I was a bit surprised when I discovered that StringIO
didn't have a __str__() method, equivalent to
getvalue(). Calling str(stringio_object) gives the
following:

>>> import StringIO
>>> s = StringIO.StringIO("hello world")
>>> print "%s" % (str(s))
<StringIO.StringIO instance at 0x401ef08c>

I had some (perhaps dodgy code) code that did:

if isinstance(data, types.FileType):
  return data.read()
else:
  return str(data)

Since StringIO doesn't subclass any file type and
doesn't implement a __str__() method, I was getting
seemingly bogus results. This was trivially worked
around by adding another "isinstance(data,
StringIO.StringIO)" case, but it was surprising
nonetheless. Thanks.

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This isn't a bug.  The module is correctly emulating the API
for file objects.

I recommend replacing your isintance() test with a try/except:

try:
.... return data.read()
except AttributeError:
.... return str(data)

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The StringIO API diverges from the file API already. It adds
API (getvalue()) doesn't implement the encoding, mode, name,
newlines, or softspace attributes, and implements isatty()
in violation of the section 2.3.9 docs.

In some contexts, it may look like a file, but it isn't a
file. And I don't see how implementing __str__() changes
that. It's equivalent to saying that Java's StringBuffer
class shouldn't override toString()...

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The StringIO API diverges from the file API already. It adds
API (getvalue()) doesn't implement the encoding, mode, name,
newlines, or softspace attributes, and implements isatty()
in violation of the section 2.3.9 docs.

In some contexts, it may look like a file, but it isn't a
file. And I don't see how implementing __str__() changes
that. It's equivalent to saying that Java's StringBuffer
class shouldn't override toString()...

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user_id=69417

Since section 2.3.9 of the docs doesn't mention anything
about an overriden meaning of FILE.__str__(), its meaning in
that API is undefined. "Correctly emulating" undefined API
is impossible.