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

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.

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

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!

Use

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

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.)

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

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.

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'})