SDK - Compiler - Quoting the predicate operands (#2043)

Fixes https://github.com/kubeflow/pipelines/issues/1950

sdk/python/kfp/compiler/compiler.py

@@ -450,6 +450,9 @@ class Compiler(object):
         condition = sub_group.condition
         operand1_value = self._resolve_value_or_reference(condition.operand1, subgroup_inputs)
         operand2_value = self._resolve_value_or_reference(condition.operand2, subgroup_inputs)
+        if condition.operator in ['==', '!=']:
+          operand1_value = '"' + operand1_value + '"'
+          operand2_value = '"' + operand2_value + '"'
         task['when'] = '{} {} {}'.format(operand1_value, condition.operator, operand2_value)
 
       # Generate dependencies section for this task.

sdk/python/tests/compiler/testdata/coin.yaml

@@ -35,7 +35,7 @@ spec:
         - flip-again
         name: condition-2
         template: condition-2
-        when: '{{tasks.flip-again.outputs.parameters.flip-again-output}} == tails'
+        when: '"{{tasks.flip-again.outputs.parameters.flip-again-output}}" == "tails"'
       - arguments:
           parameters:
           - name: flip-output
@@ -119,7 +119,7 @@ spec:
         - flip
         name: condition-1
         template: condition-1
-        when: '{{tasks.flip.outputs.parameters.flip-output}} == heads'
+        when: '"{{tasks.flip.outputs.parameters.flip-output}}" == "heads"'
       - arguments:
           parameters:
           - name: flip-again-output
@@ -131,7 +131,7 @@ spec:
         - flip
         name: condition-3
         template: condition-3
-        when: '{{tasks.flip.outputs.parameters.flip-output}} == tails'
+        when: '"{{tasks.flip.outputs.parameters.flip-output}}" == "tails"'
       - name: flip
         template: flip
     name: pipeline-flip-coin

sdk/python/tests/compiler/testdata/recursive_do_while.yaml

@@ -75,7 +75,7 @@ spec:
         - flip-3
         name: condition-2
         template: condition-2
-        when: '{{tasks.flip-3.outputs.parameters.flip-3-output}} == heads'
+        when: '"{{tasks.flip-3.outputs.parameters.flip-3-output}}" == "heads"'
       - dependencies:
         - print
         name: flip-3

sdk/python/tests/compiler/testdata/recursive_while.yaml

@@ -97,7 +97,7 @@ spec:
             value: '{{inputs.parameters.maxval}}'
         name: condition-2
         template: condition-2
-        when: '{{inputs.parameters.flip-output}} == heads'
+        when: '"{{inputs.parameters.flip-output}}" == "heads"'
     inputs:
       parameters:
       - name: flip-output