...
The metadata (the headers) which describe the structure of the data are always available. Therefore the keys in these sections are available accessible as soon as you get a message handle (e.g. via the Python function codes_bufr_new_from_file())
To access the data section, you need a special directive which instructs ecCodes to do the actual unpacking and decoding of all the data elements. This is done by setting the special key "unpack" to 1.
Here is an example of decoding only the header of a BUFR message.
And here we have an example of decoding all the data (header + data).
...