Adds ability to pass a file descriptor when opening a new Reader() object

[nkinkade]

Hello!

This PR adds the ability to pass a file descriptor (or really any file-like object) to open_database().

The general use case for these changes is situations where the MaxMind database is not located on the local filesystem, and the database has to be fetched from some remote resource. In these cases, one might not be able to pass a string file path to open_database().

The specific use case that prompted these changes is our desire to use the GeoLite2-City database in a Google App Engine environment. The standard GAE environment does not allow static files larger than 32MB to be included in the app's codebase, which gets pushed to GAE. The GeoLite2-City database file is currently around 55MB. To get around this, we could store the file in Google Cloud Storage, or could even fetch it directly from maxmind.com at runtime. To compound the issue, the GAE environment does not have a wriable filesystem, so we can't just download the file, then pass the local path to open_database(). What we can do, however, is to read the database across the network into a file-like object, then pass that to open_database().

Regarding unit testing, what I've done is a little convoluted. I went this route, though, because I noticed that the current testing runs every test in BaseTestReader() for every possible mode. Since the majority of tests include passing a string path to open_database(), and since I wanted to keep the same testing patterns, I opted to bring in mock to patch the open_database() function as needed. This may be overkill and ugly. A simpler alternative might be something like:

class TestFDReader(unittest.TestCase):
    mode = MODE_FD
    readerClass = [maxminddb.reader.Reader]

    def test_mode_fd_returns_valid_reader(self):
        with open(<test_db_path>) as mmdb_fd:
            reader = open_database(mmdb_fd, mode)
            self.assertIsInstance(reader, maxminddb.reader.Reader)

Any feedback or thoughts on these changes?

Thanks,

Nathan

[coveralls]

Coverage increased (+0.1%) to 95.192% when pulling 05c95bd on nkinkade:file-descriptor into 893403f on maxmind:master.

[nkinkade]

Ugh. I see Python 3.x tests are broken now. Looking into it now.

[oschwald]

Looks great! 💯 I just have one small request on the documentation of the new mode.

[oschwald]

Could you document that the caller is responsible for closing the file descriptor and that it may be closed immediately after creation of the reader?

[nkinkade]

Of course, and done! Since the various modes are briefly described in numerous places/files, I elected to add this bit of documentation to README.rst. Does this seem sufficient?

Also, if this PR gets merged, I've got another small one for the GeoIP2-python module to enable this feature in that wrapper as well.

[oschwald]

Yeah, that seems fine for now. Thanks!