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copied!<p>Checking <code>isinstance(something, io.IOBase)</code> only checks if <code>something</code> is an instance of an <code>io.IOBase</code> or a class derived from it — so I don't understand where you got the mistaken idea that it's the "correct" way to determine if an object is "file-like".</p> <p>A different way to do it is with an <em>Abstract Base Class</em>. Python has a number of <a href="http://docs.python.org/3/library/collections.abc.html?highlight=collections%20abstract%20base%20classes" rel="nofollow noreferrer">built-in ones</a>, but currently doesn't have one for "file-like" that could used with <code>isinstance()</code>. However you can define your own by using the <code>abc</code> module as outlined in <a href="http://www.python.org/dev/peps/pep-3119/#id34" rel="nofollow noreferrer">PEP 3119</a>.</p> <p><a href="http://pymotw.com/2" rel="nofollow noreferrer">The Python Module of the Week</a> webiste has a <a href="http://pymotw.com/2/abc" rel="nofollow noreferrer">good explanation</a> of using the <code>abc</code> module to do things like as this. And this highly rated <a href="https://stackoverflow.com/a/306222/355230">answer</a> to the question <a href="https://stackoverflow.com/questions/305359/correct-way-to-detect-sequence-parameter">Correct way to detect sequence parameter?</a> shows a similar way of defining your own ABC.</p> <p>To illustrate applying it to your case, you could define an ABC like this with all its methods abstract — thereby forcing derived classes to define all of them in order to be instantiated:</p> <pre><code>from abc import ABCMeta, abstractmethod class ABCFileLike(metaclass=ABCMeta): @abstractmethod def __init__(self): pass @abstractmethod def write(self, line): pass @abstractmethod def close(self): pass @abstractmethod def flush(self): pass </code></pre> <p>You could then derive your own concrete classes from it, making sure to supply implementations of all the abstract methods. (If you don't define them all, then a <code>TypeError</code> will be be raised if any attempts are made to instantiate it.)</p> <pre><code>class FileLike(ABCFileLike): """ Concrete implementation of a file-like class. (Meaning all the abstract methods have an implementation.) """ def __init__(self): pass def write(self, line): print("Written:", line) def close(self): pass def flush(self): pass print(isinstance(FileLike(), ABCFileLike)) # -> True </code></pre> <p>You can even add existing classes to it by registering them with the new metaclass:</p> <pre><code>import io print(isinstance(io.IOBase(), ABCFileLike)) # -> False ABCFileLike.register(io.IOBase) print(isinstance(io.IOBase(), ABCFileLike)) # -> True </code></pre>

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