Adding a new key to a nested dictionary in python

Question

I need to add a key with a value that increases by one for every item in the nested dictionary. I have been trying to use the dict['key']='value' syntax but can't get it to work for a nested dictionary. I'm sure it's a very simple.

My Dictionary:

mydict={'a':{'result':[{'key1':'value1','key2':'value2'},
                        {'key1':'value3','key2':'value4'}]}}

This is the code that will add the key to the main part of the dictionary:

for x in range(len(mydict)):
        number = 1+x
        str(number)
        mydict[d'index']=number

print mydict
  #out: {d'index':d'1',d'a'{d'result':[...]}}

I want to add the new key and value to the small dictionaries inside the square parentheses:

{'a':{'result':[{'key1':'value1',...,'index':'number'}]}}

If I try adding more layers to the last line of the for loop I get a traceback error:

Traceback (most recent call last):
  File "C:\Python27\program.py", line 34, in <module>
    main()
  File "C:\Python27\program.py", line 23, in main
    mydict['a']['result']['index']=number
TypeError: list indices must be integers, not unicode

I've tried various different ways of listing the nested items but no joy. Can anyone help me out here?

Answer 1

The problem is that mydict is not simply a collection of nested dictionaries. It contains a list as well. Breaking up the definition helps clarify the internal structure:

dictlist = [{'key1':'value1','key2':'value2'},
            {'key1':'value3','key2':'value4'}]
resultdict = {'result':dictlist}
mydict = {'a':resultdict}

So to access the innermost values, we have to do this. Working backwards:

mydict['a'] 

returns resultdict. Then this:

mydict['a']['result']

returns dictlist. Then this:

mydict['a']['result'][0]

returns the first item in dictlist. Finally, this:

mydict['a']['result'][0]['key1']

returns 'value1'

So now you just have to amend your for loop to iterate correctly over mydict. There are probably better ways, but here's a first approach:

for inner_dict in mydict['a']['result']: # remember that this returns `dictlist`
    for key in inner_dict:
        do_something(inner_dict, key)