string – Python: object of type _io.TextIOWrapper has no len()

string – Python: object of type _io.TextIOWrapper has no len()

The problem occurs due to the way you are opening the text file.
You should add dna = dna.read() to your code.
so your end code should look something like this:

def mostCommonSubstring():
    dna = open(dna.txt, r)
    dna = dna.read()
    mink = 4
    maxk = 9
    count = 0
    check = 0
    answer = 
    k = mink
    while k <= maxk:
        for i in range(len(dna)-k+1):
            sub = dna[i:i+k]
            count = 0
            for i in range(len(dna)-k+1):
                if dna[i:i+k] == sub:
                    count = count + 1
            if count >= check:
                answer = sub
                check = count
        k=k+1
    print(answer)
    print(check)

@tfabiant : I suggest this script to read and process a DNA sequence.
To run this code, in the terminal: python readfasta.py fastafile.fasta

import string, sys
##########I. To Load Fasta File##############
file = open(sys.argv[1]) 
rfile = file.readline()
seqs = {} 
##########II. To Make fasta dictionary####
tnv = #temporal name value
while rfile != :
    if > in rfile:
        tnv = string.strip(rfile)
        seqs[tnv] = 
    else:
        seqs[tnv] += string.strip(rfile)    
    rfile = file.readline()
##############III. To Make Counts########
count_what = [A, T, C, G, ATG]
for s in seqs:
    name = s
    seq = seqs[s]
    print s # to print seq name if you have a multifasta file
    for cw in count_what:
        print cw, seq.count(cw)# to print counts by seq

string – Python: object of type _io.TextIOWrapper has no len()