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This is actually pilot error (not a bug!), although it's
subtle: Python uses the platform C I/O implementation, and
in standard C mixing reads with writes yields undefined
behavior unless a file-positioning operation (typically a
seek()) occurs between switching from reading to writing (or
vice versa); here from the C standard:
 When a file is opened with update mode (â€TM+â€TM as the
 second or third character in the above list of mode
 argument values), both input and output may be
 performed on the associated stream. However, output
 shall not be directly followed by input without an
 intervening call to the fflush function or to a file
 positioning function (fseek, fsetpos, or rewind), and
 input shall not be directly followed by output
 without an intervening call to a file positioning
 function, unless the input operation encounters
 end-of-file.
In other words, the result of running your sample code is
undefined: nothing is guaranteed about its behavior, which
both can and does vary across platforms.
If you want defined behavior, then, for example, add
>>> f.seek(0)
between your write() and read() calls.