Dictionary of "structs" with functions as components?

[Roman Suzi]

On Sun, 29 Apr 2001, Eric Sandeen wrote:

>First a disclaimer - I'm pretty new to Python, and OOP is not really my
>bag, so forgive me if this is a silly question...

With Python OOP is everybody's bag, look below.

>I'm trying to create a data structure which holds information about
>several different filesystems - for example: ext2, reiserfs, and xfs. I'd
>like this to be easily extensible to add other filesystems, as well. The
>type of information I'm including is name, magic number, position of magic
>number in superblock, etc.  So far, I'm doing something like this (these
>values aren't right, just an example) :
>
>FSystems['ext2'].PrettyName = "ext2 filesystem"
>FSystems['ext2'].MagicNo = 0xce32
>FSystems['ext2'].MagicNoPos = 1080
>
>FSystems['xfs'].PrettyName = "XFS filesystem"
>FSystems['xfs'].MagicNo = "XFSB"
>FSystems['xfs'].MagicNoPos = 0

Why not to use objects?


class FS:
   def __init__(self, name, **ats):
     self.name = name
     self.__dict__.update(ats)   # ;-)

class FSystems:
   def __init__(self, list=None):
     self.fs = {}
     for k in list or []:
       self.fs[k.name] = k

FSystems([
  FS('xfs',
     PrettyName="XFS filesystem",
     MagicNo="XFSB",
     MagicNoPos=0,
     create=fs_creation("man xfs"),   # ;-)
     ),
  FS('ext2',
     PrettyName="ext2 filesystem",
     MagicNo=0xce32,
     MagicNoPos=1080,
     create=fs_creation("mkfs.ext2"),
     ),
])

(Of course, FSystems is not necessary: you can put FS(..)'s into dict  directly.)

>The problem comes when I'd like to define a unique function for each
>filesystem to actually create the filesystem, since this will vary quite a
>bit from fs to fs.  I'd like to access it via something like
>
>FSystems[<fstype>].unique_fs_function()

If I understood correctly, you need just this:

import os
def fs_creation(cmd):
  def template(keys="", os_cmd=cmd):
    os.system(os_cmd+" "+keys)
  return template

Of course, it's overkill.

You can simply do it like this:

def ext2_fs_creation(keys=""):
  return                          # to play safe
  os.system("mkfs.ext2 "+keys)    # error handling to be added!

And you will be able to use it like this:

  FS('ext2',
     PrettyName = "ext2 filesystem",
     MagicNo = 0xce32,
     MagicNoPos = 1080,
     create_fs = ext2_fs_creation,
     ),

And then you can call it:

a = FSystems.fs['ext2'].create("/dev/sda1")

>In C, I'd have a pointer to the function I want.  Is something like this
>possible in Python?

OK. Overall idea is that function is no different
from dictionary or other object in this respect: you can bind it
names you like and put it into containers you like!


Sincerely yours, Roman Suzi
-- 
_/ Russia _/ Karelia _/ Petrozavodsk _/ rnd at onego.ru _/
_/ Sunday, April 29, 2001 _/ Powered by Linux RedHat 6.2 _/
_/ "Macho does not prove Mucho." _/

Working listing:

#!/usr/bin/env python

class FS:
   def __init__(self, name, **ats):
     self.name = name
     self.__dict__.update(ats)   # ;-)

class FSystems:
   def __init__(self, list=None):
     self.fs = {}
     for k in list or []:
       self.fs[k.name] = k

import os
def fs_creation(cmd):
  def template(keys="", os_cmd=cmd):
    os.system("echo "+os_cmd+" "+keys)    # safety added!!!
  return template

# description must be compact:
myfss = FSystems([
  FS('xfs',
     PrettyName="XFS filesystem",
     MagicNo="XFSB",
     MagicNoPos=0,
     create=fs_creation("man xfs"),   # ;-)
     ),
  FS('ext2',
     PrettyName="ext2 filesystem",
     MagicNo=0xce32,
     MagicNoPos=1080,
     create=fs_creation("mkfs.ext2"),
     ),
  FS('ntfs',
     PrettyName="ntfs filesystem",
     MagicNo=0x0000,
     MagicNoPos=666,
     create=None, # except M$
     ),
])

myfss.fs['ext2'].create("/dev/sda1")