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From: Peter Otten <__peter__@web.de>
Subject: Re: isinstance(.., file) for Python 3
Date: Thu, 08 Nov 2012 14:02:23 +0100
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Xref: csiph.com comp.lang.python:32947

Ulrich Eckhardt wrote:

> Hi!
> 
> I have two problems that are related and that I'd like to solve together.
> 
> Firstly, I have code that allows either a file or a string representing
> its content as parameter. If the parameter is a file, the content is
> read from the file. In Python 2, I used "isinstance(p, file)" to
> determine whether the parameter p is a file. In Python 3, the
> returnvalue of open() is of type _io.TextIOWrapper, while the built-in
> class file doesn't exist, so I can't use that code.
> 
> Secondly, checking for the type is kind-of ugly, because it means that I
> can't use an object that fits but that doesn't have the right type. In
> other words, it breaks duck-typing. This is already broken in the Python
> 2 code, but since I have to touch the code anyway, I might as well fix
> it on the way.
> 
> If possible, I'm looking for a solution that works for Pythons 2 and 3,
> since I'm not fully through the conversion yet and have clients that
> might use the older snake for some time before shedding their skin.
> 
> Suggestions?

In order of obviousness:
hasattr(p, "read")
not isinstance(p, str)
iter(p) is p

Or you change the interface

def f(*, contents=None, file=None):
    if contents is None:
        with open(file) as f:
            contents = f.read()
    ... # work with contents