Detecting if a Node's Input has an Alpha Channel

The following expressions will check if the first input of a Node has a channel beginning with an "a".  Obviously not 100% foolproof, but should get you started.

TCL Expression:

[string first ".a" [channels [input this 0]]]>-1

Python Expression:

[python "len(\[n for n in nuke.channels(nuke.thisNode().input(0)) if n.find(\".a\") != -1])>0"]