In Python 2.7.8.10 running the following gives one result:

    >>> with open(r"C:\myfile.txt", "a+") as f:
    ...     f.tell()
    ...
    0L

But in Python 3.4.1.0 it gives a different result:

    >>> with open(r"C:\myfile.txt", "a+") as f:
    ...     f.tell()
    ...
    98

According to the man page for fopen, for a+ mode: "The initial file position for reading is at the beginning of the file." Source: http://linux.die.net/man/3/fopen