Python

# invalid
for el in s:
    if str1Ht[el] == None:
        str1Ht[el] = 1
        pass
    str1Ht[el] += 1

# valid
for el in s:
    if el not in str1Ht:
        str1Ht[el] = 1
        pass
    str1Ht[el] += 1