[GitHub] [beam] damccorm opened a new issue, #20942: Wrong process function arg order cause unexpected runtime value shift

[GitBox <gi...@apache.org>]

damccorm opened a new issue, #20942:
URL: https://github.com/apache/beam/issues/20942

   if kw args of process function are declared before DnFnParam placeholders the fulfillment will shift to wrong index unexpectedly.
   
   For example if process has signature
   ```
   
    def process(element, an_arg='x', w=DoFn.WindowParam):
      window_start_1 = an_arg.start // this is
   ok, though unexpectedly
      window_start_2 = w.start // this is not ok, also unexpectedly, value of
   w is shifted onto an_arg, and w is DoFn.WindowParam, it throws exception _DoFnParam has no attribute
   'start'.
      
      ...
   ```
   
   at runtime the window param will be mapped onto 'an_arg' instead of w, and w will not be replaced with runtime value, causing confusing behaviors. (e.g. an_arg will be an instance of GlobalWindow or FixedWindow but w is still DoFn.WindowParam).
   
   Imported from Jira [BEAM-12446](https://issues.apache.org/jira/browse/BEAM-12446). Original Jira may contain additional context.
   Reported by: yichi.


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