[kovidgoyal]

metadata download runs in a worker process (MD plugins often leak memory), that worker process does not log to console. It's log is made available via the view log button in the metadata download dialog.

You can also run your metadata plugin code from the command line with logging to a text file by putting all the code in a single __init__.py file and adding a __name__ == '__main__' section and running it with calibre-debug -e as kiwidude describes. This is the best way to develop a MD plugin.

@kiwidude: The new plugin handling code is simply

calibre-customize -b /path/to/dir/with/plugin/source/code

which automatically zips up and installs/updates the plugin into calibre. So for example, if your metadata download plugin has multiple files, you can do, (in the plugin's directory):

calibre-customize -b . && calibre-debug -e __init__.py