There is an incompatibilty between StringIO and 
cStringIO, as demonstrated below..  Passing an
integer to StringIO.write is OK while
it not OK to pass one to cStringIO.

$ python
Python 2.2.2 (#4, Oct 15 2002, 04:21:28) 
[GCC 2.95.4 20011002 (Debian prerelease)] on linux2
Type "help", "copyright", "credits" or "license" for
more information.
>>> import StringIO
>>> sfile = StringIO.StringIO()
>>> sfile.write (100)
>>> print sfile.getvalue()
100
>>> import cStringIO
>>> cfile = cStringIO.StringIO()
>>> cfile.write (100)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: write() argument 1 must be string or
read-only buffer, not int
>>> cfile.write (str(100))

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This is by design. The file protocol makes no guarantee that
you can write objects other than strings into a file, unless
the object explicitly documents the support for other objects.

cStringIO differs in many ways from StringIO, this is but
one way.

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This seems like an unnecessary incompatibility,
however I guess it is there in the interest of speed?

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It's a bug that StringIO accepts it. file.write does not
support numbers, either. Unfortunately, this bug cannot be
fixed, for backwards compatibility. The conversion of any
argument to a string was originally introduced for Python
2.2, to support arbitrary buffer objects. That it also
accepts numbers now was an unexpected side effect.

Assigning to Marc-Andre, as he introduced that feature
originally.

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If this bug cannot be fixed then shouldn't this bug report be closed?

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Indeed.