How to work around needing to update a dictionary

Question

I need to delete a k/v pair from a dictionary in a loop. After getting RuntimeError: dictionary changed size during iteration I pickled the dictionary after deleting the k/v and in one of the outer loops I try to reopen the newly pickled/updated dictionary. However, as many of you will probably know-I get the same error-I think when it reaches the top of the loop. I do not use my dictionary in the outermost loop.

So my question is-does anyone know how to get around this problem? I want to delete a k/V pair from a dictionary and use that resized dictionary on the next iteration of the loop.

to focus the problem and use the solution from Cygil

list=[27,29,23,30,3,5,40]
testDict={}
for x in range(25):
    tempDict={}
    tempDict['xsquared']=x*x
    tempDict['xinverse']=1.0/(x+1.0)
    testDict[(x,x+1)]=tempDict

for item in list:
    print 'the Dictionary now has',len(testDict.keys()), ' keys'
    for key in testDict.keys():
        if key[0]==item:

        del testDict[key]

I am doing this because I have to have some research assistants compare some observations from two data sets that could not be matched because of name variants. The idea is to throw up a name from one data set (say set A) and then based on a key match find all the names attached to that key in the other dataset (set B). One a match has been identified I don't want to show the value from B again to speed things up for them. Because there are 6,000 observations I also don't want them to have to start at the beginning of A each time they get back to work. However, I can fix that by letting them chose to enter the last key from A they worked with. But I really need to reduce B once the match has been identified

Answer 1

Without code, I'm assuming you're writing something like:

for key in dict:
    if check_condition(dict[key]):
       del dict[key]

If so, you can write

for key in list(dict.keys()):
    if key in dict and check_condition(dict[key]):
        del dict[key]

list(dict.keys()) returns a copy of the keys, not a view, which makes it possible to delete from the dictionary (you are iterating through a copy of the keys, not the keys in the dictionary itself, in this case.)

Answer 2

Delete all keys whose value is > 15:

for k in mydict.keys(): # makes a list of the keys and iterate
                        # over the list, not over the dict.
    if mydict[k] > 15:
        del mydict[k]

Answer 3

Change:

for ansSeries in notmatched:

To:

for ansSeries in notmatched.copy():