Adding new key to dictionary overwrites all previously stored keys with new keys values

Question

I\'m trying to use a for loop to generate random values for item prices, by changing the value of the item prices in a pre-defined dictionary.

The new values of this pr

Answer 1

The keys in a dictionary are unique. If a key exists in a dictionary, d[key] = other_value just changes the value for that key, it does NOT create another item.

>>> d = {'a':1, 'b':'foo'}
>>> d['b'] = 'six'
>>> d
{'b': 'six', 'a': 1}
>>> d.update([('a','bar')])
>>> d
{'b': 'six', 'a': 'bar'}
>>>

If you have data that you want to place in a dictionary and the data contains keys with multiple values, you could put the values into a list for each key. collections.defaultdict makes this easy.

>>> a
[('a', 0), ('b', 1), ('c', 2), ('d', 3), ('e', 4), ('f', 5), ('a', 100), ('c', 99), ('d', 98), ('f', 97)]
>>> import collections
>>> d = collections.defaultdict(list)
>>> for key, value in a:
    d[key].append(value)

>>> d
defaultdict(<class 'list'>, {'b': [1], 'a': [0, 100], 'e': [4], 'f': [5, 97], 'd': [3, 98], 'c': [2, 99]})
>>> 

For your problem, start with the initial values in a list then add to them.

import random

d = {'a':[0], 'b':[0], 'c':[0]}
for _ in xrange(4):
    for key in d:
        d[key].append(random.randint(1, 100))

for item in d.items():
    print item

>>>
('a', [0, 92, 45, 52, 32])
('c', [0, 51, 85, 72, 4])
('b', [0, 47, 7, 74, 59])
>>>

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How to iterate over a dictionary.