[Python-ideas] adding dictionaries

[Steven D'Aprano]

On Sun, Jul 27, 2014 at 09:34:16AM +1000, Alexander Heger wrote:

> Is there a good reason for not implementing the "+" operator for dict.update()?
[...]
> That is
> 
> B += A
> 
> should be equivalent to
> 
> B.update(A)

You're asking the wrong question. The burden is not on people to justify 
*not* adding new features, the burden is on somebody to justify adding 
them. Is there a good reason for implementing the + operator as 
dict.update? We can already write B.update(A), under what circumstances 
would you spell it B += A instead, and why?


> 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():
> 
> C = A + B
> 
> should equal to
> 
> C = dict(A)
> C.update(B)

That would be spelled C = dict(A, **B).

I'd be more inclined to enhance the dict constructor and update methods 
so you can provide multiple arguments:

dict(A, B, C, D)  # Rather than A + B + C  + D
D.update(A, B, C)  # Rather than D += A + B + C


> My apologies if this has been posted before but with a quick google
> search I could not see it; if it was, could you please point me to the
> thread?  I assume this must be a design decision that has been made a
> long time ago, but it is not obvious to me why.

I'm not sure it's so much a deliberate decision not to implement 
dictionary addition, as uncertainty as to what dictionary addition ought 
to mean. Given two dicts:

    A = {'a': 1, 'b': 1}
    B = {'a': 2, 'c': 2}

I can think of at least four things that C = A + B could do:

    # add values, defaulting to 0 for missing keys
    C = {'a': 3, 'b': 1, 'c': 2}

    # add values, raising KeyError if there are missing keys

    # shallow copy of A, update with B
    C = {'a': 2, 'b': 1, 'c': 2}  

    # shallow copy of A, insert keys from B only if not already in A
    C = {'a': 1, 'b': 1, 'c': 2}

Except for the second one, I've come across people suggesting that each 
of the other three is the one and only obvious thing for A+B to do.


-- 
Steven