stable state.

If anything, the stronger argument is that `file.close()` is not a well 
designed function because it leaves your object in an unstable state.

Which I would be inclined to agree with, but I couldn't give you the 
answer for what makes it better. Because the answer is the best one you 
can get in computer science: It depends.

The reason that it depends, is because it depends on what you want to do. 
Do you want a program that seems purely functional? Do you want a program 
that's easy to maintain? Do you want a program that more accurately models 
the "real world"?

Personally, I think the file object API in Python is about as good as it 
can get - but that's because it's working with "physical" things (i.e. 
files - bits on a platter, or flash/SSD drive...) which necessarily have a 
temporal nature. And it's much less badness to blow up on a call to `read` 
than it is to remove the `read` function and die with a NameError when the 
underlying file is in a closed state.

At least in my opinion ;)
-W

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object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-08 08:52 +0000
  Re: object.enable() anti-pattern Christian Heimes <christian@python.org> - 2013-05-08 11:51 +0200
  Re: object.enable() anti-pattern Robert Kern <robert.kern@gmail.com> - 2013-05-08 11:13 +0100
    Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-08 12:30 +0000
  Re: object.enable() anti-pattern Roy Smith <roy@panix.com> - 2013-05-08 09:17 -0400
  Re: object.enable() anti-pattern Duncan Booth <duncan.booth@invalid.invalid> - 2013-05-08 14:27 +0000
    Re: object.enable() anti-pattern Dan Sommers <dan@tombstonezero.net> - 2013-05-09 02:38 +0000
      Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-09 05:37 +0000
        Re: object.enable() anti-pattern Chris Angelico <rosuav@gmail.com> - 2013-05-09 15:52 +1000
    Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-09 03:12 +0000
  Re: object.enable() anti-pattern Dan Sommers <dan@tombstonezero.net> - 2013-05-09 02:42 +0000
    Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-09 05:23 +0000
      Re: object.enable() anti-pattern Terry Jan Reedy <tjreedy@udel.edu> - 2013-05-09 02:41 -0400
      Re: object.enable() anti-pattern Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2013-05-09 19:54 +1200
        Re: object.enable() anti-pattern Cameron Simpson <cs@zip.com.au> - 2013-05-09 18:23 +1000
          Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-09 11:30 +0000
            Re: object.enable() anti-pattern Cameron Simpson <cs@zip.com.au> - 2013-05-10 09:36 +1000
              Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-10 05:00 +0000
                Re: object.enable() anti-pattern Roy Smith <roy@panix.com> - 2013-05-10 01:50 -0400
                Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-10 09:47 +0000
                Re: object.enable() anti-pattern Roy Smith <roy@panix.com> - 2013-05-10 09:22 -0400
                Re: object.enable() anti-pattern Cameron Simpson <cs@zip.com.au> - 2013-05-11 08:25 +1000
                Re: object.enable() anti-pattern Mark Janssen <dreamingforward@gmail.com> - 2013-05-10 20:16 -0700
                Re: object.enable() anti-pattern Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2013-05-11 06:21 +0200
                Re: object.enable() anti-pattern Mark Janssen <dreamingforward@gmail.com> - 2013-05-10 21:00 -0700
                Re: object.enable() anti-pattern Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-05-11 15:04 -0400
        Re: object.enable() anti-pattern Greg Ewing <greg.ewing@canterbury.ac.nz> - 2013-05-10 10:56 +1200
      Re: object.enable() anti-pattern Roy Smith <roy@panix.com> - 2013-05-09 09:07 -0400
        Re: object.enable() anti-pattern Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-05-09 14:51 +0100
        Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-09 18:21 +0000
          Re: object.enable() anti-pattern MRAB <python@mrabarnett.plus.com> - 2013-05-09 19:34 +0100
            Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-10 02:30 +0000
              Re: object.enable() anti-pattern Roy Smith <roy@panix.com> - 2013-05-09 23:09 -0400
                Re: object.enable() anti-pattern Mark Janssen <dreamingforward@gmail.com> - 2013-05-09 20:19 -0700
                Re: object.enable() anti-pattern Chris Angelico <rosuav@gmail.com> - 2013-05-10 13:46 +1000
                Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-10 05:03 +0000
                Re: object.enable() anti-pattern Dan Sommers <dan@tombstonezero.net> - 2013-05-10 06:22 +0000
                Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-10 11:00 +0000
                Re: object.enable() anti-pattern Robert Kern <robert.kern@gmail.com> - 2013-05-10 13:19 +0100
                Re: object.enable() anti-pattern Roy Smith <roy@panix.com> - 2013-05-10 10:01 -0400
                Re: object.enable() anti-pattern Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-05-10 15:29 +0100
                Re: object.enable() anti-pattern Roy Smith <roy@panix.com> - 2013-05-10 10:37 -0400
                Re: object.enable() anti-pattern Chris Angelico <rosuav@gmail.com> - 2013-05-11 00:46 +1000
                Re: object.enable() anti-pattern Roy Smith <roy@panix.com> - 2013-05-10 10:54 -0400
                Re: object.enable() anti-pattern Chris Angelico <rosuav@gmail.com> - 2013-05-11 01:09 +1000
                Re: object.enable() anti-pattern Roy Smith <roy@panix.com> - 2013-05-10 11:21 -0400
                Re: object.enable() anti-pattern Chris Angelico <rosuav@gmail.com> - 2013-05-11 01:44 +1000
                Re: object.enable() anti-pattern Robert Kern <robert.kern@gmail.com> - 2013-05-10 16:33 +0100
                Re: object.enable() anti-pattern Serhiy Storchaka <storchaka@gmail.com> - 2013-05-10 18:44 +0300
                Re: object.enable() anti-pattern André Malo <ndparker@gmail.com> - 2013-05-11 17:33 +0200
                Re: object.enable() anti-pattern Chris Angelico <rosuav@gmail.com> - 2013-05-12 02:04 +1000
                Re: object.enable() anti-pattern Robert Kern <robert.kern@gmail.com> - 2013-05-10 18:20 +0100
                Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-11 07:51 +0000
                Re: object.enable() anti-pattern Roy Smith <roy@panix.com> - 2013-05-11 09:31 -0400
                Re: object.enable() anti-pattern Robert Kern <robert.kern@gmail.com> - 2013-05-11 20:55 +0100
                Re: object.enable() anti-pattern Chris Angelico <rosuav@gmail.com> - 2013-05-12 08:39 +1000
                Re: object.enable() anti-pattern Chris Angelico <rosuav@gmail.com> - 2013-05-11 03:24 +1000
                Re: object.enable() anti-pattern Mark Janssen <dreamingforward@gmail.com> - 2013-05-10 19:43 -0700
                Re: object.enable() anti-pattern Wayne Werner <wayne@waynewerner.com> - 2013-05-12 11:48 -0500
                Re: object.enable() anti-pattern Terry Jan Reedy <tjreedy@udel.edu> - 2013-05-12 16:23 -0400
                Re: object.enable() anti-pattern Mark Janssen <dreamingforward@gmail.com> - 2013-05-09 20:51 -0700
              Re: object.enable() anti-pattern Chris Angelico <rosuav@gmail.com> - 2013-05-10 13:08 +1000
          Re: object.enable() anti-pattern roy@panix.com (Roy Smith) - 2013-05-09 14:59 -0400
            Re: object.enable() anti-pattern Chris Angelico <rosuav@gmail.com> - 2013-05-10 07:55 +1000
      Re: object.enable() anti-pattern Nobody <nobody@nowhere.com> - 2013-05-10 17:59 +0100
        Re: object.enable() anti-pattern Roy Smith <roy@panix.com> - 2013-05-10 13:32 -0400
          Re: object.enable() anti-pattern Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-05-11 15:24 -0400
        Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-11 07:05 +0000
  Re: object.enable() anti-pattern Mark Janssen <dreamingforward@gmail.com> - 2013-05-08 19:53 -0700
  Re: object.enable() anti-pattern Mark Janssen <dreamingforward@gmail.com> - 2013-05-08 19:56 -0700
  Re: object.enable() anti-pattern Wayne Werner <wayne@waynewerner.com> - 2013-05-09 06:08 -0500
    Re: object.enable() anti-pattern Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-09 11:51 +0000
    Re: object.enable() anti-pattern Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2013-05-10 11:43 +1200
      Re: object.enable() anti-pattern Michael Speer <knomenet@gmail.com> - 2013-05-09 20:18 -0400
        Re: object.enable() anti-pattern Roy Smith <roy@panix.com> - 2013-05-09 20:50 -0400
      Re: object.enable() anti-pattern Wayne Werner <wayne@waynewerner.com> - 2013-05-12 12:14 -0500
      Re: object.enable() anti-pattern Terry Jan Reedy <tjreedy@udel.edu> - 2013-05-12 16:03 -0400
      Re: object.enable() anti-pattern Greg Ewing <greg.ewing@canterbury.ac.nz> - 2013-05-13 11:18 +1200
      Re: object.enable() anti-pattern Fábio Santos <fabiosantosart@gmail.com> - 2013-05-13 07:32 +0100
      Re: object.enable() anti-pattern Chris Angelico <rosuav@gmail.com> - 2013-05-13 17:36 +1000
      Re: object.enable() anti-pattern Fábio Santos <fabiosantosart@gmail.com> - 2013-05-13 09:09 +0100
      Re: object.enable() anti-pattern Wayne Werner <wayne@waynewerner.com> - 2013-05-13 08:15 -0500

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