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Yes, the exact behaviour depends on multiple aspects.
You should follow the C library conventions: 
http://www.cplusplus.com/reference/clibrary/cstdio/fopen.html
"""
For the modes where both read and writing (or appending) are allowed
(those which include a "+" sign), the stream should be flushed (fflush)
or repositioned (fseek, fsetpos, rewind) between either a reading
operation followed by a writing operation or a writing operation
followed by a reading operation.
"""
In your case, I suggest a call to fp.seek(0, os.SEEK_CUR) before you
start writing data. And a fp.flush() after, in case you want to read again.
Python 3.0 has a completely new I/O implementation, which may have its
own problems, but hopefully the same on all platforms. And it happens to
do the right thing in your example.