dictionary creation error

[ht-thomas]

BPO	46188
Nosy	@stevendaprano, @ht-thomas

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2021-12-28.10:12:39.779>
created_at = <Date 2021-12-28.10:00:02.916>
labels = ['type-bug', 'invalid']
title = 'dictionary creation error'
updated_at = <Date 2021-12-28.10:12:39.774>
user = 'https://github.com/ht-thomas'

bugs.python.org fields:

activity = <Date 2021-12-28.10:12:39.774>
actor = 'steven.daprano'
assignee = 'none'
closed = True
closed_date = <Date 2021-12-28.10:12:39.779>
closer = 'steven.daprano'
components = []
creation = <Date 2021-12-28.10:00:02.916>
creator = 'TobiasHT'
dependencies = []
files = []
hgrepos = []
issue_num = 46188
keywords = []
message_count = 2.0
messages = ['409251', '409252']
nosy_count = 2.0
nosy_names = ['steven.daprano', 'TobiasHT']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46188'
versions = []

[ht-thomas]

I was creating a dictionary when I noticed something weird.
I had a function responsible for creating dictionary keys and then values were assigned to the keys in a for loop.
The code goes a little like this:

>>> def key_maker(n):
...     if (n % 2) == 0:
...         return n
...     return None

>>> dictionary = {}
>>> for i in range(10):
...     dictionary[key_maker(i)] = i if key_maker(i) else "error"

>>> dictionary
{0: 'error', None: 'error', 2: 2, 4: 4, 6: 6, 8: 8}

when I tried to print out the rest of the values in the "else" clause,
I realized that 0 appeared there as well, so 0 appeared twice.

>>> for i in range(10):
...     dictionary[key_maker(i)] = i if key_maker(i) else print(i)
...
0
1
3
5
7
9
>>> dictionary
{0: 'error', None: 'error', 2: 2, 4: 4, 6: 6, 8: 8}

I still can not figure out why the first two elements are inconsistent
from the rest of the dictionary, and why they appear in the first place.

[stevendaprano]

I still can not figure out why the first two elements are inconsistent
from the rest of the dictionary, and why they appear in the first place.

Hi Tobias,

This is a bug tracker for reporting bugs in Python, not a help desk to ask for explanations. The behaviour you see is correct. Except for the last line, which is a copy-and-paste error on your part: the actual last line is

>>> dictionary
{0: None, None: None, 2: 2, 4: 4, 6: 6, 8: 8}

So there is no error here, everything is working as expected.

If you need help understanding why the code is correct, please take the discussion to the Python Discuss forum where you will be sure to get answers.