How to get `ReturnsNotZero` in CFEngine?

Question

There's ReturnsZero function in CFEngine but there's no ReturnsNotZero.

If I do need to define a class depending on non-zero return value of a command, what should I do?

Examples in CFEngine2 is preferred but CFEngine3 is fine :-)

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Example for CFEngine2

( which I don't think it can be found in any official document. It takes me about half an hour to find this trick in CFEngine2's source code )

classes:
    has_python27 = ( ReturnsZero(/usr/bin/which python27) )
    no_python27 = ( !ReturnsZero(/usr/bin/which python27) )

Hope this helps someone with lagacy CFEngine

Example for CFEngine3

Please read zzamboni's answer below :-)

Answer 1

With CFEngine 3, you can simply use not on the result from returnszero():

classes:
    "someclass" not => returnszero("/some/command");