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Re: Returning a custom file object (Python 3)

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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 ‘open’ implementation doesn't allow 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 = MyFile
>
> Also, in other modules, you can:
>
>    from myfile import open

Well, you can...

>>> import builtins
>>> builtins.open
<built-in function open>
>>> builtins.open = "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 = open("/tmp/dummy", "w", encoding="utf-8")
>>> f
<_io.TextIOWrapper name='/tmp/dummy' mode='w' encoding='utf-8'>
>>> f.__class__
<class '_io.TextIOWrapper'>
>>> f.__class__ = 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 = open
>>> def open(*args, **kw):
...   cls = kw.pop("use_class", None)
...   f = _orig_open(*args, **kw)
...   if cls is not None: f.__class__ = cls
...   return f
...
>>> f = open("/tmp/dummy", "w", encoding="utf-8", use_class=MyFile)
>>> f
<_io.TextIOWrapper name='/tmp/dummy' mode='w' encoding='utf-8'>
>>> f.demo()
Hello, world!

But again, I'm really not sure this is a good way to do things.

ChrisA

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Thread

Returning a custom file object (Python 3) Steven D'Aprano <steve@pearwood.info> - 2015-05-28 12:16 +1000
  Re: Returning a custom file object (Python 3) Ben Finney <ben+python@benfinney.id.au> - 2015-05-28 12:40 +1000
    Re: Returning a custom file object (Python 3) Marko Rauhamaa <marko@pacujo.net> - 2015-05-28 08:29 +0300
      Re: Returning a custom file object (Python 3) Chris Angelico <rosuav@gmail.com> - 2015-05-28 15:49 +1000
        Re: Returning a custom file object (Python 3) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-28 17:04 +1000
          Re: Returning a custom file object (Python 3) Chris Angelico <rosuav@gmail.com> - 2015-05-28 19:06 +1000
      Re: Returning a custom file object (Python 3) Gary Herron <gherron@digipen.edu> - 2015-05-27 22:56 -0700
        Re: Returning a custom file object (Python 3) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-28 16:52 +1000
  Re: Returning a custom file object (Python 3) Ben Finney <ben+python@benfinney.id.au> - 2015-05-28 12:34 +1000
  Re: Returning a custom file object (Python 3) Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2015-05-28 10:29 +0100

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