Python: Add forceDefaults opt to python Builder (#5564)

* Add forceDefaults opt to python Builder

* Add test functions for force_default option for python builder

* Simplify

* Add force default test for UOffsetTFlags

python/flatbuffers/builder.py

@@ -103,7 +103,7 @@ class Builder(object):
 
     ## @cond FLATBUFFERS_INTENRAL
     __slots__ = ("Bytes", "current_vtable", "head", "minalign", "objectEnd",
-                 "vtables", "nested", "finished")
+                 "vtables", "nested", "forceDefaults", "finished")
 
     """Maximum buffer size constant, in bytes.
 
@@ -131,10 +131,10 @@ class Builder(object):
         self.objectEnd = None
         self.vtables = {}
         self.nested = False
+        self.forceDefaults = False
         ## @endcond
         self.finished = False
 
-
     def Output(self):
         """Return the portion of the buffer that has been used for writing data.
 
@@ -552,7 +552,7 @@ class Builder(object):
     def PrependSlot(self, flags, o, x, d):
         N.enforce_number(x, flags)
         N.enforce_number(d, flags)
-        if x != d:
+        if x != d or self.forceDefaults:
             self.Prepend(flags, x)
             self.Slot(o)
 
@@ -589,7 +589,7 @@ class Builder(object):
         be set to zero and no other data will be written.
         """
 
-        if x != d:
+        if x != d or self.forceDefaults:
             self.PrependUOffsetTRelative(x)
             self.Slot(o)
 
@@ -691,6 +691,15 @@ class Builder(object):
         """
         self.Prepend(N.Float64Flags, x)
 
+    def ForceDefaults(self, forceDefaults):
+        """
+        In order to save space, fields that are set to their default value
+        don't get serialized into the buffer. Forcing defaults provides a
+        way to manually disable this optimization. When set to `True`, will
+        always serialize default values.
+        """
+        self.forceDefaults = forceDefaults
+
 ##############################################################
 
     ## @cond FLATBUFFERS_INTERNAL