def transforma(c):
    if c.isalpha():
        r = c.lower()
    elif c.isspace():
        r = c
    else:
        r = ' '
    return r


def filtra_paraules_1(text):
    caracters = [transforma(c) for c in text]
    nou_text = ''.join(caracters)
    pars_text = nou_text.split()
    r = ' '.join(pars_text)
    return r


def filtra_paraules_2(text):
    caracters = []
    for c in text:
        caracters.append(transforma(c))
    nou_text = ''.join(caracters)
    pars_text = nou_text.split()
    r = ' '.join(pars_text)
    return r

# Tria la solució que vulguis provar
# filtra_paraules = filtra_paraules_1
filtra_paraules = filtra_paraules_2