Shuffle : Improve ChannelPlug compatibility

And add test demonstrating that expressions can still be made using the old names for plugs. This is necessary for an internal expression created by a node at Cinesite.

Changes.md

@@ -22,6 +22,7 @@ Fixes
   - Fixed loading of 2 channel images [^1].
   - Fixed error message to include filename [^1].
 - Expression : `setExpression()` now respects configs that provide backwards compatibility for old plug names.
+- Shuffle : Fixed default name for plugs constructed via the legacy `ChannelPlug( out, in )` constructor [^1].
 
 API
 ---

python/GafferImageTest/ShuffleTest.py

@@ -412,5 +412,25 @@ def testIgnoreMissingSourceDoesnCreateChannels( self ) :
 		shuffle["missingSourceMode"].setValue( shuffle.MissingSourceMode.Ignore )
 		self.assertEqual( shuffle["out"].channelNames(), IECore.StringVectorData( [ "R", "G", "B", "A" ] ) )
 
+	def testLegacyChannelPlugConstructor( self ) :
+
+		p = GafferImage.Shuffle.ChannelPlug( "R", "R" )
+		self.assertEqual( p.getName(), "channel" )
+
+	def testCreateExpressionWithLegacyNames( self ) :
+
+		script = Gaffer.ScriptNode()
+		script["shuffle"] = GafferImage.Shuffle()
+		script["shuffle"]["shuffles"].addChild( GafferImage.Shuffle.ChannelPlug( "R", "R" ) )
+		script["shuffle"]["shuffles"].addChild( GafferImage.Shuffle.ChannelPlug( "G", "G" ) )
+
+		script["expression"] = Gaffer.Expression()
+		script["expression"].setExpression(
+			'parent["shuffle"]["channels"]["channel"]["in"] = "X"; parent["shuffle"]["channels"]["channel1"]["in"] = "Y"'
+		)
+
+		self.assertEqual( script["shuffle"]["shuffles"][0]["source"].getValue(), "X" )
+		self.assertEqual( script["shuffle"]["shuffles"][1]["source"].getValue(), "Y" )
+
 if __name__ == "__main__":
 	unittest.main()

startup/GafferImage/shuffleCompatibility.py

@@ -46,6 +46,7 @@ def __init__( self, *args, **kw ) :
 			and isinstance( args[0], str ) and isinstance( args[1], str )
 		) :
 			Gaffer.ShufflePlug.__init__( self, args[1], args[0] )
+			self.setName( "channel" )
 		else :
 			Gaffer.ShufflePlug.__init__( self, *args, **kw )