Const taken over ConstAdapter and Magic.

construct/__init__.py

@@ -22,14 +22,14 @@
     Container, FieldError, FormatField, LazyBound, LazyContainer, ListContainer, MetaArray, MetaField, OnDemand, 
     OverwriteError, Packer, Pass, Peek, Pointer, Range, RangeError, Reconfig, RepeatUntil, Restream, Select, 
     SelectError, Sequence, SizeofError, StaticField, Struct, Subconstruct, Switch, SwitchError, Terminator, 
-    TerminatorError, ULInt24, Union, Computed, Padding, PaddingError)
-from construct.adapters import (BitIntegerAdapter, BitIntegerError, CStringAdapter, ConstAdapter, ConstError, 
+    TerminatorError, ULInt24, Union, Computed, Padding, PaddingError, Const, ConstError)
+from construct.adapters import (BitIntegerAdapter, BitIntegerError, CStringAdapter, 
     ExprAdapter, FlagsAdapter, FlagsContainer, HexDumpAdapter, HexString, IndexingAdapter, LengthValueAdapter, 
     MappingAdapter, MappingError, NoneOf, OneOf, PaddedStringAdapter, SlicingAdapter, 
     StringAdapter, TunnelAdapter, ValidationError, Validator) 
 from construct.macros import (Alias, Aligned, AlignedStruct, Array, BFloat32, BFloat64, Bit, BitField, 
     BitStreamReader, BitStreamWriter, BitStruct, Bitwise, CString, Embedded, EmbeddedBitStruct, Enum, Field,
-    Flag, FlagsEnum, GreedyRange, If, IfThenElse, LFloat32, LFloat64, Magic, NFloat32, NFloat64, Nibble, Octet, 
+    Flag, FlagsEnum, GreedyRange, If, IfThenElse, LFloat32, LFloat64, NFloat32, NFloat64, Nibble, Octet, 
     OnDemandPointer, OpenRange, Optional, OptionalGreedyRange, PascalString, PrefixedArray, 
     Rename, SBInt16, SBInt32, SBInt64, SBInt8, SLInt16, SLInt32, SLInt64, SLInt8, SNInt16, SNInt32, SNInt64, 
     SNInt8, SeqOfOne, String, SymmetricMapping, UBInt16, UBInt32, UBInt64, UBInt8, ULInt16, ULInt32, ULInt64, 
@@ -50,7 +50,6 @@
 Bits = BitField
 Byte = UBInt8
 Bytes = Field
-Const = ConstAdapter
 Tunnel = TunnelAdapter
 Embed = Embedded
 
@@ -60,11 +59,11 @@
 __all__ = [
     'AdaptationError', 'Adapter', 'Alias', 'Aligned', 'AlignedStruct', 'Anchor', 'Array', 'ArrayError', 
     'BFloat32', 'BFloat64', 'Bit', 'BitField', 'BitIntegerAdapter', 'BitIntegerError', 'BitStreamReader', 
-    'BitStreamWriter', 'BitStruct', 'Bitwise', 'Buffered', 'CString', 'CStringAdapter', 'ConstAdapter', 
-    'ConstError', 'Construct', 'ConstructError', 'Container', 'Debugger', 'Embedded', 'EmbeddedBitStruct', 
+    'BitStreamWriter', 'BitStruct', 'Bitwise', 'Buffered', 'CString', 'CStringAdapter', 
+    'Construct', 'ConstructError', 'Container', 'Debugger', 'Embedded', 'EmbeddedBitStruct', 
     'Enum', 'ExprAdapter', 'Field', 'FieldError', 'Flag', 'FlagsAdapter', 'FlagsContainer', 'FlagsEnum', 
     'FormatField', 'GreedyRange', 'HexDumpAdapter', 'HexString', 'If', 'IfThenElse', 'IndexingAdapter', 
-    'LFloat32', 'LFloat64', 'LazyBound', 'LazyContainer', 'LengthValueAdapter', 'ListContainer', 'Magic', 
+    'LFloat32', 'LFloat64', 'LazyBound', 'LazyContainer', 'LengthValueAdapter', 'ListContainer', 
     'MappingAdapter', 'MappingError', 'MetaArray', 'MetaField', 'NFloat32', 'NFloat64', 'Nibble', 'NoneOf', 
     'Octet', 'OnDemand', 'OnDemandPointer', 'OneOf', 'OpenRange', 'Optional', 'OptionalGreedyRange', 
     'OverwriteError', 'Packer', 'PaddedStringAdapter', 'Padding', 'PaddingError', 
@@ -75,5 +74,5 @@
     'Subconstruct', 'Switch', 'SwitchError', 'SymmetricMapping', 'Terminator', 'TerminatorError', 
     'TunnelAdapter', 'UBInt16', 'UBInt32', 'UBInt64', 'UBInt8', 'ULInt16', 'ULInt24', 'ULInt32', 'ULInt64', 
     'ULInt8', 'UNInt16', 'UNInt32', 'UNInt64', 'UNInt8', 'Union', 'ValidationError', 'Validator', 'Computed', 
-    'this', 'Bits', 'Byte', 'Bytes', 'Const', 'Tunnel', 'Embed',
+    'this', 'Bits', 'Byte', 'Bytes', 'Tunnel', 'Embed', 'Const', 'ConstError', 
 ]

construct/adapters.py

@@ -12,8 +12,6 @@ class BitIntegerError(AdaptationError):
     pass
 class MappingError(AdaptationError):
     pass
-class ConstError(AdaptationError):
-    pass
 class ValidationError(AdaptationError):
     pass
 
@@ -306,32 +304,6 @@ def _encode(self, obj, context):
     def _decode(self, obj, context):
         return HexString(obj, linesize = self.linesize)
 
-class ConstAdapter(Adapter):
-    """
-    Adapter for enforcing a constant value ("magic numbers"). When decoding,
-    the return value is checked; when building, the value is substituted in.
-
-    :param subcon: the subcon to validate
-    :param value: the expected value
-
-    Example::
-
-        Const(Field("signature", 2), "MZ")
-    """
-    __slots__ = ["value"]
-    def __init__(self, subcon, value):
-        super(ConstAdapter, self).__init__(subcon)
-        self.value = value
-    def _encode(self, obj, context):
-        if obj is None or obj == self.value:
-            return self.value
-        else:
-            raise ConstError("expected %r, found %r" % (self.value, obj))
-    def _decode(self, obj, context):
-        if obj != self.value:
-            raise ConstError("expected %r, found %r" % (self.value, obj))
-        return obj
-
 class SlicingAdapter(Adapter):
     """
     Adapter for slicing a list (getting a slice from that list)

construct/core.py

@@ -31,6 +31,8 @@ class OverwriteError(ValueError):
     pass
 class PaddingError(ConstructError):
     pass
+class ConstError(ConstructError):
+    pass
 
 #===============================================================================
 # abstract constructs
@@ -1455,3 +1457,41 @@ def _build(self, obj, stream, context):
     def _sizeof(self, context):
         length = self.length(context) if callable(self.length) else self.length
         return length
+
+class Const(Construct):
+    r"""
+    Constant field enforcing a constant value. It is used for file signatures, 
+    to validate that the given pattern exists. When parsed, the value must match.
+
+    :param data: a bytes object
+    :param subcon: the subcon to validate
+    :param value: the expected value
+
+    Example::
+
+        Const(b"IHDR")
+        
+        Const("signature", b"IHDR")
+
+        Const(ULInt64("signature"), 123)
+
+    """
+    __slots__ = ["subcon", "value"]
+    def __init__(self, subcon, value=None):
+        if value is None:
+            subcon, value = StaticField(None, len(subcon)), subcon
+        if isinstance(subcon, str):
+            subcon, value = StaticField(subcon, len(value)), value
+        super(Const, self).__init__(subcon.name)
+        self.subcon = subcon
+        self.value = value
+    def _parse(self, stream, context):
+        obj = self.subcon._parse(stream, context)
+        if obj != self.value:
+            raise ConstError("expected %r but parsed %r" % (self.value,obj))
+        return obj
+    def _build(self, obj, stream, context):
+        return self.subcon._build(self.value, stream, context)
+    def _sizeof(self, context):
+        return self.subcon._sizeof(context)
+

construct/formats/graphics/png.py

@@ -317,7 +317,7 @@ def splt_info_data_length(ctx):
 
 image_header_chunk = Struct("image_header",
     UBInt32("length"),
-    Magic(b"IHDR"),
+    Const(b"IHDR"),
     UBInt32("width"),
     UBInt32("height"),
     UBInt8("bit_depth"),
@@ -348,7 +348,7 @@ def splt_info_data_length(ctx):
 # the complete PNG file
 #===============================================================================
 png_file = Struct("png",
-    Magic(b"\x89PNG\r\n\x1a\n"),
+    Const(b"\x89PNG\r\n\x1a\n"),
     image_header_chunk,
     Rename("chunks", GreedyRange(chunk)),
 )

construct/macros.py

@@ -2,8 +2,8 @@
 
 from construct.lib.py3compat import int2byte
 from construct.lib import BitStreamReader, BitStreamWriter, encode_bin, decode_bin
-from construct.core import Struct, MetaField, StaticField, FormatField, OnDemand, Pointer, Switch, Computed, RepeatUntil, MetaArray, Sequence, Range, Select, Pass, SizeofError, Buffered, Restream, Reconfig, Padding
-from construct.adapters import BitIntegerAdapter, ConstAdapter, CStringAdapter, LengthValueAdapter, IndexingAdapter, PaddedStringAdapter, FlagsAdapter, StringAdapter, MappingAdapter
+from construct.core import Struct, MetaField, StaticField, FormatField, OnDemand, Pointer, Switch, Computed, RepeatUntil, MetaArray, Sequence, Range, Select, Pass, SizeofError, Buffered, Restream, Reconfig, Padding, Const
+from construct.adapters import BitIntegerAdapter, CStringAdapter, LengthValueAdapter, IndexingAdapter, PaddedStringAdapter, FlagsAdapter, StringAdapter, MappingAdapter
 
 
 #===============================================================================
@@ -664,18 +664,3 @@ def OnDemandPointer(offsetfunc, subcon, force_build=True):
         force_build = force_build
     )
 
-def Magic(data):
-    """A 'magic number' construct. It is used for file signatures, to validate
-    that the given pattern exists. When parsed. the value must match.
-
-    :param data: a bytes object
-
-    Example::
-
-        elf_header = Struct("elf_header",
-            Magic("\x7fELF"),
-            # ...
-        )
-    """
-    return ConstAdapter(Field(None, len(data)), data)
-

tests/test_all.py

@@ -189,12 +189,19 @@ class ZlibCodec(object):
     [TunnelAdapter(PascalString("data", encoding = ZlibCodec), GreedyRange(UBInt16("elements"))).build, 
         [3] * 100, b"\rx\x9cc`f\x18\x16\x10\x00u\xf8\x01-", None],
 
-    [Const(Field("const", 2), b"MZ").parse, b"MZ", b"MZ", None],
-    [Const(Field("const", 2), b"MZ").parse, b"MS", None, ConstError],
-    [Const(Field("const", 2), b"MZ").build, b"MZ", b"MZ", None],
-    [Const(Field("const", 2), b"MZ").build, b"MS", None, ConstError],
-    [Const(Field("const", 2), b"MZ").build, None, b"MZ", None],
-
+    [Const(b"MZ").parse, b"MZ", b"MZ", None],
+    [Const(b"MZ").parse, b"ELF", None, ConstError],
+    [Const(b"MZ").build, None, b"MZ", None],
+    [Const(b"MZ").build, b"MZ", b"MZ", None],
+    [Const("const", b"MZ").parse, b"MZ", b"MZ", None],
+    [Const("const", b"MZ").parse, b"ELF", None, ConstError],
+    [Const("const", b"MZ").build, None, b"MZ", None],
+    [Const("const", b"MZ").build, b"MZ", b"MZ", None],
+    [Const(ULInt32("const"), 255).parse, b"\xff\x00\x00\x00", 255, None],
+    [Const(ULInt32("const"), 255).parse, b"\x00\x00\x00\x00", 255, ConstError],
+    [Const(ULInt32("const"), 255).build, None, b"\xff\x00\x00\x00", None],
+    [Const(ULInt32("const"), 255).build, 255, b"\xff\x00\x00\x00", None],
+
     [ExprAdapter(UBInt8("expradapter"), 
         encoder = lambda obj, ctx: obj // 7, 
         decoder = lambda obj, ctx: obj * 7).parse, 
@@ -266,10 +273,6 @@ class ZlibCodec(object):
     [IfThenElse("ifthenelse", lambda ctx: False, UBInt8("then"), UBInt16("else")).parse, b"\x00\x01", 1, None],
     [IfThenElse("ifthenelse", lambda ctx: True, UBInt8("then"), UBInt16("else")).build, 1, b"\x01", None],
     [IfThenElse("ifthenelse", lambda ctx: False, UBInt8("then"), UBInt16("else")).build, 1, b"\x00\x01", None],
-
-    [Magic(b"MZ").parse, b"MZ", b"MZ", None],
-    [Magic(b"MZ").parse, b"ELF", None, ConstError],
-    [Magic(b"MZ").build, None, b"MZ", None],
 ]
 
 
@@ -291,6 +294,7 @@ def _run_tests(self, tests):
                     continue
             else:
                 if exctype is not None:
+                    print("Testing: %r", (i, func, args, res, exctype))
                     errors.append("%s: expected exception %r" % (func, exctype))
                     continue
                 if r != res: