Appendix B — Practice Problem Solutions — Chapter 6

Solution Section 6.9.1

try:
    value = float("abc")
except ValueError:
    print("Not a valid number")

Solution Section 6.9.2

counts = {"A": 1, "C": 2, "G": 0, "T": 4}
total = sum(counts.values())

try:
    n_ratio = counts["N"] / total
except KeyError:
    print("N is not present in the counts dictionary")
    n_ratio = None

Solution Section 6.9.3

try:
    silly_divide(5, 0)
except ZeroDivisionError:
    print("you can't divide by zero!")
except Exception as error:
    print(f"a mysterious error occurred: {error=}")

Solution Section 6.9.4

def fold_change(expression_1, expression_2)
    try:
        return expression_1 / expression_2
    except ZeroDivisionError:
        print("expression_2 was zero!")
        return None

Solution Section 6.9.5

class SequenceLengthError(Exception):
    pass

MIN_LENGTH = 50
MAX_LENGTH = 150

def validate_sequence_length(sequence):
    sequence_length = len(sequence)

    if sequence_length < MIN_LENGTH:
        raise SequenceLengthError(f"sequence length {sequence_length} was too short!")

    if sequence_length > MAX_LENGTH:
        raise SequenceLengthError(f"sequence length {sequence_length} was too long!")

    return None

Solution Section 6.9.6

def run_simulation(max_turns):
    if max_turns < 1:
        raise ValueError(f"Expected at least 1 iteration, but got {max_turns=}")

    if max_turns > 1000:
        raise ValueError(f"Expected at most 1000 iterations, but got {max_turns=}")

    # Simulation code would follow
    pass