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Date: Thu, 28 May 2015 15:49:19 +1000
Subject: Re: Returning a custom file object (Python 3)
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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On Thu, May 28, 2015 at 3:29 PM, Marko Rauhamaa <marko@pacujo.net> wrote:
> Ben Finney <ben+python@benfinney.id.au>:
>
>> It seems the existing =E2=80=98open=E2=80=99 implementation doesn't allo=
w you to
>> override the type of object returned.
>
> The question is, can you assign to the builtin namespace. I'm guessing
> you can't.
>
> Within a module, you can simply do:
>
>    open =3D MyFile
>
> Also, in other modules, you can:
>
>    from myfile import open

Well, you can...

>>> import builtins
>>> builtins.open
<built-in function open>
>>> builtins.open =3D "not your grandfather's open() function"
>>> open
"not your grandfather's open() function"

... but I don't think replacing all of open() is what Steven has in
mind; it's a function that does a lot of work, most of which is what's
wanted.

Depending on how brutal you want to be, though, you _could_ hack around it.

>>> from io import TextIOWrapper
>>> class MyFile(TextIOWrapper):
...     def demo(self): print("Hello, world!")
...
>>> f =3D open("/tmp/dummy", "w", encoding=3D"utf-8")
>>> f
<_io.TextIOWrapper name=3D'/tmp/dummy' mode=3D'w' encoding=3D'utf-8'>
>>> f.__class__
<class '_io.TextIOWrapper'>
>>> f.__class__ =3D MyFile
>>> f.demo()
Hello, world!

This does appear to work. Whether or not it's a good idea is a
separate question. And of course, you could replace open() with
something like this:

>>> _orig_open =3D open
>>> def open(*args, **kw):
...   cls =3D kw.pop("use_class", None)
...   f =3D _orig_open(*args, **kw)
...   if cls is not None: f.__class__ =3D cls
...   return f
...
>>> f =3D open("/tmp/dummy", "w", encoding=3D"utf-8", use_class=3DMyFile)
>>> f
<_io.TextIOWrapper name=3D'/tmp/dummy' mode=3D'w' encoding=3D'utf-8'>
>>> f.demo()
Hello, world!

But again, I'm really not sure this is a good way to do things.

ChrisA