Python: Added support for file_identifiers (#5123)

* Python: Added support for file_identifiers

* Added tests. Fixed file_identifier code.

* Python: Fixed excessive padding of file_identifier. Repaired tests.

* Python: Made code compatible with python2.7

* Python: Typo fix in @endcond

* whitespace normaalization

* Stylistic change from if(not X is None) to if(X is not None). Added comment to type string.

* Python: Added support for automatic code generation of file_identifiers. Added tests for said code generation.

* converted sprintf to snprintf

* Bugfix, added snprint deffinition for MSVC

* changed snprint deffinition for MSVC to sprint_s

* changed scanf to IntToStringHex. Renamed HasFileIdentifier to GenHasFileIdentifier.

* Added updated genereated code to commit

* Python bugix: flatc no longer produces HasFileIdentfier for shcemas with no file identifier

* Added tests to verify `MonsterBufferHasIdentifier` returns false on no Identifier

* Python: added tests for GetBufferIdentifier and BufferHasIdentifier
Python: removed unessasary parenethesis in if statements
Minor format changes.

* Python : correceted instances of keyword arguments being called as positional arguments

* fixed typos and grammer in comments

* Minor style fixes

* Indentation fix

* Equals style changes

* Python: Fixed Alignment Issues. Changed test code to test against atual output

* Ran make(forgot to run make last commit)

* Python: Style changes

* Style changes

* indentation and style

* readded CONTRIBUTING.md

* Formatting tweak

Mostly to make CI run again

* More formatting fixes

* More formatting fixes

* More formatting fixes

* More formatting fixes

* Formatting fix

* More formatting fixes

* Formatting

* ran generate_code.sh

python/flatbuffers/builder.py

@@ -510,13 +510,21 @@ class Builder(object):
         self.current_vtable[slotnum] = self.Offset()
     ## @endcond
 
-    def __Finish(self, rootTable, sizePrefix):
+    def __Finish(self, rootTable, sizePrefix, file_identifier=None):
         """Finish finalizes a buffer, pointing to the given `rootTable`."""
         N.enforce_number(rootTable, N.UOffsetTFlags)
-        prepSize = N.UOffsetTFlags.bytewidth
-        if sizePrefix:
-            prepSize += N.Int32Flags.bytewidth
-        self.Prep(self.minalign, prepSize)
+
+        if file_identifier is not None:
+            self.Prep(N.UOffsetTFlags.bytewidth, N.Uint8Flags.bytewidth*4)
+            
+            # Convert bytes object file_identifier to an array of 4 8-bit integers,
+            # and use big-endian to enforce size compliance.
+            # https://docs.python.org/2/library/struct.html#format-characters
+            file_identifier = N.struct.unpack(">BBBB", file_identifier)
+            for i in range(encode.FILE_IDENTIFIER_LENGTH-1, -1, -1):
+                # Place the bytes of the file_identifer in reverse order:
+                self.Place(file_identifier[i], N.Uint8Flags)   
+                
         self.PrependUOffsetTRelative(rootTable)
         if sizePrefix:
             size = len(self.Bytes) - self.Head()
@@ -525,16 +533,16 @@ class Builder(object):
         self.finished = True
         return self.Head()
 
-    def Finish(self, rootTable):
+    def Finish(self, rootTable, file_identifier=None):
         """Finish finalizes a buffer, pointing to the given `rootTable`."""
-        return self.__Finish(rootTable, False)
+        return self.__Finish(rootTable, False, file_identifier=file_identifier)
 
-    def FinishSizePrefixed(self, rootTable):
+    def FinishSizePrefixed(self, rootTable, file_identifier=None):
         """
         Finish finalizes a buffer, pointing to the given `rootTable`,
         with the size prefixed.
         """
-        return self.__Finish(rootTable, True)
+        return self.__Finish(rootTable, True, file_identifier=file_identifier)
 
     ## @cond FLATBUFFERS_INTERNAL
     def Prepend(self, flags, off):

python/flatbuffers/encode.py

@@ -19,6 +19,8 @@ from .compat import import_numpy, NumpyRequiredForThisFeature
 
 np = import_numpy()
 
+FILE_IDENTIFIER_LENGTH=4
+
 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]

python/flatbuffers/util.py

@@ -20,6 +20,21 @@ def GetSizePrefix(buf, offset):
 	"""Extract the size prefix from a buffer."""
 	return encode.Get(packer.int32, buf, offset)
 
+def GetBufferIdentifier(buf, offset, size_prefixed=False):
+        """Extract the file_identifier from a buffer"""
+        if size_prefixed:
+            # increase offset by size of UOffsetTFlags
+            offset += number_types.UOffsetTFlags.bytewidth
+        # increase offset by size of root table pointer
+        offset += number_types.UOffsetTFlags.bytewidth
+        # end of FILE_IDENTIFIER
+        end = offset + encode.FILE_IDENTIFIER_LENGTH
+        return buf[offset:end]
+
+def BufferHasIdentifier(buf, offset, file_identifier, size_prefixed=False):
+        got = GetBufferIdentifier(buf, offset, size_prefixed=size_prefixed)
+        return got == file_identifier
+
 def RemoveSizePrefix(buf, offset):
 	"""
 	Create a slice of a size-prefixed buffer that has