python: Default whence argument for Data() to SEEK_SET.

* lang/python/gpgme.i: copied signature from gpgme.h and defaulted the
value to SEEK_SET.
* lang/python/tests/t-data.py: Added a test for no second argument
--

Having to import the os package when wanting to read a Data object is a
slight annoyance. With SWIG, we can define default parameters. This
change defaults the whence argument to SEEK_SET which is how StringIO
and BytesIO behave.

Signed-off-by: Tobias Mueller <muelli@cryptobitch.de>

lang/python/gpgme.i

@@ -557,6 +557,11 @@ gpgme_error_t gpgme_op_keylist_start (gpgme_ctx_t ctx,
                       const char *pattern="",
                       int secret_only=0);
 
+/* The whence argument is surprising in Python-land,
+   because BytesIO or StringIO objects do not require it.
+   It defaults to SEEK_SET. Let's do that for Data objects, too */
+off_t gpgme_data_seek (gpgme_data_t dh, off_t offset, int whence=SEEK_SET);
+
 /* Include the unmodified <gpgme.h> for cc, and the cleaned-up local
    version for SWIG.  We do, however, want to hide certain fields on
    some structs, which we provide prior to including the version for

lang/python/tests/t-data.py

@@ -51,6 +51,12 @@ data.write(b'Hello world!')
 data.seek(0, os.SEEK_SET)
 assert data.read() == b'Hello world!'
 
+data = gpg.Data()
+data.write(b'Hello world!')
+# We expect the second argument to default to SEEK_SET
+data.seek(0)
+assert data.read() == b'Hello world!'
+
 binjunk = bytes(range(256))
 data = gpg.Data()
 data.write(binjunk)