Re: [Python-ideas] Dict joining using + and +=
I’m having issues to understand the semantics of d1 + d2. I think mappings are more complicated than sequences it some things seems not obvious to me. What would be OrderedDict1 + OrderedDict2, in which positions would be the resulting keys, which value would be used if the same key is present in both? What would be defaultdict1 + defaultdict2? It seems to me that subclasses of dict are complex mappings for which « merging » may be less obvious than for sequences.
On 01/03/2019 14:06, Rémi Lapeyre wrote:
I’m having issues to understand the semantics of d1 + d2.
That's understandable, clouds of confusion have been raised. As far as I can tell it's pretty straightforward: d = d1 + d2 is equivalent to:
d = d1.copy() d.update(d2)
All of your subsequent questions then become "What does DictSubclassInQuestion.update() do?" which should be well defined. -- Rhodri James *-* Kynesim Ltd
Rémi Lapeyre schrieb am 01.03.19 um 15:06:
The only reasonable answer I can come up with is: 1) unique keys from OrderedDict1 are in the same order as before 2) duplicate keys and new keys from OrderedDict2 come after the keys from d1, in their original order in d2 since they replace keys in d1. Basically, the expression says: "take a copy of d1 and add the items from d2 to it". That's exactly what you should get, whether the mappings are ordered or not (and dict are ordered by insertion in Py3.6+).
What would be defaultdict1 + defaultdict2?
No surprises here, the result is a copy of defaultdict1 (using the same missing-key function) with all items from defaultdict2 added. Remember that the order of the two operands matters. The first always defines the type of the result, the second is only added to it.
It seems to me that subclasses of dict are complex mappings for which « merging » may be less obvious than for sequences.
It's the same for subclasses of sequences. Stefan
On 01/03/2019 14:06, Rémi Lapeyre wrote:
I’m having issues to understand the semantics of d1 + d2.
That's understandable, clouds of confusion have been raised. As far as I can tell it's pretty straightforward: d = d1 + d2 is equivalent to:
d = d1.copy() d.update(d2)
All of your subsequent questions then become "What does DictSubclassInQuestion.update() do?" which should be well defined. -- Rhodri James *-* Kynesim Ltd
Rémi Lapeyre schrieb am 01.03.19 um 15:06:
The only reasonable answer I can come up with is: 1) unique keys from OrderedDict1 are in the same order as before 2) duplicate keys and new keys from OrderedDict2 come after the keys from d1, in their original order in d2 since they replace keys in d1. Basically, the expression says: "take a copy of d1 and add the items from d2 to it". That's exactly what you should get, whether the mappings are ordered or not (and dict are ordered by insertion in Py3.6+).
What would be defaultdict1 + defaultdict2?
No surprises here, the result is a copy of defaultdict1 (using the same missing-key function) with all items from defaultdict2 added. Remember that the order of the two operands matters. The first always defines the type of the result, the second is only added to it.
It seems to me that subclasses of dict are complex mappings for which « merging » may be less obvious than for sequences.
It's the same for subclasses of sequences. Stefan
participants (3)
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Rhodri James -
Rémi Lapeyre -
Stefan Behnel