Add vectorNumElements attribute to Builder for simpler vector creation. (#6328)

* Add vectorNumElements attribute to Builder for simpler vector creation.

This adds a default to EndVector which should simplify its use.

* Update tutorial to reflect new default arg in Python EndVector.

* Remove optional argument to Python EndVector.

* Add generated files.

* Unset Builder.vectorNumElems when not in use.

python/flatbuffers/builder.py

@@ -103,7 +103,7 @@ class Builder(object):
 
     ## @cond FLATBUFFERS_INTENRAL
     __slots__ = ("Bytes", "current_vtable", "head", "minalign", "objectEnd",
-                 "vtables", "nested", "forceDefaults", "finished")
+                 "vtables", "nested", "forceDefaults", "finished", "vectorNumElems")
 
     """Maximum buffer size constant, in bytes.
 
@@ -371,12 +371,13 @@ class Builder(object):
 
         self.assertNotNested()
         self.nested = True
+        self.vectorNumElems = numElems
         self.Prep(N.Uint32Flags.bytewidth, elemSize*numElems)
         self.Prep(alignment, elemSize*numElems)  # In case alignment > int.
         return self.Offset()
     ## @endcond
 
-    def EndVector(self, vectorNumElems):
+    def EndVector(self):
         """EndVector writes data necessary to finish vector construction."""
 
         self.assertNested()
@@ -384,7 +385,8 @@ class Builder(object):
         self.nested = False
         ## @endcond
         # we already made space for this, so write without PrependUint32
-        self.PlaceUOffsetT(vectorNumElems)
+        self.PlaceUOffsetT(self.vectorNumElems)
+        self.vectorNumElems = None
         return self.Offset()
 
     def CreateString(self, s, encoding='utf-8', errors='strict'):
@@ -411,7 +413,8 @@ class Builder(object):
         ## @endcond
         self.Bytes[self.Head():self.Head()+l] = x
 
-        return self.EndVector(len(x))
+        self.vectorNumElems = len(x)
+        return self.EndVector()
 
     def CreateByteVector(self, x):
         """CreateString writes a byte vector."""
@@ -432,7 +435,8 @@ class Builder(object):
         ## @endcond
         self.Bytes[self.Head():self.Head()+l] = x
 
-        return self.EndVector(len(x))
+        self.vectorNumElems = len(x)
+        return self.EndVector()
 
     def CreateNumpyVector(self, x):
         """CreateNumpyVector writes a numpy array into the buffer."""
@@ -467,7 +471,8 @@ class Builder(object):
         # tobytes ensures c_contiguous ordering
         self.Bytes[self.Head():self.Head()+l] = x_lend.tobytes(order='C')
 
-        return self.EndVector(x.size)
+        self.vectorNumElems = x.size
+        return self.EndVector()
 
     ## @cond FLATBUFFERS_INTERNAL
     def assertNested(self):