a2.py

def get_length(dna):
    """ (str) -> int

    Return the length of the DNA sequence dna.

    >>> get_length('ATCGAT')
    6
    >>> get_length('ATCG')
    4
    """
    return len(dna)


def is_longer(dna1, dna2):
    """ (str, str) -> bool

    Return True if and only if DNA sequence dna1 is longer than DNA sequence
    dna2.

    >>> is_longer('ATCG', 'AT')
    True
    >>> is_longer('ATCG', 'ATCGGA')
    False
    """
    return len(dna1)>len(dna2)


def count_nucleotides(dna, nucleotide):
    """ (str, str) -> int

    Return the number of occurrences of nucleotide in the DNA sequence dna.

    >>> count_nucleotides('ATCGGC', 'G')
    2
    >>> count_nucleotides('ATCTA', 'G')
    0
    """
    dna = dna.upper()
    nucleotide = nucleotide.upper()
    return dna.count(nucleotide)


def contains_sequence(dna1, dna2):
    """ (str, str) -> bool

    Return True if and only if DNA sequence dna2 occurs in the DNA sequence
    dna1.

    >>> contains_sequence('ATCGGC', 'GG')
    True
    >>> contains_sequence('ATCGGC', 'GT')
    False

    """
    dna1 = dna1.upper()
    dna2 = dna2.upper()
    return dna2 in dna1


def is_valid_sequence(dna):
    '''(str) -> bool

    Return True if and only if DNA sequence is valid.
    
    >>> is_valid_sequence('TCATGT')
    True
    >>> is_valid_sequence('TCAXGT')
    False
    >>> is_valid_sequence('')
    True
    >>> is_valid_sequence('tcatgt')
    False
    '''
    
    if dna.isupper() or (len(dna)==0):
        count = 0
        for char in dna:
           if char not in 'ATCG':
               count += 1
        if count>0:
            return False
        else:
            return True
    else:
        return False
    

def insert_sequence(dna1, dna2, x):
        
    ''' (str,str,int)-> str

    Return the DNA sequence obtained by inserting the
    second DNA sequence into the first DNA sequence at the given index.
    
    >>> insert_sequence('CCGG','AT',2)
    'CCATGG'
    >>> insert_sequence('TCATGT','CAG',3)
    'TCACAGTGT'    
    '''
    return (dna1[0:x]+dna2+dna1[x:])
    
def get_complement(nucleotide):
    '''(str) -> str

    Return the nucleotide's complement.

    >>> get_complement('A')
    T
    >>> get_complement('T')
    A
    >>> get_complement('G')
    C
    >>> get_complement('C')
    G
    '''
    if nucleotide in 'ATGC':
        if nucleotide == 'A':
            return 'T'
        elif nucleotide == 'T':
            return 'A'
        elif nucleotide == 'G':
            return 'C'
        elif nucleotide == 'C':
            return 'G'
    else:
        return False

def get_complementary_sequence(dna):
    
    ''' (str) -> str

    Return the DNA sequence that is complementary to the given DNA sequence.
    
    >>> get_complementary_sequence('AT')
    'TA'
    >>> get_complementary_sequence('GC')
    'CG'
    >>> get_complementary_sequence('CC')
    False
    '''

    new_list = ''
    for char in dna:
        new_list += get_complement(char)
    return new_list