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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 (’+’ 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.