how to only get the alphabet letter from a string in python [closed]

Question

I'm trying to create a program that takes in text from a file and outputs the frequency of each letter. The output also shows the frequency of '.' but I only want to output the frequency of letters. I'm trying to use isalpha() but I'm not sure where to put it. This is my code so far:

def count(str):
    d = dict()
    word = str.lower()

    for i in word:
        for j in i:
            if j in d:
                d[j] = d[j] + 1
            else:
                d[j] = 1
    return d

print(count("Once upon a time there lived a princess who was too beautiful and kind to her subjects. The poets and artists were all in praise of her beauty and their works were inspired by her. Once a charming prince from another kingdom came to meet the princess and soon they became friends and slowly they fell in love")) 

I tried putting isalpha() here:

for i in word.isalpha():

but it just gives me an error.

Answer 1

from string import ascii_lowercase as alphabet 
# or simply declare: alphabet = 'abcdefghijklmnopqrstuvwxyz' 
def letter_frequency(text):    
    frequency = {letter:0 for letter in alphabet}
    for character in text.lower():
        try:
            frequency[character] += 1
        except KeyError:
            pass
    return frequency

Answer 2

import string

alphabet_string = string.ascii_lowercase
alphabet_list = list(alphabet_string)

letter_count = {}


string = (
    "Once upon a time there lived a princess who was too beautiful and" 
    "kind to her subjects.The poets and artists were all in praise of" 
    "her beauty and their works were inspired"
)

for char in alphabet_list:
    letter_count[char] = 0
for char in string:
    if char in alphabet_list:
        letter_count[char] += 1
print (letter_count)