Checar múltiplos tipos com isinstance()

def dobrar(valor):
    if isinstance(valor, (int, float)):
        return valor * 2
    elif isinstance(valor, str):
        return valor * 2
    return None

print(dobrar(5))       # 10
print(dobrar(3.14))    # 6.28
print(dobrar('Oi'))    # OiOi
print(dobrar([1, 2]))  # None