Q: I want to delete non comon items of two lists I want to create a function that can be use to  delete the non comon elements( I should delete Audi an Mercedes of two lists) :
marcas = [
(1, 'Audi'),
(2, 'Nissan'),
(3, 'Mercedes'),]
marcas2 = [] 

coches = [
{
    'modelo': 'Audi C3',
    'marca': 1,
    'precio': 25000,
    'ano': 2017,
}]


def delbrand(marcas):

for y in coches:
    for x in marcas:
        if y['marca'] == x[0]:
            if x not in marcas2:
                marcas2.append(x)
m = 0

for i in marcas:
    if i not in marcas2:
        del marcaslist[m]
    m = m+1

would you mind to help me with this question?

A: It is not very clear what are you asking for. I modified your code a bit: 
brands = ['Audi', 'Nissan', 'Mercedes']

cars = [
{
    'modelo': 'Audi C3',
    'marca': 1,
    'precio': 25000,
    'ano': 2017,
}]

not_existing_brands = set()

def find_not_common(brands, cars):

    for brand in brands:
        for car in cars:
            if brand not in car['modelo']:
                not_existing_brands.add(brand)

find_not_common(brands, cars)

print brands
print list(not_existing_brands)

for item in list(not_existing_brands):
    brands.remove(item)

# final brands list
print brands

Output: 
['Audi', 'Nissan', 'Mercedes']
['Mercedes', 'Nissan']
['Audi']

