frequency of letters in column python

Question

I want to calculate the frequency of occurrence of each letter in all columns: for example I have this three sequences :

seq1=AATC
seq2=GCCT
seq3=ATCA
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Answer 1

Here:

sequences = ['AATC',
             'GCCT',
             'ATCA']
f = zip(*sequences)
counts = [{letter: column.count(letter) for letter in column} for column in f]
print(counts)

Output (reformatted):

[{'A': 2, 'G': 1}, 
 {'A': 1, 'C': 1, 'T': 1}, 
 {'C': 2, 'T': 1}, 
 {'A': 1, 'C': 1, 'T': 1}]

Salient features:

• Rather than explicitly naming seq1, seq2, etc., we put them into a list.
• We unpack the list with the * operator.
• We use a dict comprehension inside a list comprehension to generate the counts for each letter in each column. It's basically what you did for the one-sequence case, but more readable (IMO).