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I think that certain FileIO methods should raise IOError instead of
ValueError when a file operation is attempted with the wrong mode.
The methods of IOBase are documented to raise IOError in these situations.
>>> import io
>>> f = io.open("testfile", "w")
>>> f.read()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
IOError: not readable
>>> 
>>> f = io.FileIO("testfile", "w")
>>> f.read()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: File not open for reading

All of them should probably raise io.UnsupportedOperation instead (which inherits from both IOError and ValueError).

See #9293, where there's a patch.