Flatbuffers Python Object API (#5616)

* Flatbuffers Python Object API

Implement the logic to generate the Python object API that can
unpack the data from a buf class into an object class, and pack
the data of an object class to a buf class.

* Fix the build issues

Remove unused parameters and replace auto in the for-loop statement
with std::string to make it compatible with VS2010.

* Fix the build issues.

* Add support for Array type

Added logic to handle Array type in Python Object API. Updated the
generated code accordingly.

* Fix the old style casting from int to char

* Fixed another conversion from int to char

* Fixed the import for typing

Importing typing may cause errors when a machine do not have the
moduel typing installed. This PR fixes the issue by guarding
"import typing" with the "try/except" statement.

* Fix issue of iterating the vector of import list

* Update the generated examples using generate_code.sh

* Fix the import order for typing

The import list was stored in unordered_set, so that each generated
codes may have different import order. Therefore, it failed in the
consistency test where two generated copies need to have exactly the
same apperance.

* Optimize unpack using numpy

Use numpy to unpack vector whenever it is possible to improve unpack
performance.

Also, added codegen command for Python specificly in generate_code.sh,
because --no-includes cannot be turn on for Python.

* Fix the import order

* Update generate_code.bat for windows accordingly

* Replace error message with pass

Avoid printing error message for every Python2 users about typing.
Replace it with pass.

tests/MyGame/Example/ArrayStruct.py

@@ -3,6 +3,8 @@
 # namespace: Example
 
 import flatbuffers
+from flatbuffers.compat import import_numpy
+np = import_numpy()
 
 class ArrayStruct(object):
     __slots__ = ['_tab']
@@ -15,6 +17,18 @@ class ArrayStruct(object):
     def A(self): return self._tab.Get(flatbuffers.number_types.Float32Flags, self._tab.Pos + flatbuffers.number_types.UOffsetTFlags.py_type(0))
     # ArrayStruct
     def B(self): return [self._tab.Get(flatbuffers.number_types.Int32Flags, self._tab.Pos + flatbuffers.number_types.UOffsetTFlags.py_type(4 + i * 4)) for i in range(15)]
+    # ArrayStruct
+    def BLength(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(4))
+        if o != 0:
+            return self._tab.VectorLen(o)
+        return 0
+
+    # ArrayStruct
+    def BIsNone(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(4))
+        return o == 0
+
     # ArrayStruct
     def C(self): return self._tab.Get(flatbuffers.number_types.Int8Flags, self._tab.Pos + flatbuffers.number_types.UOffsetTFlags.py_type(64))
     # ArrayStruct
@@ -22,10 +36,34 @@ class ArrayStruct(object):
         obj.Init(self._tab.Bytes, self._tab.Pos + 72 + i * 32)
         return obj
 
+    # ArrayStruct
+    def DLength(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(72))
+        if o != 0:
+            return self._tab.VectorLen(o)
+        return 0
+
+    # ArrayStruct
+    def DIsNone(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(72))
+        return o == 0
+
     # ArrayStruct
     def E(self): return self._tab.Get(flatbuffers.number_types.Int32Flags, self._tab.Pos + flatbuffers.number_types.UOffsetTFlags.py_type(136))
     # ArrayStruct
     def F(self): return [self._tab.Get(flatbuffers.number_types.Int64Flags, self._tab.Pos + flatbuffers.number_types.UOffsetTFlags.py_type(144 + i * 8)) for i in range(2)]
+    # ArrayStruct
+    def FLength(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(144))
+        if o != 0:
+            return self._tab.VectorLen(o)
+        return 0
+
+    # ArrayStruct
+    def FIsNone(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(144))
+        return o == 0
+
 
 def CreateArrayStruct(builder, a, b, c, d_a, d_b, d_c, d_d, e, f):
     builder.Prep(8, 160)
@@ -49,3 +87,62 @@ def CreateArrayStruct(builder, a, b, c, d_a, d_b, d_c, d_d, e, f):
         builder.PrependInt32(b[_idx0-1])
     builder.PrependFloat32(a)
     return builder.Offset()
+
+import MyGame.Example.NestedStruct
+try:
+    from typing import List
+except:
+    pass
+
+class ArrayStructT(object):
+
+    # ArrayStructT
+    def __init__(self):
+        self.a = 0.0  # type: float
+        self.b = None  # type: List[int]
+        self.c = 0  # type: int
+        self.d = None  # type: List[MyGame.Example.NestedStruct.NestedStructT]
+        self.e = 0  # type: int
+        self.f = None  # type: List[int]
+
+    @classmethod
+    def InitFromBuf(cls, buf, pos):
+        arrayStruct = ArrayStruct()
+        arrayStruct.Init(buf, pos)
+        return cls.InitFromObj(arrayStruct)
+
+    @classmethod
+    def InitFromObj(cls, arrayStruct):
+        x = ArrayStructT()
+        x._UnPack(arrayStruct)
+        return x
+
+    # ArrayStructT
+    def _UnPack(self, arrayStruct):
+        if arrayStruct is None:
+            return
+        self.a = arrayStruct.A()
+        if not arrayStruct.BIsNone():
+            if np is None:
+                self.b = []
+                for i in range(arrayStruct.BLength()):
+                    self.b.append(arrayStruct.B(i))
+            else:
+                self.b = arrayStruct.BAsNumpy()
+        self.c = arrayStruct.C()
+        if not arrayStruct.DIsNone():
+            self.d = []
+            for i in range(arrayStruct.DLength()):
+                self.d.append(arrayStruct.D(i))
+        self.e = arrayStruct.E()
+        if not arrayStruct.FIsNone():
+            if np is None:
+                self.f = []
+                for i in range(arrayStruct.FLength()):
+                    self.f.append(arrayStruct.F(i))
+            else:
+                self.f = arrayStruct.FAsNumpy()
+
+    # ArrayStructT
+    def Pack(self, builder):
+        return CreateArrayStruct(builder, self.a, self.b, self.c, self.d.a, self.d.b, self.d.c, self.d.d, self.e, self.f)