[Python-ideas] PEP: Dict addition and subtraction

[James Lu]

On Mar 4, 2019, at 11:25 AM, Steven D'Aprano <steve at pearwood.info> wrote:

>> How many situations would you need to make a copy of a dictionary and 
>> then update that copy and override old keys from a new dictionary?
>> 
> 
> Very frequently.
> 
> That's why we have a dict.update method, which if I remember correctly, 
> was introduced in Python 1.5 because people were frequently re-inventing 
> the same wheel:
> 
>    def update(d1, d2):
>        for key in d2.keys():
>            d1[key] in d2[key]
> 
> 
> You should have a look at how many times it is used in the standard 
> library:
> 
> [steve at ando cpython]$ cd Lib/
> [steve at ando Lib]$ grep -U "\.update[(]" *.py */*.py | wc -l
> 373
> 
> Now some of those are false positives (docstrings, comments, non-dicts, 
> etc) but that still leaves a lot of examples of wanting to override old 
> keys. This is a very common need. Wanting an exception if the key 
> already exists is, as far as I can tell, very rare.
It is very rare when you want to modify an existing dictionary. It’s not rare at all when you’re creating a new one.
> 
> It is true that many of the examples in the std lib involve updating an 
> existing dict, not creating a new one. But that's only to be expected: 
> since Python didn't provide an obvious functional version of update, 
> only an in-place version, naturally people get used to writing 
> in-place code.
My question was “How many situations would you need to make a copy of a dictionary and then update that copy and override old keys from a new dictionary?” Try to really think about my question, instead of giving answering with half of it to dismiss my point.
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