string  usd_iprimvarnames(<stage>stage, string primpath)
This function returns the primvar names that are available directly on the given primitive or are inherited from primitive’s ancestors.
When running in the context of a node (such as a wrangle LOP), this argument can be an integer representing the input number (starting at 0) to read the stage from. The integer is equivalent to the string form referencing a particular input, e.g., “opinput:0”.
The path to the primitive.
String array containing the names of the primvars of the primitive and its ancestors.
// Get the primvar names from the primitive and its ancestors. string primvar_names = usd_iprimvarnames(0, "/geo/src_sphere");