Question

I am iterating through a list and comparing each element to 2 dictionaries. The list elements are keys in the dictionaries. Some elements are in the 2 dictionaries, some are in one, some are in none.

for i in range(0,len(mylist)-1):
    if mylist[i] == mydict[mylist[i]]:
        print 'in dict 1'
    elif mylist[i] == mydict2[mylist[i]]:
        print 'in dict 2'
    else: print 'not in dictionaries'

My problem is it isn’t getting past the first elif statement. If it doesn’t find the list element in the 2 dictionaries, it prints a key error. I can’t understand it because I have another loop in another part of the code that’s very similar to this and works perfectly. If a key isn’t in a dictionary I want the else statement printed. Not a key error

Answer 1

Problem 1, as mgilson said, is that = is assignment, == is equality. However, even with your question edit, if you are looking to find out if a key is in a dictionary, you should be using the in operator, not the equals. In other words, the form if key in dict:. So:

for key in mylist:
    if key in mydict1:
        print 'Key %s in dict 1' % key
    elif key in mydict2:
        print 'Key %s in dict 2' % key
    else:
        print 'Key %s not in dictionaries' % key

You could abstract this further to handle an arbitrary set of dicts with a function, if that would prove helpful in the long run (though for a small number of dicts, like 2, you are probably better of with the above hardcoded checking):

def print_keys_from_list_in_dicts(key_list, dict_list):
    indexed_dict_list = enumerate(dict_list)
    for key in key_list:
        found_in_list = []
        for index, dict in indexed_dict_list:
            if key in dict:
                found_in_list.append(index)
        print 'Key %s found in dicts %s.' % (key, found_in_list)

print_keys_from_list_in_dicts(mylist, [mydict1, mydict2])