Issue 33572: Better document mixed-type comparison of set items, dict keys (original) (raw)
Created on 2018-05-18 15:19 by Janusz Harkot, last changed 2022-04-11 14:59 by admin.
Messages (6)
Author: Janusz Harkot (Janusz Harkot)
Date: 2018-05-18 15:19
using boolean (True/False) as dictionary keys, coerce them to integers - is this behavior documented somewhere?
I know that asking to fix this is not easy fix, but shouldn't this be highlighted everywhere with red flags and warnings, so people will know that this is expected?
Python 3.6.5 (default, Mar 29 2018, 03:28:50) [GCC 5.4.0 20160609] on linux Type "help", "copyright", "credits" or "license" for more information.
dta = {False: 'false', True: 'true', 0: 'zero', 1: 'one'} print(dta[False]) zero
Author: Janusz Harkot (Janusz Harkot)
Date: 2018-05-18 15:22
Python 3.6.5 (default, Mar 29 2018, 03:28:50) [GCC 5.4.0 20160609] on linux Type "help", "copyright", "credits" or "license" for more information.
dta = {False: 'false', True: 'true', 0: 'zero', 1: 'one'} print(dta[False]) zero dta {False: 'zero', True: 'one'}
Author: Mark Dickinson (mark.dickinson) * 
Date: 2018-05-18 15:26
It's documented here: https://docs.python.org/3/library/stdtypes.html#mapping-types-dict
Numeric types used for keys obey the normal rules for numeric comparison: if two numbers compare equal (such as 1 and 1.0) then they can be used interchangeably to index the same dictionary entry.
Since False == 0, False and 0 are interchangeable as dictionary keys. Similarly for True and 1. Note that the dictionary you created only actually has two entries:
dta = {False: 'false', True: 'true', 0: 'zero', 1: 'one'} dta {False: 'zero', True: 'one'}
Though calling out numeric types in particular in the docs does feel a little odd to me: the rule is that any two hashable objects that compare equal should be interchangeable for the purposes of dictionary lookup (or set membership, come to that). There's nothing particularly special about numbers in this context.
Author: Terry J. Reedy (terry.reedy) *
Date: 2018-05-18 18:13
Mark, are you suggesting a doc addition (and a change of Components) or should we close this as 'not a bug'?
Author: Tim Peters (tim.peters) *
Date: 2018-05-18 18:29
I expect these docs date back to when ints, longs, and floats were the only hashable language-supplied types for which mixed-type comparison could ever return True.
They could stand some updates ;-) fractions.Fraction and decimal.Decimal are more language-supplied numeric types that participate now, and the Boolean singletons are instances of (a subclass of) int. I think it would be good to point that out, especially the latter (in many other languages Booleans can't be compared to ints).
So +1 both to Mark's more-general explanation, and to explicitly naming more specific cases for illustration.
Author: Terry J. Reedy (terry.reedy) *
Date: 2018-05-18 19:44
With Tim's addition
from fractions import Fraction as F from decimal import Decimal as D s = {0, 1, 0.0, 1.0, F(0,1), F(1, 1), D(0), D(1), False, True} s {0, 1}
I think we should consider moving the main discussion of the general comparison and hashing principle and example to the set entry. (Sets ars simpler and are when people new to Python already know about.) Point out that for displays, the first of equals is kept and not replaced (this is also true of dicts). Then, in the dict entry, say that dict keys are treated like set items, give an equivalent dict example, and link to the discussion of set items.
x = None d = {0:x, 1:x, 0.0:x, 1.0:x, F(0,1):x, F(1, 1):x, D(0):x, D(1):x, False:x, True:x} d {0: None, 1: None}
History
Date
User
Action
Args
2022-04-11 14:59:00
admin
set
github: 77753
2018-05-18 19:44:10
terry.reedy
set
messages: +
title: False/True as dictionary keys treated as integers -> Better document mixed-type comparison of set items, dict keys
2018-05-18 19:21:44
terry.reedy
set
assignee: docs@python
components: + Documentation, - Interpreter Core
nosy: + docs@python
2018-05-18 18:29:27
tim.peters
set
nosy: + tim.peters
messages: +
2018-05-18 18:13:21
terry.reedy
set
nosy: + terry.reedy
messages: +
2018-05-18 15:26:48
mark.dickinson
set
nosy: + mark.dickinson
messages: +
2018-05-18 15:22:01
Janusz Harkot
set
messages: +
2018-05-18 15:19:50
Janusz Harkot
create