[Python-ideas] adding dictionaries

[Terry Reedy]

On 7/26/2014 7:34 PM, Alexander Heger wrote:
> Is there a good reason for not implementing the "+" operator for dict.update()?

As you immediate noticed, this is an incoherent request as stated. A op 
B should be a new object.

> A = dict(a=1, b=1)
> B = dict(a=2, c=2)
> B += A

Since "B op= A" is *defined* as resulting in B having the value of "B op 
A", with the operations possibly being done in-place if B is mutable, we 
would first have to define addition on dicts.

> B
> dict(a=1, b=1, c=2)
>
> That is
>
> B += A
>
> should be equivalent to
>
> B.update(A)
>
> It would be even better if there was also a regular "addition"
> operator that is equivalent to creating a shallow copy and then
> calling update():

You have this backwards. Dict addition would have to come first, and 
there are multiple possible and contextually useful definitions. The 
idea of choosing anyone of them as '+' has been rejected.

As indicated, augmented dict addition would follow from the choice of 
dict addition. It would not necessarily be equivalent to .update.  The 
addition needed to make this true would be asymmetric, like catenation.

But unlike sequence catenation, information is erased in that items in 
the updated dict get subtracted. Conceptually, update is replacement 
rather than just addition.

> My apologies if this has been posted

Multiple dict additions have been proposed and discussed here on 
python-ideas and probably on python-list.

-- 
Terry Jan Reedy