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Port FlatBuffers to Python.

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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)
This commit is contained in:
rw
2014-12-16 00:32:11 -08:00
parent 4d213c2d06
commit 48dfc69ee6
35 changed files with 3608 additions and 35 deletions
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17
python/flatbuffers/__init__.py Normal file
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# 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 .builder import Builder
from .table import Table
from .compat import range_func as compat_range

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python/flatbuffers/builder.py Normal file
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# 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 number_types as N
from .number_types import (UOffsetTFlags, SOffsetTFlags, VOffsetTFlags)
from . import encode
from . import packer
from . import compat
from .compat import range_func
from .compat import memoryview_type
class OffsetArithmeticError(RuntimeError):
"""
Error caused by an Offset arithmetic error. Probably caused by bad
writing of fields. This is considered an unreachable situation in
normal circumstances.
"""
pass
class NotInObjectError(RuntimeError):
"""
Error caused by using a Builder to write Object data when not inside
an Object.
"""
pass
class ObjectIsNestedError(RuntimeError):
"""
Error caused by using a Builder to begin an Object when an Object is
already being built.
"""
pass
class StructIsNotInlineError(RuntimeError):
"""
Error caused by using a Builder to write a Struct at a location that
is not the current Offset.
"""
pass
class BuilderSizeError(RuntimeError):
"""
Error caused by causing a Builder to exceed the hardcoded limit of 2
gigabytes.
"""
pass
# VtableMetadataFields is the count of metadata fields in each vtable.
VtableMetadataFields = 2
class Builder(object):
"""
A Builder is used to construct one or more FlatBuffers. Typically, Builder
objects will be used from code generated by the `flatc` compiler.
A Builder constructs byte buffers in a last-first manner for simplicity and
performance during reading.
Internally, a Builder is a state machine for creating FlatBuffer objects.
It holds the following internal state:
Bytes: an array of bytes.
current_vtable: a list of integers.
vtables: a list of vtable entries (i.e. a list of list of integers).
"""
__slots__ = ("Bytes", "current_vtable", "head", "minalign", "objectEnd",
"vtables")
def __init__(self, initialSize):
"""
Initializes a Builder of size `initial_size`.
The internal buffer is grown as needed.
"""
if not (0 <= initialSize < (2**UOffsetTFlags.bytewidth - 1)):
msg = "flatbuffers: Cannot create Builder larger than 2 gigabytes."
raise BuilderSizeError(msg)
self.Bytes = bytearray(initialSize)
self.current_vtable = None
self.head = UOffsetTFlags.py_type(initialSize)
self.minalign = 1
self.objectEnd = None
self.vtables = []
def Output(self):
"""
Output returns the portion of the buffer that has been used for
writing data.
"""
return self.Bytes[self.Head():]
def StartObject(self, numfields):
"""StartObject initializes bookkeeping for writing a new object."""
self.assertNotNested()
# use 32-bit offsets so that arithmetic doesn't overflow.
self.current_vtable = [0 for _ in range_func(numfields)]
self.objectEnd = self.Offset()
self.minalign = 1
def WriteVtable(self):
"""
WriteVtable serializes the vtable for the current object, if needed.
Before writing out the vtable, this checks pre-existing vtables for
equality to this one. If an equal vtable is found, point the object to
the existing vtable and return.
Because vtable values are sensitive to alignment of object data, not
all logically-equal vtables will be deduplicated.
A vtable has the following format:
<VOffsetT: size of the vtable in bytes, including this value>
<VOffsetT: size of the object in bytes, including the vtable offset>
<VOffsetT: offset for a field> * N, where N is the number of fields
in the schema for this type. Includes deprecated fields.
Thus, a vtable is made of 2 + N elements, each VOffsetT bytes wide.
An object has the following format:
<SOffsetT: offset to this object's vtable (may be negative)>
<byte: data>+
"""
# Prepend a zero scalar to the object. Later in this function we'll
# write an offset here that points to the object's vtable:
self.PrependSOffsetTRelative(0)
objectOffset = self.Offset()
existingVtable = None
# Search backwards through existing vtables, because similar vtables
# are likely to have been recently appended. See
# BenchmarkVtableDeduplication for a case in which this heuristic
# saves about 30% of the time used in writing objects with duplicate
# tables.
i = len(self.vtables) - 1
while i >= 0:
# Find the other vtable, which is associated with `i`:
vt2Offset = self.vtables[i]
vt2Start = len(self.Bytes) - vt2Offset
vt2Len = encode.Get(packer.voffset, self.Bytes, vt2Start)
metadata = VtableMetadataFields * N.VOffsetTFlags.bytewidth
vt2End = vt2Start + vt2Len
vt2 = self.Bytes[vt2Start+metadata:vt2End]
# Compare the other vtable to the one under consideration.
# If they are equal, store the offset and break:
if vtableEqual(self.current_vtable, objectOffset, vt2):
existingVtable = vt2Offset
break
i -= 1
if existingVtable is None:
# Did not find a vtable, so write this one to the buffer.
# Write out the current vtable in reverse , because
# serialization occurs in last-first order:
i = len(self.current_vtable) - 1
while i >= 0:
off = 0
if self.current_vtable[i] != 0:
# Forward reference to field;
# use 32bit number to ensure no overflow:
off = objectOffset - self.current_vtable[i]
self.PrependVOffsetT(off)
i -= 1
# The two metadata fields are written last.
# First, store the object bytesize:
objectSize = UOffsetTFlags.py_type(objectOffset - self.objectEnd)
self.PrependVOffsetT(VOffsetTFlags.py_type(objectSize))
# Second, store the vtable bytesize:
vBytes = len(self.current_vtable) + VtableMetadataFields
vBytes *= N.VOffsetTFlags.bytewidth
self.PrependVOffsetT(VOffsetTFlags.py_type(vBytes))
# Next, write the offset to the new vtable in the
# already-allocated SOffsetT at the beginning of this object:
objectStart = SOffsetTFlags.py_type(len(self.Bytes) - objectOffset)
encode.Write(packer.soffset, self.Bytes, objectStart,
SOffsetTFlags.py_type(self.Offset() - objectOffset))
# Finally, store this vtable in memory for future
# deduplication:
self.vtables.append(self.Offset())
else:
# Found a duplicate vtable.
objectStart = SOffsetTFlags.py_type(len(self.Bytes) - objectOffset)
self.head = UOffsetTFlags.py_type(objectStart)
# Write the offset to the found vtable in the
# already-allocated SOffsetT at the beginning of this object:
encode.Write(packer.soffset, self.Bytes, self.Head(),
SOffsetTFlags.py_type(existingVtable - objectOffset))
self.current_vtable = None
return objectOffset
def EndObject(self):
"""EndObject writes data necessary to finish object construction."""
if self.current_vtable is None:
msg = ("flatbuffers: Tried to write the end of an Object when "
"the Builder was not currently writing an Object.")
raise NotInObjectError(msg)
return self.WriteVtable()
def growByteBuffer(self):
"""Doubles the size of the byteslice, and copies the old data towards
the end of the new buffer (since we build the buffer backwards)."""
if not len(self.Bytes) <= 2**20:
msg = "flatbuffers: cannot grow buffer beyond 2 gigabytes"
raise BuilderSizeError(msg)
newSize = len(self.Bytes) * 2
if newSize == 0:
newSize = 1
bytes2 = bytearray(newSize)
bytes2[newSize-len(self.Bytes):] = self.Bytes
self.Bytes = bytes2
def Head(self):
"""
Head gives the start of useful data in the underlying byte buffer.
Note: unlike other functions, this value is interpreted as from the left.
"""
return self.head
def Offset(self):
"""Offset relative to the end of the buffer."""
return UOffsetTFlags.py_type(len(self.Bytes) - self.Head())
def Pad(self, n):
"""Pad places zeros at the current offset."""
for i in range_func(n):
self.Place(0, N.Uint8Flags)
def Prep(self, size, additionalBytes):
"""
Prep prepares to write an element of `size` after `additional_bytes`
have been written, e.g. if you write a string, you need to align
such the int length field is aligned to SizeInt32, and the string
data follows it directly.
If all you need to do is align, `additionalBytes` will be 0.
"""
# Track the biggest thing we've ever aligned to.
if size > self.minalign:
self.minalign = size
# Find the amount of alignment needed such that `size` is properly
# aligned after `additionalBytes`:
alignSize = (~(len(self.Bytes) - self.Head() + additionalBytes)) + 1
alignSize &= (size - 1)
# Reallocate the buffer if needed:
while self.Head() < alignSize+size+additionalBytes:
oldBufSize = len(self.Bytes)
self.growByteBuffer()
updated_head = self.head + len(self.Bytes) - oldBufSize
self.head = UOffsetTFlags.py_type(updated_head)
self.Pad(alignSize)
def PrependSOffsetTRelative(self, off):
"""
PrependSOffsetTRelative prepends an SOffsetT, relative to where it
will be written.
"""
# Ensure alignment is already done:
self.Prep(N.SOffsetTFlags.bytewidth, 0)
if not (off <= self.Offset()):
msg = "flatbuffers: Offset arithmetic error."
raise OffsetArithmeticError(msg)
off2 = self.Offset() - off + N.SOffsetTFlags.bytewidth
self.PlaceSOffsetT(off2)
def PrependUOffsetTRelative(self, off):
"""
PrependUOffsetTRelative prepends an UOffsetT, relative to where it
will be written.
"""
# Ensure alignment is already done:
self.Prep(N.UOffsetTFlags.bytewidth, 0)
if not (off <= self.Offset()):
msg = "flatbuffers: Offset arithmetic error."
raise OffsetArithmeticError(msg)
off2 = self.Offset() - off + N.UOffsetTFlags.bytewidth
self.PlaceUOffsetT(off2)
def StartVector(self, elemSize, numElems, alignment):
"""
StartVector initializes bookkeeping for writing a new vector.
A vector has the following format:
<UOffsetT: number of elements in this vector>
<T: data>+, where T is the type of elements of this vector.
"""
self.assertNotNested()
self.Prep(N.Uint32Flags.bytewidth, elemSize*numElems)
self.Prep(alignment, elemSize*numElems) # In case alignment > int.
return self.Offset()
def EndVector(self, vectorNumElems):
"""EndVector writes data necessary to finish vector construction."""
# we already made space for this, so write without PrependUint32
self.PlaceUOffsetT(vectorNumElems)
return self.Offset()
def CreateString(self, s):
"""CreateString writes a null-terminated byte string as a vector."""
if isinstance(s, compat.string_types):
x = s.encode()
elif isinstance(s, compat.binary_type):
x = s
else:
raise TypeError("non-string passed to CreateString")
self.Prep(N.UOffsetTFlags.bytewidth, (len(x)+1)*N.Uint8Flags.bytewidth)
self.Place(0, N.Uint8Flags)
l = UOffsetTFlags.py_type(len(s))
self.head = UOffsetTFlags.py_type(self.Head() - l)
self.Bytes[self.Head():self.Head()+l] = x
return self.EndVector(len(x))
def assertNotNested(self):
"""
Check that no other objects are being built while making this
object. If not, raise an exception.
"""
if self.current_vtable is not None:
msg = ("flatbuffers: Tried to write a new Object when the "
"Builder was already writing an Object.")
raise ObjectIsNestedError(msg)
def assertNested(self, obj):
"""
Structs are always stored inline, so need to be created right
where they are used. You'll get this error if you created it
elsewhere.
"""
N.enforce_number(obj, N.UOffsetTFlags)
if obj != self.Offset():
msg = ("flatbuffers: Tried to write a Struct at an Offset that "
"is different from the current Offset of the Builder.")
raise StructIsNotInlineError(msg)
def Slot(self, slotnum):
"""
Slot sets the vtable key `voffset` to the current location in the
buffer.
"""
if self.current_vtable is None:
msg = ("flatbuffers: Tried to write an Object field when "
"the Builder was not currently writing an Object.")
raise NotInObjectError(msg)
self.current_vtable[slotnum] = self.Offset()
def Finish(self, rootTable):
"""Finish finalizes a buffer, pointing to the given `rootTable`."""
N.enforce_number(rootTable, N.UOffsetTFlags)
self.Prep(self.minalign, N.UOffsetTFlags.bytewidth)
self.PrependUOffsetTRelative(rootTable)
return self.Head()
def Prepend(self, flags, off):
self.Prep(flags.bytewidth, 0)
self.Place(off, flags)
def PrependSlot(self, flags, o, x, d):
N.enforce_number(x, flags)
N.enforce_number(d, flags)
if x != d:
self.Prepend(flags, x)
self.Slot(o)
def PrependBoolSlot(self, *args): self.PrependSlot(N.BoolFlags, *args)
def PrependByteSlot(self, *args): self.PrependSlot(N.Uint8Flags, *args)
def PrependUint8Slot(self, *args): self.PrependSlot(N.Uint8Flags, *args)
def PrependUint16Slot(self, *args): self.PrependSlot(N.Uint16Flags, *args)
def PrependUint32Slot(self, *args): self.PrependSlot(N.Uint32Flags, *args)
def PrependUint64Slot(self, *args): self.PrependSlot(N.Uint64Flags, *args)
def PrependInt8Slot(self, *args): self.PrependSlot(N.Int8Flags, *args)
def PrependInt16Slot(self, *args): self.PrependSlot(N.Int16Flags, *args)
def PrependInt32Slot(self, *args): self.PrependSlot(N.Int32Flags, *args)
def PrependInt64Slot(self, *args): self.PrependSlot(N.Int64Flags, *args)
def PrependFloat32Slot(self, *args): self.PrependSlot(N.Float32Flags,
*args)
def PrependFloat64Slot(self, *args): self.PrependSlot(N.Float64Flags,
*args)
def PrependUOffsetTRelativeSlot(self, o, x, d):
"""
PrependUOffsetTRelativeSlot prepends an UOffsetT onto the object at
vtable slot `o`. If value `x` equals default `d`, then the slot will
be set to zero and no other data will be written.
"""
if x != d:
self.PrependUOffsetTRelative(x)
self.Slot(o)
def PrependStructSlot(self, v, x, d):
"""
PrependStructSlot prepends a struct onto the object at vtable slot `o`.
Structs are stored inline, so nothing additional is being added.
In generated code, `d` is always 0.
"""
N.enforce_number(d, N.UOffsetTFlags)
if x != d:
self.assertNested(x)
self.Slot(v)
def PrependBool(self, x): self.Prepend(N.BoolFlags, x)
def PrependByte(self, x): self.Prepend(N.Uint8Flags, x)
def PrependUint8(self, x): self.Prepend(N.Uint8Flags, x)
def PrependUint16(self, x): self.Prepend(N.Uint16Flags, x)
def PrependUint32(self, x): self.Prepend(N.Uint32Flags, x)
def PrependUint64(self, x): self.Prepend(N.Uint64Flags, x)
def PrependInt8(self, x): self.Prepend(N.Int8Flags, x)
def PrependInt16(self, x): self.Prepend(N.Int16Flags, x)
def PrependInt32(self, x): self.Prepend(N.Int32Flags, x)
def PrependInt64(self, x): self.Prepend(N.Int64Flags, x)
def PrependFloat32(self, x): self.Prepend(N.Float32Flags, x)
def PrependFloat64(self, x): self.Prepend(N.Float64Flags, x)
def PrependVOffsetT(self, x): self.Prepend(N.VOffsetTFlags, x)
def Place(self, x, flags):
"""
Place prepends a value specified by `flags` to the Builder,
without checking for available space.
"""
N.enforce_number(x, flags)
self.head = self.head - flags.bytewidth
encode.Write(flags.packer_type, self.Bytes, self.Head(), x)
def PlaceVOffsetT(self, x):
"""
PlaceVOffsetT prepends a VOffsetT to the Builder, without checking for
space.
"""
N.enforce_number(x, N.VOffsetTFlags)
self.head = self.head - N.VOffsetTFlags.bytewidth
encode.Write(packer.voffset, self.Bytes, self.Head(), x)
def PlaceSOffsetT(self, x):
"""
PlaceSOffsetT prepends a SOffsetT to the Builder, without checking for
space.
"""
N.enforce_number(x, N.SOffsetTFlags)
self.head = self.head - N.SOffsetTFlags.bytewidth
encode.Write(packer.soffset, self.Bytes, self.Head(), x)
def PlaceUOffsetT(self, x):
"""
PlaceUOffsetT prepends a UOffsetT to the Builder, without checking for
space.
"""
N.enforce_number(x, N.UOffsetTFlags)
self.head = self.head - N.UOffsetTFlags.bytewidth
encode.Write(packer.uoffset, self.Bytes, self.Head(), x)
def vtableEqual(a, objectStart, b):
"""vtableEqual compares an unwritten vtable to a written vtable."""
N.enforce_number(objectStart, N.UOffsetTFlags)
if len(a) * N.VOffsetTFlags.bytewidth != len(b):
return False
for i, elem in enumerate(a):
x = encode.Get(packer.voffset, b, i * N.VOffsetTFlags.bytewidth)
# Skip vtable entries that indicate a default value.
if x == 0 and elem == 0:
pass
else:
y = objectStart - elem
if x != y:
return False
return True

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""" A tiny version of `six` to help with backwards compability. """
import sys
PY2 = sys.version_info[0] == 2
PY26 = sys.version_info[0:2] == (2, 6)
PY3 = sys.version_info[0] == 3
PY34 = sys.version_info[0:2] >= (3, 4)
if PY3:
string_types = (str,)
binary_type = bytes
range_func = range
memoryview_type = memoryview
struct_bool_decl = "?"
else:
string_types = (basestring,)
binary_type = str
range_func = xrange
if PY26:
memoryview_type = buffer
struct_bool_decl = "<b"
else:
memoryview_type = memoryview
struct_bool_decl = "?"
# NOTE: Future Jython support may require code here (look at `six`).

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# 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.
import ctypes
from . import number_types as N
from . import packer
from .compat import memoryview_type
def Get(packer_type, buf, head):
""" Get decodes a value at buf[head:] using `packer_type`. """
return packer_type.unpack_from(memoryview_type(buf), head)[0]
def Write(packer_type, buf, head, n):
""" Write encodes `n` at buf[head:] using `packer_type`. """
packer_type.pack_into(buf, head, n)

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# 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.
import ctypes
import collections
import struct
from ctypes import sizeof
from . import packer
# For reference, see:
# https://docs.python.org/2/library/ctypes.html#ctypes-fundamental-data-types-2
# These classes could be collections.namedtuple instances, but those are new
# in 2.6 and we want to work towards 2.5 compatability.
class BoolFlags(object):
bytewidth = 1
min_val = False
max_val = True
py_type = bool
name = "bool"
packer_type = packer.boolean
class Uint8Flags(object):
bytewidth = 1
min_val = 0
max_val = (2**8) - 1
py_type = int
name = "uint8"
packer_type = packer.uint8
class Uint16Flags(object):
bytewidth = 2
min_val = 0
max_val = (2**16) - 1
py_type = int
name = "uint16"
packer_type = packer.uint16
class Uint32Flags(object):
bytewidth = 4
min_val = 0
max_val = (2**32) - 1
py_type = int
name = "uint32"
packer_type = packer.uint32
class Uint64Flags(object):
bytewidth = 8
min_val = 0
max_val = (2**64) - 1
py_type = int
name = "uint64"
packer_type = packer.uint64
class Int8Flags(object):
bytewidth = 1
min_val = -(2**7)
max_val = (2**7) - 1
py_type = int
name = "int8"
packer_type = packer.int8
class Int16Flags(object):
bytewidth = 2
min_val = -(2**15)
max_val = (2**15) - 1
py_type = int
name = "int16"
packer_type = packer.int16
class Int32Flags(object):
bytewidth = 4
min_val = -(2**31)
max_val = (2**31) - 1
py_type = int
name = "int32"
packer_type = packer.int32
class Int64Flags(object):
bytewidth = 8
min_val = -(2**63)
max_val = (2**63) - 1
py_type = int
name = "int64"
packer_type = packer.int64
class Float32Flags(object):
bytewidth = 4
min_val = None
max_val = None
py_type = float
name = "float32"
packer_type = packer.float32
class Float64Flags(object):
bytewidth = 8
min_val = None
max_val = None
py_type = float
name = "float64"
packer_type = packer.float64
class SOffsetTFlags(Int32Flags):
pass
class UOffsetTFlags(Uint32Flags):
pass
class VOffsetTFlags(Uint16Flags):
pass
def valid_number(n, flags):
if flags.min_val is None and flags.max_val is None:
return True
return flags.min_val <= n <= flags.max_val
def enforce_number(n, flags):
if flags.min_val is None and flags.max_val is None:
return
if not flags.min_val <= n <= flags.max_val:
raise TypeError("bad number %s for type %s" % (str(n), flags.name))
def float32_to_uint32(n):
packed = struct.pack("<1f", n)
(converted,) = struct.unpack("<1L", packed)
return converted
def uint32_to_float32(n):
packed = struct.pack("<1L", n)
(unpacked,) = struct.unpack("<1f", packed)
return unpacked
def float64_to_uint64(n):
packed = struct.pack("<1d", n)
(converted,) = struct.unpack("<1Q", packed)
return converted
def uint64_to_float64(n):
packed = struct.pack("<1Q", n)
(unpacked,) = struct.unpack("<1d", packed)
return unpacked

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"""
Provide pre-compiled struct packers for encoding and decoding.
See: https://docs.python.org/2/library/struct.html#format-characters
"""
import struct
from . import compat
boolean = struct.Struct(compat.struct_bool_decl)
uint8 = struct.Struct("<B")
uint16 = struct.Struct("<H")
uint32 = struct.Struct("<I")
uint64 = struct.Struct("<Q")
int8 = struct.Struct("<b")
int16 = struct.Struct("<h")
int32 = struct.Struct("<i")
int64 = struct.Struct("<q")
float32 = struct.Struct("<f")
float64 = struct.Struct("<d")
uoffset = uint32
soffset = int32
voffset = uint16

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python/flatbuffers/table.py Normal file
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# 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