How can I create an array/list of dictionaries in python?

Question

I have a dictionary as follows:

{'A':0,'C':0,'G':0,'T':0}

I want to create an array with many dictionaries in it, as follows:

[{'A':0,'C':0,'G':0,'T':0},{'A':0,'C':0,'G':0,'T':0},{'A':0,'C':0,'G':0,'T':0},...]

This is my code:

weightMatrix = []
for k in range(motifWidth):
    weightMatrix[k] = {'A':0,'C':0,'G':0,'T':0}

But of course it isn't working. Can someone give me a hint? Thanks.

Does this answer your question? [Create list of single item repeated N times](https://stackoverflow.com/questions/3459098/create-list-of-single-item-repeated-n-times) — Georgy, Oct 31 '20 at 12:40

score 76 · Answer 1 · answered Nov 25 '12 at 11:03

76

This is how I did it and it works:

dictlist = [dict() for x in range(n)]

It gives you a list of n empty dictionaries.

answered Nov 25 '12 at 11:03

user1850980

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that wasn't what he asked... and it was already answered 2 years ago – yprez Nov 25 '12 at 11:21
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Yes, he should have asked a different question. – Derek Aug 22 '16 at 20:50
The BEST solution ever. Works for Python3. – Jeff Hu Mar 08 '17 at 16:13

score 32 · Answer 2 · edited Mar 07 '10 at 20:21

32

weightMatrix = [{'A':0,'C':0,'G':0,'T':0} for k in range(motifWidth)]

edited Mar 07 '10 at 20:21

Alex Martelli

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answered Mar 07 '10 at 20:12

dan04

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+1 because list comprehension is the way to go (more readable, elegant and concise than a loop of `.append` calls), even though the `k in motifWidth` in the original answer was an obvious, horrible bug (I've edited the Answer to fix that!-). – Alex Martelli Mar 07 '10 at 20:21
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@devon: No, they aren't. Note, for example, that if you run `weightMatrix[0]['A'] += 1`, then `weightMatrix[1]` still has all 0's in its dictionary values. The list comprehension works differently from `[{'A':0,'C':0,'G':0,'T':0}] * motifWidth`, which _would_ create multiple copies of the same `dict` object. – dan04 Apr 16 '19 at 19:18

score 21 · Answer 3 · answered Mar 07 '10 at 20:11

21

Use

weightMatrix = []
for k in range(motifWidth):
    weightMatrix.append({'A':0,'C':0,'G':0,'T':0})

answered Mar 07 '10 at 20:11

Tim Pietzcker

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score 12 · Answer 4 · answered Nov 15 '16 at 21:59

Minor variation to user1850980's answer (for the question "How to initialize a list of empty dictionaries") using list constructor:

dictlistGOOD = list( {} for i in xrange(listsize) )

I found out to my chagrin, this does NOT work:

dictlistFAIL = [{}] * listsize  # FAIL!

as it creates a list of references to the same empty dictionary, so that if you update one dictionary in the list, all the other references get updated too.

Try these updates to see the difference:

dictlistGOOD[0]["key"] = "value"
dictlistFAIL[0]["key"] = "value"

(I was actually looking for user1850980's answer to the question asked, so his/her answer was helpful.)

score 9 · Answer 5 · answered Jan 28 '18 at 07:49

Try this:

lst = []
##use append to add items to the list.

lst.append({'A':0,'C':0,'G':0,'T':0})
lst.append({'A':1,'C':1,'G':1,'T':1})

##if u need to add n no of items to the list, use range with append:
for i in range(n):
    lst.append({'A':0,'C':0,'G':0,'T':0})

print lst

score 4 · Answer 6 · answered Mar 07 '10 at 20:12

I assume that motifWidth contains an integer.

In Python, lists do not change size unless you tell them to. Hence, Python throws an exception when you try to change an element that isn't there. I believe you want:

weightMatrix = []
for k in range(motifWidth):
    weightMatrix.append({'A':0,'C':0,'G':0,'T':0})

For what it's worth, when asking questions in the future, it would help if you included the stack trace showing the error that you're getting rather than just saying "it isn't working". That would help us directly figure out the cause of the problem, rather than trying to puzzle it out from your code.

Hope that helps!

+1 I'd add the hint "Beware of `matrix = [{'A':0}, ...] * motifWidth` as this creates a list of items referencing the same object. — Johannes Charra, Mar 07 '10 at 20:18

score 0 · Answer 7 · answered May 24 '23 at 04:12

0

You can use import json, e.g.

weightMatrix = json.loads("""
[{"A":0,"C":0,"G":0,"T":0},
 {"A":0,"C":0,"G":0,"T":0},
 {"A":0,"C":0,"G":0,"T":0}]
""")

answered May 24 '23 at 04:12

Stephen Quan

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score -8 · Answer 8 · edited Nov 27 '15 at 08:17

-8

Dictionary:

dict = {'a':'a','b':'b','c':'c'}

array of dictionary

arr = (dict,dict,dict)
arr
({'a': 'a', 'c': 'c', 'b': 'b'}, {'a': 'a', 'c': 'c', 'b': 'b'}, {'a': 'a', 'c': 'c', 'b': 'b'})

edited Nov 27 '15 at 08:17

Jiri Tousek

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answered Nov 30 '13 at 14:42

fly_man

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First, you should never use `dict` as a variable name, and second, you're creating a tuple (not a list) that contains the *same* dictionary three times, so every change you make to one of them affects all the others. – Tim Pietzcker Dec 08 '14 at 10:26

How can I create an array/list of dictionaries in python?

8 Answers8