SUMMARY
This article offers some clarification on parameter expressions and how you can add a custom object to them through a custom function example.
MORE INFORMATION
Parameter expressions only support Python expressions rather than full Python scripts.
These are analogous to the body of a lambda, or the right-hand side of a variable assignment. You can't use ordinary if/else blocks or loops, but you could use ternary operators and list comprehensions.
You can register custom objects that you can then use within a parameter expression. Here's an example showing how you can register a custom function and use this in a parameter expression:
In the Python tab, define and register your function like so:
def triple(value):
return value * 3
NodegraphAPI.SetExpressionGlobalValue("triple", triple)
Then from a parameter expression, you could use the function like so:
triple(5) + 1
The parameter's value would then be 16. The important thing to note is that your expression must evaluate to some value rather than return some value.
The purpose of a parameter expression is to compute a parameter's value programmatically. A parameter expression should not make any modifications to your project as part of its evaluation.
If you need to drive the values of many parameters, consider using a Shelf Script or a Script Button, for which you can find more information in our Katana User Guide's chapters on Shelf Item Scripts, User Parameters, and Widget Types.
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