With the 'dynamic' flag enabled the function that this flag provides doesn't work by default. It provides ability for dynamic playlists or autoDJ functions but by default when you press 'start dynamic playlist' nothing happens.

I found this - https://code.google.com/p/cantata/issues/detail?id=240&can=1&q=dynamic however just installing dev-perl/URI is not enough, dev-lang/perl needs to be compiled with 'ithreads' use flag enabled. The ebuild reflects none of this which is why, if you're lucky, it may work but most likely will not.

To reproduce:

emerge media-sound/cantata with dynamic enabled
run cantata, click on Dynamic on the left hand side (or go in to preferences and enable it to be shown if it is not there already).
Create a simple dynamic playlist.
Press the play button on the playlist you created or right click, start dynamic playlist.

What happens: nothing. It will clear the now playing list if you have something there but nothing will be left blank.

What is expected: it should load songs based on the rules you set up.

It can also be tested with:

/usr/share/cantata/scripts/cantata-dynamic start

for me it gave:

/usr/share/cantata/scripts/cantata-dynamic start
This Perl not built to support threads
Compilation failed in require at /usr/share/cantata/scripts/cantata-dynamic line 31.
BEGIN failed--compilation aborted at /usr/share/cantata/scripts/cantata-dynamic line 31

After enabling ithreads in perl I then got:

Can't locate URI/Escape.pm in @INC (you may need to install the URI::Escape module) (@INC contains: /etc/perl /usr/local/lib64/perl5/5.20.1/x86_64-linux-thread-multi /usr/local/lib64/perl5/5.20.1 /usr/lib64/perl5/vendor_perl/5.20.1/x86_64-linux-thread-multi /usr/lib64/perl5/vendor_perl/5.20.1 /usr/local/lib64/perl5 /usr/lib64/perl5/vendor_perl /usr/lib64/perl5/5.20.1/x86_64-linux-thread-multi /usr/lib64/perl5/5.20.1 .) at /usr/share/cantata/scripts/cantata-dynamic line 33.
BEGIN failed--compilation aborted at /usr/share/cantata/scripts/cantata-dynamic line 33.

Then, after emerging dev-perl/URI, it finally works as it should. Again, this is not mentioned in the ebuild and as far as I can see the flag doesn't have any requirements or dependencies attached to it.