CODEWITHSUNDEEP
pythondictionary
user2966752
user2966752

Reputation: 3

How to update values for specific keys in dict in Python

I have created a dictionary by using this:

a = dict.fromkeys([round(x*0.1,1) for x in range(10)], [0,0])

it will give me the following results:

>>> a
{0.0: [0, 0], 0.5: [0, 0], 0.2: [0, 0], 0.4: [0, 0], 0.8: [0, 0], 0.6: [0, 0], 0.3: [0, 0], 0.1: [0, 0], 0.9: [0, 0], 0.7: [0, 0]}

I only want to update the second value for key=0.5, for example. I was using the following code:

a[0.5][1]=a[0.5][1]+10

However, it turns out that it updated all second values for all keys.

>>> a
{0.0: [0, 10], 0.5: [0, 10], 0.2: [0, 10], 0.4: [0, 10], 0.8: [0, 10], 0.6: [0, 10], 0.3: [0, 10], 0.1: [0, 10], 0.9: [0, 10], 0.7: [0, 10]}

I'm wondering if there is a way to do that?

Many thanks in advance!

Upvotes: 0

Views: 2544

Answers (4)

Ashwini Chaudhary
Ashwini Chaudhary

Reputation: 250951

Don't use dict.fromkeys with a mutable value, it simply copies the reference to the same list object to all the keys. So, changing any one the reference is going to affect all the lists.

>>> d = dict.fromkeys('abcdef', [])
>>> [id(x) for x in d.values()]
[164654156, 164654156, 164654156, 164654156, 164654156, 164654156]

Use a dict comprehension instead:

>>> d = {k:[] for k in  'abcdef'}
>>> [id(x) for x in d.values()]
[164621484, 164653580, 164331340, 164653804, 164653900, 164653836]

For your code it is going to be:

a = {round(x*0.1,1): [0, 0] for _ in range(10)}

Upvotes: 4

Pedro Werneck
Pedro Werneck

Reputation: 41898

You're using the same list instance as value for all keys. Instead of:

a=dict.fromkeys([round(x*0.1,1) for x in range(10)], [0,0])

Initialize it like this:

a=dict((round(x*0.1,1), [0, 0]) for x in range(10))

Upvotes: 0

Mark Reed
Mark Reed

Reputation: 95252

All of the dictionary entries reference the same list. You would be better off creating the dictionary differently:

a = {}
for x in range(10):
    a[round(x*0.1,1)] = [0,0]

Upvotes: 0

Eevee
Eevee

Reputation: 48546

That because you used the same list for every value in your dict.

Don't use fromkeys with a mutable value. Use a dict comprehension:

a = {round(x*0.1, 1): [0, 0] for x in range(10)}

Or, if you're stuck with 2.6, a fake dict comprehension:

a = dict((round(x*0.1, 1), [0, 0]) for x in range(10))

Upvotes: 0

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