Subject: Re: iterating over DictRow
To: Karsten Hilbert <Karsten.Hilbert@gmx.net>, psycopg@lists.postgresql.org
References: <20200923215444.GA25406@hermes.hilbert.loc>
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 <9f9f7536-9d5f-d95b-b1eb-eed7ec1b83f6@aklaver.com>
 <20200925074820.GC2138@hermes.hilbert.loc>
From: Adrian Klaver <adrian.klaver@aklaver.com>
Message-ID: <4975719e-8981-c2ff-1408-c348730b31c5@aklaver.com>
Date: Fri, 25 Sep 2020 09:06:43 -0700
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On 9/25/20 12:48 AM, Karsten Hilbert wrote:
> Adrian,
> 
> thanks for tracing my misunderstanding. I should have
> confirmed my suspicion here:
> 
>> https://github.com/psycopg/psycopg2/blob/fbba461052ae6ebc43167ab69ad91cadb7914c83/lib/extras.py
> 
>> class DictRow(list):
> 
>> 	def __getitem__(self, x):
>>          	if not isinstance(x, (int, slice)):
>>              		x = self._index[x]
>> 	        return super(DictRow, self).__getitem__(x)
>>
>> So if the value passed to __getitem__() is a integer or slice it does a list
>> index.
> 
> Indeed. I wonder whether that should be mentioned in the
> psycopg2 docs somewhere as it might be considered to violate
> the Principle Of Least Astonishment.
> 
> Or rather, this issues seems unfortunate fallout from python3:
> 
> In py2 one *had* to do DictRow.keys() to iterate over the
> keys. In py3
> 
> 	for key in DictRow:
> 
> is the suggested idiom for that which, however, iterates over
> DictRow as a list (as it always did).
> 
> DictRow.keys() still exists on dicts in py3 (and is not
> deprec(i?)ated to my knowledge) but now returns a memoryview
> (dict_keys, that is) rather than a list, which brings with it
> its own set of issues (dict and keys "list" are not
> independant objects anymore).
> 
> So, neither using py2's
> 
> 	for key in DictRow.keys():
> 
> under py3 nor changing to py3's
> 
> 	for key in DictRow:			# beep: variable wrongly named
> 
> leads to fully equivalent code. So this is a py2/py3 Gotcha
> in psycopg2.

Well you can do, borrowing from previous example:

for ky in r0._index:
     print(ky)

line_id
category
cell_per
ts_insert
ts_update
user_insert
user_update
plant_type
season
short_category

for ky in r0._index:
     print(r0[ky])

5
H PREM 3.5
18
None
2004-06-02 15:11:26
None
postgres
herb
none
HP3


Where _index is a substitute for *.keys().


> 
> Not that I complain, but worth a mention in the DictRow docs
> somewhere ?
> 
> Karsten
> --
> GPG  40BE 5B0E C98E 1713 AFA6  5BC0 3BEA AC80 7D4F C89B
> 
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com