1.

Convert this string

123-456-789

into this string

321:654:987

s = '123-456-789'

Method 1

':'.join(item[::-1] for item in s.split('-'))

'321:654:987'

Method 2

s.translate(str.maketrans('123456789-', '321654987:'))

'321:654:987'

2.

Read the DNA sequence in the file dan.fasta and find the longest run of a single nucleotide. For example, the longest run in GATTACA is T with a length of 2. Ignore lines beginning with >.

lines = []
with open('dna.fasta') as f:
    for line in f:
        if not line.startswith('>'):
            lines.append(line.strip())
dna = ''.join(lines)

dna

'ATGAATAAAATATACTACATAATCTTTTTAAGCGCCCAGTGCCTTGTGCACATTGGGAAGTGCGGGCGAAACCATAAGCCGAGCAGGCTGACCCGTAGCGCCAACAACGTTCTACTGGAAAAGGGGCCTACCGTTGAGAGAAGCACACGAATGAGTAACCCCTGGAAAGCGTTCATGGAAAAATACGACATCGAAAGAACACACAGTTCTGGGGTTCGAGTGGATTTAGGGGAAGATGCAGAAGTGGAAAATGCAAAGTACAGAATTCCAGCTGGAAGATGTCCTGTTTTTGGAAAGGGTATCGTCATAGAGAATTCCGCTGTTAGCTTCTTAACCCCTGTGGCTACAGGAGATCAGAGGCTGAAGGATGGAGGTTTCGCCTTCCCCAAAGCGGATGACCATATCTCCCCCATGACATTAGCGAACCTTAAGGAAAGGTATAAAGACAATGTAGAGATGATGAAGTTAAACGATATAGCTTTGTGCAGAACCCACGCAGCTAGCTTTGTCATGGCAGGGGATCAAAATTCGTCCTACAGACACCCAGCTGTATACGACGAAAAGGAAAAAACATGCCACATGTTGTATTTATCAGCGCAGGAAAATATGGGTCCGAGGTACTGCAGCTCAGATGCACAAAATAGAGATGCCGTGTTCTGCTTCAAGCCAGATAAAAATGTAGATTTTGAAAACCTGGTGTATTTGAGCAAAAATGTGCGTAATGATTGGGATAAAAAATGTCCCCGTAAAAATTTAGGAAACGCCAAGTTCGGATTATGGGTGGATGGGAACTGCGAAGAAATTCCATACGTTAAAGAAGTGGAGGCAAAGGATCTGCGCGAATGCAACCGAATCGTTTTCGAAGCGAGTGCCTCAGATCAACCAACTCAGTATGAAGAAGAAATGACGGATTATCAAAAAATACAACAAGGGTTTAGACAAAACAACCGAGAGATGATTAAAAGTGCCTTTCTTCCAGTGGGTGCATTCAACTCGGATAATTTCAAAAGTAAAGGAAGAGGATTTAACTGGGCAAATTTCGATTCTGTAAAAAAGAAGTGTTACATTTTTAATACCAAACCGACTTGCCTCATTAATGACAAAAATTTTATTGCAACAACGGCGTTATCTCACCCACAAGAAGTAGACCGGGATTTCCCCTGCAGCATATATAAAGACGAAATTGAAAGAGAAATTAGGAAACAATCGAGGAACATGAATCTGTACAGTGTTGATGGGGAACGCATTGTCCTGCCGAGGATATTTATCTCCAACGATAAGGAGAGTATCAAATGTCCCTGCGAACCTGAGCACATTTCCAACAGTACCTGCAACTTTTACGTTTGTAACTGTGTAGAGAAAAGGGCGGAAATTAAGGAAAATAACCAAGTTGTTATAAAGGAAGAATTTAGGGATTATTACGAAAATGGGGAGGAAAAATCGAACAAGCAGATGCTACTAATCATTATCGGAATAACTGGTGGCGTGTGCGTCGTCGCGCTGGCCTCTATGGCCTACTTCAAGAAGAAGGCTAACAATGATAAGTATGACAAGATGGACCAGGCAGAGGGGTACGGGAAGCCCACCACCAGGAAGGACGAGATGCTCGACCCCGAGGCCTCCTTCTGGGGCGAGGACAAGCGGGCCTCCCACACCACGCCCGTGCTGATGGAGAAGCCGTACTACTGA'

Method 1

Using a finite state machine with two states (INCR and RESET)

best = 0
nuc = dna[0]
count = 1
best_nuc = None
for x in dna[1:]:
    # INCR
    if x == nuc:
        count += 1
    # RESET
    else:
        if count > best:
            best = count
            best_nuc = nuc
        nuc = x
        count =1

best_nuc, best

('A', 6)

Method 2

Using regular expressions

import re

runs = re.findall('(A+|T+|C+|G+)', dna)

max(runs, key=len)

'AAAAAA'