Github user zero323 commented on the issue:

    https://github.com/apache/spark/pull/16537
  
    For me it is all about the bigger picture. I've been working with Python 
for quite a while right now (probably to long for my own good) and I am used to 
two things:
    
    - Language is relatively forgiving when it comes to types. I am more used 
to thinking about abstract base classes than concrete types.
    - Language which communicates failures in a clear way. If there is problem 
with incompatible types or interfaces I expect to receive clear feedback. And 
of course interactive debugger on top of that if something goes particularly 
wrong.
    
    PySpark is not there yet. We get Py4j exceptions (albeit it improved a lot 
in 2.x), we get runtime exceptions with huge JVM tracebacks, when it is 
possible to fail fast (on the driver), and finally we get silent errors (like 
returning `int` from and UDF with declared type `float`). 
    
    It is not always possible or practical to avoid these failures but I 
believe that in cases where:
    
    - We have very strict requirements regarding types.
    - Cost of checking is low (O(1) not for example O(N)).
    - We fail early and prevent an expensive failure in the middle of a 
pipeline.
    
    it is a good idea to be proactive.



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