[Python-ideas] Adding "+" and "+=" operators to dict

Ian Lee ianlee1521 at gmail.com
Wed Feb 11 08:21:20 CET 2015

I mentioned this on the python-dev list [1] originally as a +1 to someone
else suggesting the idea [2]. It also came up in a response to my post that
I can't seem to find in the archives, so I've quoted it below [3].

As the subject says, the idea would be to add a "+" and "+=" operator to
dict that would provide the following behavior:

>>> {'x': 1, 'y': 2} + {'z': 3}
{'x': 1, 'y': 2, 'z': 3}

With the only potentially non obvious case I can see then is when there are
duplicate keys, in which case the syntax could just be defined that last
setter wins, e.g.:

>>> {'x': 1, 'y': 2} + {'x': 3}
{'x': 3, 'y': 2}

Which is analogous to the example:

>>> new_dict = dict1.copy()
>>> new_dict.update(dict2)

With "+=" then essentially ending up being an alias for

I'd be happy to champion this as a PEP if the feedback / public opinion
heads in that direction.

[1] https://mail.python.org/pipermail/python-dev/2015-February/138150.html
[2] https://mail.python.org/pipermail/python-dev/2015-February/138116.html
[3] John Wong --

> Well looking at just list
> a + b yields new list
> a += b yields modified a
> then there is also .extend in list. etc.
> so do we want to follow list's footstep? I like + because + is more
> natural to read. Maybe this needs to be a separate thread. I am actually
> amazed to remember dict + dict is not possible... there must be a reason
> (performance??) for this...


~ Ian Lee
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