%%file calc_gc.py

def calc_gc(dna):
    dna = dna.upper()
    gc = dna.count('G') + dna.count('C')
    at = dna.count('A') + dna.count('T')
    
    total = gc + at
    if total == 0:
        return 0.0
    
    frac = gc / float(total)
    return frac

def test_correct_counting_gc():
    x = calc_gc('ATGC')
    assert x == 0.5, x
    
def test_correct_counting_at():
    x = calc_gc('AAAA')
    assert x == 0.0, x

def test_3():
    x = calc_gc('atgc')
    assert x == 0.5, x
    
def test_4():
    x = calc_gc('AtgCNN')
    assert x == 0.5, x
    
def test_5():
    x = calc_gc('NNNNNN')
    assert x == 0.0, x

!nosetests -v calc_gc.py

!pip install git+https://github.com/ged-lab/screed.git