Const updated: parameters reordered

construct/core.py

@@ -1247,14 +1247,14 @@ def _sizeof(self, context, path):
 #===============================================================================
 class Const(Subconstruct):
     r"""
-    Field enforcing a constant value. It is used for file signatures, to validate that the given pattern exists. When parsed, the value must match.
+    Field enforcing a constant value. It is used for file signatures, to validate that the given pattern exists. When parsed, the value must strictly match.
 
     Usually a member of a Struct, where it can be anonymous (so it does not appear in parsed dictionary for simplicity).
 
     Note that a variable length subcon may still provide positive verification. Const does not consume a precomputed amount of bytes (and hence does NOT require a fixed sized lenghtfield), but depends on the subcon to read the appropriate amount (eg. VarInt is acceptable).
 
-    :param subcon: the subcon used to build value from, or a bytes value
-    :param value: optional, the expected value
+    :param value: the expected value, or a bytes literal
+    :param subcon: optional, the subcon used to build value from, Bytes if value was a bytes literal
 
     :raises ConstError: when parsed data does not match specified value, or building from wrong value
 
@@ -1266,23 +1266,21 @@ class Const(Subconstruct):
         >>> d.parse(b"JPEG")
         construct.core.ConstError: expected b'IHDR' but parsed b'JPEG'
 
-        >>> d = Const(Int32ul, 16)
+        >>> d = Const(16, Int32ul)
         >>> d.build(None)
         b'\x10\x00\x00\x00'
     """
     __slots__ = ["value"]
-    def __init__(self, subcon, value=None):
-        if value is None:
-            subcon, value = Bytes(len(subcon)), subcon
-        if isinstance(subcon, str):
-            subcon, value = Bytes(len(value)), value
+    def __init__(self, value, subcon=None):
+        if subcon is None:
+            subcon = Bytes(len(value))
         super(Const, self).__init__(subcon)
         self.value = value
         self.flagbuildnone = True
     def _parse(self, stream, context, path):
         obj = self.subcon._parse(stream, context, path)
         if obj != self.value:
-            raise ConstError("parsing expected %r but got %r" % (self.value, obj))
+            raise ConstError("parsing expected %r but parsed %r" % (self.value, obj))
         return obj
     def _build(self, obj, stream, context, path):
         if obj not in (None, self.value):

construct/examples/formats/graphics/emf.py

@@ -111,7 +111,7 @@
 )
 
 header_record = Struct(
-    Const(record_type, "HEADER"),
+    Const("HEADER", record_type),
     "record_size" / Int32ul,              # Size of the record in bytes 
     "bounds_left" / Int32sl,              # Left inclusive bounds 
     "bounds_right" / Int32sl,             # Right inclusive bounds 
@@ -121,7 +121,7 @@
     "frame_right" / Int32sl,              # Right side of inclusive picture frame 
     "frame_top" / Int32sl,                # Top side of inclusive picture frame 
     "frame_bottom" / Int32sl,             # Bottom side of inclusive picture frame 
-    "signature" / Const(Int32ul, 0x464D4520),
+    "signature" / Const(0x464D4520, Int32ul),
     "version" / Int32ul,                  # Version of the metafile 
     "size" / Int32ul,                     # Size of the metafile in bytes 
     "num_of_records" / Int32ul,           # Number of records in the metafile 

construct/examples/formats/graphics/gif.py

@@ -38,7 +38,7 @@
 )
 
 application_extension = Struct(
-    "block_size" / Const(Int8ul, 11),
+    "block_size" / Const(11, Int8ul),
     "application_identifier" / String(8),
     "application_auth_code" / String(3),
     "data_sub_block" / data_sub_block,
@@ -51,7 +51,7 @@
 )
 
 graphic_control_extension = Struct(
-    "block_size" / Const(Int8ul, 4),
+    "block_size" / Const(4, Int8ul),
     "flags" / BitStruct(
         "reserved" / BitsInteger(3),
         "disposal_method" / BitsInteger(3),
@@ -64,7 +64,7 @@
 )
 
 plain_text_extension = Struct(
-    "block_size" / Const(Int8ul, 12),
+    "block_size" / Const(12, Int8ul),
     "text_left" / Int16ul,
     "text_top" / Int16ul,
     "text_width" / Int16ul,

construct/examples/formats/graphics/wmf.py

@@ -98,7 +98,7 @@
 )
 
 wmf_placeable_header = Struct(
-  "key" / Const(Int32ul, 0x9AC6CDD7),
+  "key" / Const(0x9AC6CDD7, Int32ul),
   "handle" / Int16ul,
   "left" / Int16sl,
   "top" / Int16sl,
@@ -118,7 +118,7 @@
         InMemory = 0,
         File = 1,
     ),
-    "header_size" / Const(Int16ul, 9),
+    "header_size" / Const(9, Int16ul),
     "version" / Int16ul,
     "size" / Int32ul, # file size is in words
     "number_of_objects" / Int16ul,

construct/examples/protocols/ipstack.py

@@ -126,7 +126,7 @@
 
 ipv4_header = Struct(
     EmbeddedBitStruct(
-        "version" / Const(Nibble, 4),
+        "version" / Const(4, Nibble),
         "header_length" / ExprAdapter(Nibble,
             decoder = lambda obj, ctx: obj * 4,
             encoder = lambda obj, ctx: obj // 4,

tests/test_all.py

@@ -343,11 +343,11 @@ def test_pointer(self):
 
     def test_const(self):
         common(Const(b"MZ"), b"MZ", b"MZ", 2)
-        common(Const(Bytes(4), b"****"), b"****", b"****", 4)
-        common(Const(Int32ul, 255), b"\xff\x00\x00\x00", 255, 4)
+        common(Const(b"****", Bytes(4)), b"****", b"****", 4)
+        common(Const(255, Int32ul), b"\xff\x00\x00\x00", 255, 4)
         assert raises(Const(b"MZ").parse, b"ELF") == ConstError
         assert raises(Const(b"MZ").build, b"???") == ConstError
-        assert raises(Const(Int32ul, 255).parse, b"\x00\x00\x00\x00") == ConstError
+        assert raises(Const(255, Int32ul).parse, b"\x00\x00\x00\x00") == ConstError
         common(Struct(sig=Const(b"MZ")), b"MZ", Container(sig=b"MZ"), 2)
         assert Struct(sig=Const(b"MZ")).build({}) == b"MZ"