How to subclass file

[Gabriel Genellina]

En Wed, 14 May 2008 21:23:26 -0300, Yves Dorfsman <yves at zioup.com>  
escribió:

> I want to create a subclass of 'file' but need to open the file with  
> os.open
> (because I want to open it in exclusive mode), and need an additional  
> method.
>
> Because I need an additional method, I truly need a object of my sublass.
> If I do something like
>
> class myFile(file):
>
>   def __new__(cls, filename):
>     import os
>     fd = os.open(filename, os.O_WRONLY | os.O_CREAT | os.O_EXCL)
>    return os.fdoen(fd, 'w')
>
>   def myMethod(self):
>     do_something
>
>
> then x = myFile('somefilename') is of type file, not myFile, and  
> therefore
> does not have myMethod as a valid method.

Use delegation instead of inheritance.

 from __future__ import with_statement
import os

class myFile(object):
     __slots__ = ['_file']

     def __init__(self, filename):
         fd = os.open(filename, os.O_WRONLY | os.O_CREAT | os.O_EXCL)
         f = os.fdopen(fd, 'w')
         object.__setattr__(self, '_file', f)

     def __getattr__(self, name):
         return getattr(self._file, name)

     def __setattr__(self, name, value):
         setattr(self._file, name, value)

     def mymethod(self):
         print "anything"


<output>
py> regular_file = open(r"c:\\temp\\regular.txt", "wt")
py> special_file = myFile(r"c:\\temp\\special.txt")
py> print regular_file
<open file 'c:\\temp\\regular.txt', mode 'wt' at 0x00A411D0>
py> print special_file
<__main__.myFile object at 0x00A3D1D0>
py> print dir(regular_file)
['__class__', '__delattr__', '__doc__', '__enter__', '__exit__',  
'__getattribute
__', '__hash__', '__init__', '__iter__', '__new__', '__reduce__',  
'__reduce_ex__
', '__repr__', '__setattr__', '__str__', 'close', 'closed', 'encoding',  
'fileno'
, 'flush', 'isatty', 'mode', 'name', 'newlines', 'next', 'read',  
'readinto', 're
adline', 'readlines', 'seek', 'softspace', 'tell', 'truncate', 'write',  
'writeli
nes', 'xreadlines']
py> print dir(special_file)
['__class__', '__delattr__', '__doc__', '__getattr__', '__getattribute__',  
'__ha
sh__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__',  
'__re
pr__', '__setattr__', '__slots__', '__str__', '_file', 'mymethod']
py> with special_file:
...   special_file.write("hello\n")
...   special_file.mymethod()
...
anything
py> print "closed?", special_file.closed
closed? True
</output>

(note that __enter__/__exit__ -used by the with statement- work fine, even  
if not listed by dir(); also the "closed" attribute exists and is set  
correctly)

Note also that myFile is *not* a subclass of file:

py> isinstance(special_file, file)
False

but it has all methods and attributes of file objects, even if  
dir(special_files) doesn't list them. Duck typing in action - typical  
Python code should work fine with this myFile object instead of a true  
file object, but if you actually need a file subclass, I think you'll have  
to write a C extension.

-- 
Gabriel Genellina