ltp-a2

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
    """
    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

    """
    return dna2 in dna1

def is_valid_sequence(dna):

        nucleotide = 'ATCG'
        for d in dna:
            if d not in nucleotide:
                return False
        return True

def insert_sequence(dna1, dna2, index):
    """ (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('CON', 'PO', 2)
   COPON

   """

    return dna1[:index] + dna2 + dna1[index:]



def get_complement(dna):
    """ (str) -> str

   Return the nucleotide's complement ('A', 'T', 'C', 'G')

   >>>get_complement('PATA')
   'TAT'
   >>>get_complement('CGCG')
   'GCGC'

   """

    nucletoide = ''


    for char in dna:
        if char == 'A':
            nucletoide = nucletoide + 'T'
        elif char == 'T':
            nucletoide = nucletoide + 'A'
        elif char == 'C':
            nucletoide = nucletoide + 'G'
        elif char == 'G':
            nucletoide = nucletoide + 'C'

    return nucletoide

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'

   """

    nucletoide = ''

    for char in dna:
        if char == 'A':
            nucletoide = nucletoide + 'T'
        elif char == 'T':
            nucletoide = nucletoide + 'A'
        elif char == 'C':
            nucletoide = nucletoide + 'G'
        elif char == 'G':
            nucletoide = nucletoide + 'C'

    return nucletoide