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