Add partial flushing of ZipStreams

I use this to flush partial zips as files are streamed into
 them from requests, and then at the end add manifest and
 error files to the end of the archive

I've also added a related test and example of use

Author: kageurufu

README.md

@@ -113,6 +113,27 @@ def zipball():
     response = Response(z, mimetype='application/zip')
     response.headers['Content-Disposition'] = 'attachment; filename={}'.format('files.zip')
     return response
+
+# Partial flushing of the zip before closing
+
+@app.route('/package.zip', methods=['GET'], endpoint='zipball')
+def zipball():
+    def generate_zip_with_manifest():
+        z = zipstream.ZipFile(mode='w', compression=ZIP_DEFLATED)
+
+        manifest = []
+        for filename in os.listdir('/path/to/files'):
+            z.write(os.path.join('/path/to/files', filename), arcname=filename)
+            yield from z.flush()
+            manifest.append(filename)
+
+        z.write_str('manifest.json', json.dumps(manifest).encode())
+
+        yield from z
+
+    response = Response(z, mimetype='application/zip')
+    response.headers['Content-Disposition'] = 'attachment; filename={}'.format('files.zip')
+    return response
 ```
 
 ### django 1.5+

tests/test_zipstream.py

@@ -92,6 +92,31 @@ def test_writestr(self):
 
         os.remove(f.name)
 
+    def test_partial_writes(self):
+        z = zipstream.ZipFile(mode='w')
+        f = tempfile.NamedTemporaryFile(suffix='zip', delete=False)
+
+        with open(SAMPLE_FILE_RTF, 'rb') as fp:
+            z.writestr('sample1.rtf', fp.read())
+
+        for chunk in z.flush():
+            f.write(chunk)
+
+        with open(SAMPLE_FILE_RTF, 'rb') as fp:
+            z.writestr('sample2.rtf', fp.read())
+
+        for chunk in z.flush():
+            f.write(chunk)
+
+        for chunk in z:
+            f.write(chunk)
+        
+        f.close()
+        z2 = zipfile.ZipFile(f.name, 'r')
+        self.assertFalse(z2.testzip())
+
+        os.remove(f.name)
+
     def test_write_iterable_no_archive(self):
         z = zipstream.ZipFile(mode='w')
         self.assertRaises(TypeError, z.write_iter, iterable=range(10))

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py26, py27, py32, py33, py34, py35, pypy
+envlist = py26, py27, py32, py33, py34, py35, py36, py37, pypy
 
 [testenv]
 deps=nose

zipstream/__init__.py

@@ -178,9 +178,8 @@ def __init__(self, fileobj=None, mode='w', compression=ZIP_STORED, allowZip64=Fa
         self.paths_to_write = []
 
     def __iter__(self):
-        for kwargs in self.paths_to_write:
-            for data in self.__write(**kwargs):
-                yield data
+        for data in self.flush():
+            yield data
         for data in self.__close():
             yield data
 
@@ -190,6 +189,12 @@ def __enter__(self):
     def __exit__(self, type, value, traceback):
         self.close()
 
+    def flush(self):
+        while self.paths_to_write:
+            kwargs = self.paths_to_write.pop()
+            for data in self.__write(**kwargs):
+                yield data
+
     @property
     def comment(self):
         """The comment text associated with the ZIP file."""