CODEWITHSUNDEEP
python-3.x
AE-H
AE-H

Reputation: 23

Problem with the order of dictionary entries

I was showing off some python data types and their functions and attributes about them and one of them was a dictionary and I wanted to reassign the value at the first key and it did reassign it but it also moved the key and its value to a different position and I can't figure out why

I tried this on the Python shell and it gave me the expected result but when I was trying it on GDB online debugger, it gave me the unexpected and wrong results.

This is the snippet of the code that's acting unexpectedly:

a=[1, 2, 3, 3, 4]
b={'f':1, 's':2, 't':3, 'fo':4}
b['f']=0
a[0]=0
print(a, b)

What I expected was:

[1, 2, 3, 3, 4] {'f':0, 's':2, 't':3, 'fo':4}

What actually came out:

[0, 2, 3, 3, 4] {'s':2, 't':3, 'fo':4, 'f':0}

Upvotes: 1

Views: 48

Answers (2)

0x5453
0x5453

Reputation: 13589

In Python <= 3.6, dicts are unordered. You should not rely on the ordering of their elements.

In Python >= 3.7, dicts are insertion-ordered. Updating existing values in the dict should not affect the order.

In all versions >= 2.7, you can use collections.OrderedDict if you need stable ordering.

Based on the behavior you described, I would bet that the GDB online version of Python is older than your local version, or you just got lucky when running locally.

Upvotes: 2

Mureinik
Mureinik

Reputation: 311188

Dictionaries in python don't have any guarantee about their order, so any order you see is purely coincedental. If you want to use a dictionary that preserves the order of elements, you could use an OrderedDict:

from collections import OrderedDict
b = OrderedDict((('f', 1), ('s', 2), ('t', 3), ('fo', 4)))

Upvotes: 0

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