Django : can't interrupt update function with redirect. Is it possible?

Question

I use a function for updating a Model.

def update_mapping(request, pk):
    flow = Flow.objects.get(pk=pk)
    mappings = MappingField.objects.filter(fl_id=pk)
    headers_samples = GetCsvHeadersAndSamples(request, pk)
    [...]

In this function, I call another one (GetCsvHeadersAndSamples) for getting datas from a CSV. Later, I use those datas with JS in the template.

def GetCsvHeadersAndSamples(request, flow_id):
    get_file_and_attribs = get_csv(request, flow_id)
    file = get_file_and_attribs[0]
    separator = get_file_and_attribs[1]
    encoding = get_file_and_attribs[2]
    with open(file, newline='') as f:
        reader = csv.reader(f, delimiter=separator, 
  encoding=encoding)
        headers = next(reader)
        samples = next(itertools.islice(csv.reader(f), 1, None))
    headersAndSamples = {'headers': headers, 'samples': samples}
    return headersAndSamples

For accessing CSV datas, I use another function for checking if the CSV still exists, in which case, I retrieve datas in it.

def get_csv(request, flow_id):
    flow = Flow.objects.get(pk=flow_id)
    file = flow.fl_file_name
    separator = flow.fl_separator
    media_folder = settings.MEDIA_ROOT
    file = os.path.join(media_folder, str(file))
    if os.path.isfile(file):
        file_2_test = urllib.request.urlopen('file://' + file).read()
        encoding = (chardet.detect(file_2_test))['encoding']
        return (file, separator, encoding)
    else:
        # print('No file')
        messages.error(request, "File not found or corrupted.")
        return HttpResponseRedirect(reverse('mappings-list', args=(flow_id,)))

I hoped that the return would "break" my original function and would redirect to the 'mappings-list' page with the message.error. But it continues and returns to GetCsvHeadersAndSamples function that generates an error because CSV datas were not found. Nota: the commented print however shows well that the file is not found.

It seems that the way I'm doing things is not the good one.