Port FlatBuffers to Python.

Implement code generation and self-contained runtime library for Python.

The test suite verifies:
  - Correctness of generated Python code by comparing output to that of
    the other language ports.
  - The exact bytes in the Builder buffer during many scenarios.
  - Vtable deduplication correctness.
  - Edge cases for table construction, via a fuzzer derived from the Go
    implementation.
  - All code is simultaneously valid in Python 2.6, 2.7, and 3.4.

The test suite includes benchmarks for:
  - Building 'gold' data.
  - Parsing 'gold' data.
  - Deduplicating vtables.

All tests pass on this author's system for the following Python
implementations:
  - CPython 2.6.7
  - CPython 2.7.8
  - CPython 3.4.2
  - PyPy 2.5.0 (CPython 2.7.8 compatible)

python/flatbuffers/table.py

@@ -0,0 +1,117 @@
+# Copyright 2014 Google Inc. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from . import encode
+from . import number_types as N
+
+
+class Table(object):
+    """Table wraps a byte slice and provides read access to its data.
+
+    The variable `Pos` indicates the root of the FlatBuffers object therein."""
+
+    __slots__ = ("Bytes", "Pos")
+
+    def __init__(self, buf, pos):
+        N.enforce_number(pos, N.UOffsetTFlags)
+
+        self.Bytes = buf
+        self.Pos = pos
+
+    def Offset(self, vtableOffset):
+        """Offset provides access into the Table's vtable.
+
+        Deprecated fields are ignored by checking the vtable's length."""
+
+        vtable = self.Pos - self.Get(N.SOffsetTFlags, self.Pos)
+        vtableEnd = self.Get(N.VOffsetTFlags, vtable)
+        if vtableOffset < vtableEnd:
+            return self.Get(N.VOffsetTFlags, vtable + vtableOffset)
+        return 0
+
+    def Indirect(self, off):
+        """Indirect retrieves the relative offset stored at `offset`."""
+        N.enforce_number(off, N.UOffsetTFlags)
+        return off + encode.Get(N.UOffsetTFlags.packer_type, self.Bytes, off)
+
+    def String(self, off):
+        """String gets a string from data stored inside the flatbuffer."""
+        N.enforce_number(off, N.UOffsetTFlags)
+        off += encode.Get(N.UOffsetTFlags.packer_type, self.Bytes, off)
+        start = off + N.UOffsetTFlags.bytewidth
+        length = encode.Get(N.UOffsetTFlags.packer_type, self.Bytes, off)
+        return bytes(self.Bytes[start:start+length])
+
+    def VectorLen(self, off):
+        """VectorLen retrieves the length of the vector whose offset is stored
+           at "off" in this object."""
+        N.enforce_number(off, N.UOffsetTFlags)
+
+        off += self.Pos
+        off += encode.Get(N.UOffsetTFlags.packer_type, self.Bytes, off)
+        ret = encode.Get(N.UOffsetTFlags.packer_type, self.Bytes, off)
+        return ret
+
+    def Vector(self, off):
+        """Vector retrieves the start of data of the vector whose offset is
+           stored at "off" in this object."""
+        N.enforce_number(off, N.UOffsetTFlags)
+
+        off += self.Pos
+        x = off + self.Get(N.UOffsetTFlags, off)
+        # data starts after metadata containing the vector length
+        x += N.UOffsetTFlags.bytewidth
+        return x
+
+    def Union(self, t2, off):
+        """Union initializes any Table-derived type to point to the union at
+           the given offset."""
+        assert type(t2) is Table
+        N.enforce_number(off, N.UOffsetTFlags)
+
+        off += self.Pos
+        t2.Pos = off + self.Get(N.UOffsetTFlags, off)
+        t2.Bytes = self.Bytes
+
+    def Get(self, flags, off):
+        """
+        Get retrieves a value of the type specified by `flags`  at the
+        given offset.
+        """
+        N.enforce_number(off, N.UOffsetTFlags)
+        return flags.py_type(encode.Get(flags.packer_type, self.Bytes, off))
+
+    def GetSlot(self, slot, d, validator_flags):
+        N.enforce_number(slot, N.VOffsetTFlags)
+        if validator_flags is not None:
+            N.enforce_number(d, validator_flags)
+        off = self.Offset(slot)
+        if off == 0:
+            return d
+        return self.Get(validator_flags, self.Pos + off)
+
+    def GetVOffsetTSlot(self, slot, d):
+        """
+        GetVOffsetTSlot retrieves the VOffsetT that the given vtable location
+        points to. If the vtable value is zero, the default value `d`
+        will be returned.
+        """
+
+        N.enforce_number(slot, N.VOffsetTFlags)
+        N.enforce_number(d, N.VOffsetTFlags)
+
+        off = self.Offset(slot)
+        if off == 0:
+                return d
+        return off