Using 'mock_open' to unit test a code that uses the 'seek()' function for the file object does not work. This would be a great addition for mock_open functionality as this use case happens occasionally.

Test file contains:
def test_one(self):
    with patch("builtins.open", mock_open(read_data="#123")):
        script.main()

Example script:
def main():
    with open('some/file', 'r') as f:
        print(f.read(1))
        f.seek(0)
        print(f.read(1))

if __name__ == "__main__":
    main()

Output will be:
#
1

While the expected output is:
#
#

See also https://bugs.python.org/issue25690 . https://github.com/nivbend/mock-open has support for seek.

As stated in the other issue supporting all operations of a filesystem might add more complexity.