How to create a list of dictionaries in python?

Question

I am trying to make a list of dictionaries in python. Why don't these three methods produce the same results?

A = [{}]*2
A[0]['first_name'] = 'Tom'
A[1]['first_name'] = 'Nancy'
print A

B = [{},{}]
B[0]['first_name'] = 'Tom'
B[1]['first_name'] = 'Nancy'
print B


C = [None]*2
C[0] = {}
C[1] = {}
C[0]['first_name'] = 'Tom'
C[1]['first_name'] = 'Nancy'
print C

this is what I get:

[{'first_name': 'Nancy'}, {'first_name': 'Nancy'}]
[{'first_name': 'Tom'}, {'first_name': 'Nancy'}]
[{'first_name': 'Tom'}, {'first_name': 'Nancy'}]

In the first case you are creating a list with two elements referenceing the same dict. — Jan Christoph Terasa
– Jan Christoph Terasa, Commented May 6, 2019 at 16:59
What do you expect? Dictionaries in Python are unordered. If you want to keep the keys ordered, take a look on collections.OrderedDict(). — accdias
– accdias, Commented May 6, 2019 at 17:04

Barmar · Accepted Answer · 2019-05-06 16:58:58Z

3

Your first method is only creating one dictionary. It's equivalent to:

templist = [{}]
A = templist + templist

This extends the list, but doesn't make a copy of the dictionary that's in it. It's also equivalent to:

tempdict = {}
A = []
A.append(tempdict)
A.append(tempdict)

All the list elements are references to the same tempdict object.

answered May 6, 2019 at 16:58

Barmar

789k57 gold badges555 silver badges669 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Kevin Hoopes Over a year ago

That makes sense. Thank you!

user6035995 · Accepted Answer · 2019-05-06 17:44:32Z

0

Barmar has a very good answer why. I say you how to do it well :)

If you want to generate a list of empty dicts use a generator like this:

A = [{}for n in range(2)]
A[0]['first_name'] = 'Tom'
A[1]['first_name'] = 'Nancy'
print (A)

answered May 6, 2019 at 17:44

user6035995

Collectives™ on Stack Overflow

How to create a list of dictionaries in python?

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related