Iterating through and mutating dictionary

Question

I am running into issues iterating through, and modifying, a dictionary...

Say I have a dictionary:

dict1 = {'A' : 'first', 'B' : 'second', 'C' : 'third', 'D' : 'fourth'}

I want to iterate through dict1, using the data within to build up a second dictionary. Once finished with each entry in dict1, I delete it.

In pseudocode:

dict2 = {}

for an entry in dict1:
    if key is A or B:
        dict2[key] = dict1[key]   # copy the dictionary entry
    if key is C:
        do this...
    otherwise:
        do something else...
    del dict1[key]

I know that altering the length of an iterable in a loop causes problems and the above may not be simple to achieve.

The answer to this question to this question seems to indicate that I can use the keys() function as it returns a dynamic object. I've thus tried:

for k in dict1.keys():
    if k == A or k == B:
        dict2[k] = dict1[k]
    elif k == C:
        dothis()
    else:
        dosomethingelse()
    del dict1[k]

But, this simply gives:

'RuntimeError: dictionary changed size during iteration'

after the first deletion. I have also tried using iter(dict1.keys()) but got the same error.

I'm thus a little confused and could do with some advice. Thanks

It appears that dict1 will always be empty when the loop finishes. So could you problem be solved by simply overwriting dict1 when you have completed your loop? — Jonas, Apr 19 '17 at 22:12
http://stackoverflow.com/questions/12753351/removing-one-or-multiple-keys-from-a-dictionary http://stackoverflow.com/questions/8995611/removing-multiple-keys-from-a-dictionary-safely — Kenny Ostrom, Apr 19 '17 at 22:19
iterate over `list(dict1.keys())` But why must you delete the keys one-by-one? Just `dict1.clear()` after the loop. — juanpa.arrivillaga, Apr 19 '17 at 23:03

score 5 · Accepted Answer · answered Apr 20 '17 at 01:31

Just use the .keys() method to create an independent list of keys.

Here is a working version of your code for Python 2.7:

>>> dict1 = {'A' : 'first', 'B' : 'second', 'C' : 'third', 'D' : 'fourth'}
>>> dict2 = {}
>>> for key in dict1.keys():     # this makes a separate list of keys
        if key in ('A', 'B'):
            dict2[key] = dict1[key]
        elif key == 'C':
            print 'Do this!'
        else:
            print 'Do something else'
        del dict1[key]

Do this!
Do something else
>>> dict1
{}
>>> dict2
{'A': 'first', 'B': 'second'}

For Python 3, add list() around the .keys() and use the print-function:

>>> dict1 = {'A' : 'first', 'B' : 'second', 'C' : 'third', 'D' : 'fourth'}
>>> dict2 = {}
>>> for key in list(dict1.keys()):     # this makes a separate list of keys
        if key in ('A', 'B'):
            dict2[key] = dict1[key]
        elif key == 'C':
            print('Do this!')
        else:
            print('Do something else')
        del dict1[key]

Do this!
Do something else
>>> dict1
{}
>>> dict2
{'A': 'first', 'B': 'second'}

score 3 · Answer 2 · answered Apr 19 '17 at 22:18

why not simply dict1.clear()? notice that on your loop you delete each element per iteration?

one simplified (and naive) solution I can think about is

delkeys=[]
dict2 = {}

for an entry in dict1:
  if key is A or B:
    dict2[key] = dict1[key]         # copy the dictionary entry
  if key is C:
    do this...
  elif:
    do something else...
  delkeys.append(key)

for x in delkeys:
   del dict1[x]

Iterating through and mutating dictionary

2 Answers2