items = []
for k in data:
    items.append( (k, data[k]) )

items = [ (k,data[k]) for k in data ]

Here is one possible approach...

if 'CA' in captial:
    val = capital['CA']
else:
    capital['CA'] = 'Springfield'
    val in 'Springfield'

if 'CA' not in capital:
    captial['CA'] = 'Springfield'
val = capital['CA']

Here is a typical approach using a condition to detect newly encountered characters.

def letterFrequency(word):
    freq = {}
    for c in word:
	if c not in freq:
	    freq[c] = 1    # first time
	else:
	    freq[c] += 1   # one additional time
    return freq

Here is a more preferable approach that better leverages the get method of the dictionary to retrieve a previous count, yet with 0 as the result for something not found.

def letterFrequency(word):
    freq = {}
    for c in word:
	freq[c] = 1 + freq.get(c,0)
    return freq