For this problem, you will write a program to form a frequency table of the letters in a text file, that is, how many times each letter appears in the file In Python 3.7.

Such a frequency table might be useful for compressing a text file. Because different letters appear with different frequencies, we can compress a file by using shorter codes for common letters and longer codes for letters that appear less frequently.

Dictionaries provide an elegant way to generate a frequency table:

>>> letter_counts = {}

>>> for letter in "Mississippi":

... letter_counts[letter] = letter_counts.get(letter, 0) + 1

...

>>> letter_counts

{'M': 1, 's': 4, 'p': 2, 'i': 4}

We start with an empty dictionary. For each letter in the string, we find the current count (possibly zero) and increment it. At the end, the dictionary contains pairs of letters and their frequencies.

It might be more appealing to display the frequency table in alphabetical order. We can do that with the items and sort methods:

>>> letter_items = list(letter_counts.items())

>>> letter_items.sort()

>>> print(letter_items)

[('M', 1), ('i', 4), ('p', 2), ('s', 4)]

Notice in the first line we had to call the type conversion function list. That turns the promise we get from items into a list, a step that is needed before we can use the lists sort method.