[Python-ideas] Dict joining using + and +=

Steven D'Aprano steve at pearwood.info
Fri Mar 1 00:49:56 EST 2019

On Thu, Feb 28, 2019 at 08:59:30PM -0800, Hasan Diwan wrote:

> Do we really need a "+" and a "-" operation on dictionaries?
> [dictinstance.update({k:v}) for k,v in dictinstance.items()] does handle
> merges already.

I don;t think that does what you intended. That merges dictinstance with 
itself (a no-op!), but one item at a time, so in the slowest, most 
inefficient way possible.

Writing a comprehension for its side-effects is an anti-pattern that 
should be avoided. You are creating a (potentially large) list of Nones 
which has to be created, then garbage collected.

> And I'm assuming that "-" should return the difference --
> set(d1.keys()) - set(d2.keys()), right?

No. That throws away the values associated with the keys.

P.S. As per Guido's ~~command~~ request *wink* I'm writing a PEP for 
this. I should have a draft ready later this evening.


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