Problem deriving a class from the built-in file object

[Peter Otten]

Pierre Rouleau wrote:

> I'm trying to extend the functionality of the file object by creating a
> class that derives from file.  MyFile class re-implements __init__(),
> write(), writelines() and close() to augment the capabilities of file.
> 
> All works fine, except for one thing:  'print >> myfile'  does not
> execute Myfile.write(), it executes the file.write().   If I execute
> myfile.write() explicitly, then Myfile.write() is called as expected.

As a workaround, you can use delegation instead of inheritance:

>>> class File(object):
...     def __init__(self, *args):
...             self.file = file(*args)
...     def __getattr__(self, name):
...             return getattr(self.file, name)
...     def write(self, s):
...             print "writing", s
...             self.file.write(s)
...
>>> f = File("tmp.txt", "w")
>>> for s in ["alpha", "beta", "gamma"]:
...     print >> f, s
...
writing alpha
writing

writing beta
writing

writing gamma
writing

>>> f.close()
>>> File("tmp.txt").read()
'alpha\nbeta\ngamma\n'
 
> I was not expecting that behaviour.  I though that 'print >> afileobject
> ' would execute the afileobject.write() as you can easily obtain by
> defining a simple file-like class that implements write() and writeline().
> 
> I am running Python 2.3.4.  Can't move to 2.4 yet.

Nothing has changed with 2.4 in that respect.
 
> Is it the expected behavior?

I certainly didn't expect it either when I saw it for the first time.

Peter