public inbox for [email protected]  

help / color / mirror / Atom feed

From: Karsten Hilbert <[email protected]>
To: [email protected]
Subject: Re: iterating over DictRow
Date: Fri, 25 Sep 2020 09:48:20 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<AM0PR07MB4036EA8B58205084F30DA2AC87390@AM0PR07MB4036.eurprd07.prod.outlook.com>
	<[email protected]>
	<[email protected]>

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.

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

view thread (12+ messages)  latest in thread

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected]
  Subject: Re: iterating over DictRow
  In-Reply-To: <[email protected]>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox