in memory zip with python

myzip.py

from zipfile import ZipFile


def append_file(memory_stream, file_name, file_content):
    with ZipFile(memory_stream, 'a') as zf:
        zf.writestr(file_name, file_content)

myzip_test.py

from io import BytesIO
import unittest
from zipfile import ZipFile
import myzip


class TestZip(unittest.TestCase):
    def test_zip(self):
        memory_stream = BytesIO()

        myzip.append_file(memory_stream, 'filename1', 'content of file')
        myzip.append_file(memory_stream, 'filename2', 'content of another file')

        with ZipFile(memory_stream) as zf:
            self.assertEqual(2, len(zf.filelist))
            self.assertEqual('filename1', zf.filelist[0].filename)
            self.assertEqual('filename2', zf.filelist[1].filename)
            self.assertEqual('content of file', zf.read('filename1'))
            self.assertEqual('content of another file', zf.read('filename2'))

And this doesn't work :

def create_zip(memory_stream):
    with ZipFile(memory_stream, 'a') as zf:
         zf.writestr('filename1', 'content of file')
         zf.writestr('filename2', 'content of file2')
         return zf

zip = create_zip(memory_stream)
# then do stuff with zip