I really like this idea.  Once you've already decided to raise an exception, does it really cost much to try to raise a more helpful one?  And helpful exception messages make programming a lot less painful, and a lot more of a joy.

On Tue, Oct 22, 2019 at 2:43 PM Mike Miller <python-ideas@mgmiller.net> wrote:

On 2019-10-18 10:23, Ricky Teachey wrote:
> but i'm -0 because i am very concerned it will not be obvious to new learners,
> without constantly looking it up, whether adding two mappings together would either:

The big trade off I'm gathering from this mega-thread is that the |, |=
operators are more accurate, but less obvious to newcomers, who will first try
+, += instead.

I've tried them in this order myself several times over the years.

Had an idea, why not choose the more accurate syntax: |, |= after all?  Then, to
help newcomers and forgetful pros a custom error message is implemented for +,
+=.  In pseudo C/Python, something like this:

     class dict:

         def __add__(self, other):

             if isinstance(other, dict):
                 raise TypeError(
                     'unsupported operand type(s) for +: … '
                     'Dictionary merging leads to last-value-wins data '
                     'loss. If acceptable, use the union "|" operator.'
                 raise TypeError(std_err_msg)

I think it is worth it to lead the newcomer to a moment's reflection on why
dictionary combining/merging is potentially lossy.  Everyone is informed with
the proper mental model, then on their way and left alone afterward.

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