When StringIO is instantiated with an intial string:
StringIO("Hello, world!")
it's read-only; the write methods don't exist.  The
Python Library Reference should mention this.

If you instantiate StringIO without an initial string:
StringIO()
you get both read and write methods.

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Did you import StringIO, or cStringIO as StringIO ?

This behaviour occurs only in cStringIO... if you do

m= cStringIO.StringIO("hello there")
n= cStringIO.StringIO()

you see that type(m) is cStringIO.StringI and n is 
cStringIO.StringO.

This is documented in the cStringIO module documentation.

 

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Did you import StringIO, or cStringIO as StringIO ?

This behaviour occurs only in cStringIO... if you do

m= cStringIO.StringIO("hello there")
n= cStringIO.StringIO()

you see that type(m) is cStringIO.StringI and n is 
cStringIO.StringO.

This is documented in the cStringIO module documentation.

 

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Yes, it's only cStringIO that does it.

However, the statenent about the type does not say anything
about which methods are available.  Many readers would
think, "I never have to compare the type to anything; this
doesn't apply to me."  How about a statement:

If you instantiate cStringIO.StringIO() without an argument,
the string is read-write.  But if you instantiate it with a
string argument: cStringIO.StringIO("abc"), it's read-only
and the write methods do not exist.  This applies only to
cStringIO; the corresponding objects in the StringIO module
are read-write in both cases.  [Then the paragraph about
InputType and OutputType.]

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I find your suggestion informative, appropriate and nice :)

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Fixed the docs.
See Doc/lib/libstringio.tex 1.7