for loop skipping sections of dictionary in python

Question

GeoFigs =  {'Populus':0, 'Tristidia':1, 'Albus':2, 'Fortuna Major':3, 'Rubeus':4,
            'Acquisitio':5, 'Conjunctivo':6, 'Caput Draconis':7, 'Laetita':8, 'Carcer':9, 
            'Amissio':10, 'Puella':11, 'Fortuna Minor':12, 'Puer':13, 'Cauda Draconis':14, 'Via':15}
KeyGeo = dict.copy(GeoFigs)
for key in KeyGeo:
    print KeyGeo[key]
    keychange = KeyGeo[key]
    newValue = key
    del KeyGeo[key]
    KeyGeo[keychange] =  newValue

When I run the for loop it skips over some of the keys producing

(0, 'Populus'), (1, 'Tristidia'), ('Carcer', 9), (3, 'Fortuna Major'), (4, 'Rubeus'),
(5, 'Acquisitio'), (6, 'Conjunctivo'), (7, 'Caput Draconis'), ('Puer', 13), (10, 'Amissio'),
(11, 'Puella'), (12, 'Fortuna Minor'), (2, 'Albus'), (14, 'Cauda Draconis'), (15, 'Via'), ('Laetita', 8)]

Any idea why it's skipping the 3rd and the 9th only?

Dictionaries are normally hash based implementations. So, the order in which the values are stored may not be the same always. — thefourtheye, Dec 14 '14 at 13:42
You are deleting elements from a collection as you iterate it. Don't do this. Never. — tobias_k, Dec 14 '14 at 13:44

score 2 · Answer 1 · edited May 23 '17 at 10:26

2

There are two issues at play here:

The ordering of keys in a dict is not defined. If you care about the ordering, using something like OrderedDict.
You are making structural changes to the dict while iterating over it. See Modifying a Python dict while iterating over it

edited May 23 '17 at 10:26

Community

1
1

answered Dec 14 '14 at 13:44

NPE

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score 0 · Answer 2 · answered Dec 14 '14 at 15:33

Others have nailed the problem - you are changing the dict as you iterate. You solve the problem by getting a copy of the keys before you start. Its a bit different in python 2 and 3. In python 2, keys is a list:

for key in KeyGeo.keys():
    ...

In python 3, keys is an iterator so you have to iterate before you start

for key in list(KeyGeo.keys()):
    ...

for loop skipping sections of dictionary in python

2 Answers2