List of dictionaries

Question

Iam trying to create a list of dictionaries, but seems to me, that i am doing something wrong:

So, I have a list of tuples, something like this:

dict = {}
lst = []
cats = [(u'cat1', u'Matilda'),(u'cat2', u'Mew')]
for line in cats:
    dict['cat_num'] = line[0]
    dict['name'] = line[1]
    lst.append(dict)
print lst

As a result I am getting this list:

[{'cat_num': u'cat2', 'name': u'Mew'}, {'cat_num': u'cat2', 'name': u'Mew'}]

Can anyone tell me where is my mistake?

Thank you.

score 5 · Answer 1 · answered Feb 21 '11 at 15:00

Move the definition of dct inside the loop (and don't call it dict, since this is the name of the class):

lst = []
cats = [(u'cat1', u'Matilda'),(u'cat2', u'Mew')]
for line in cats:
    dct = {}
    dct['cat_num'] = line[0]
    dct['name'] = line[1]
    lst.append(dct)
print lst

score 4 · Answer 2 · answered Feb 21 '11 at 15:25

Remember that dict() is actually a builtin Python function that builds dictionaries, so it is probably not a good idea to rename it. Why not do something like,

cats = [(u'cat1', u'Matilda'),(u'cat2', u'Mew')]
lst = [dict(cat_num=c,name=n) for c,n in cats]

score 1 · Answer 3 · answered Feb 21 '11 at 15:00

First you need to create a dictionary for every tuple. Before you were using only one, shared dictionary:

lst = []
cats = [(u'cat1', u'Matilda'),(u'cat2', u'Mew')]
for line in cats:
    d = {}
    d['cat_num'] = line[0]
    d['name'] = line[1]
    lst.append(d)
print lst

score 0 · Answer 4 · edited May 23 '17 at 12:04

Since dict has already been created you are appending a reference to the dictionary to the array (lst) and altering it for every line in cats.

To see this, simply print your dictionary for each iteration:

tmp = {}
final_results = []
cats = [(u'cat1', u'Matilda'),(u'cat2', u'Mew')]
for line in cats:
    tmp['cat_num'] = line[0]
    tmp['name'] = line[1]
    print "For", line, "the dictionary is", tmp
    final_results.append(dict)

print "The final list is:", final_results

All you need to do is create a new dictionary each time, and your problems will be solved:

final_results = []
cats = [(u'cat1', u'Matilda'),(u'cat2', u'Mew')]
for line in cats:
    final_results.append( \
        {'cat_num': line[0],
        'name': line[1]} \
     )

SEE ALSO: "Least Astonishment" and the Mutable Default Argument for other places where this behavior might surprise you.

List of dictionaries

4 Answers4