Pointer updated, added stream optional parameter

construct/core.py

@@ -4044,6 +4044,7 @@ class Pointer(Subconstruct):
 
     :param offset: integer or context lambda, positive or negative
     :param subcon: Construct instance
+    :param stream: None to use original stream (default), or context lambda to provide a different stream
 
     :raises StreamError: requested reading negative amount, could not read enough bytes, requested writing different amount than actual data, or could not write all bytes
     :raises StreamError: stream is not seekable and tellable
@@ -4059,20 +4060,23 @@ class Pointer(Subconstruct):
         b'\x00\x00\x00\x00\x00\x00\x00\x00Z'
     """
 
-    def __init__(self, offset, subcon):
+    def __init__(self, offset, subcon, stream=None):
         super(Pointer, self).__init__(subcon)
         self.offset = offset
+        self.stream = stream
 
     def _parse(self, stream, context, path):
-        offset = self.offset(context) if callable(self.offset) else self.offset
+        offset = evaluate(self.offset, context)
+        stream = evaluate(self.stream, context) or stream
         fallback = stream_tell(stream)
         stream_seek(stream, offset, 2 if offset < 0 else 0)
         obj = self.subcon._parsereport(stream, context, path)
         stream_seek(stream, fallback)
         return obj
 
     def _build(self, obj, stream, context, path):
-        offset = self.offset(context) if callable(self.offset) else self.offset
+        offset = evaluate(self.offset, context)
+        stream = evaluate(self.stream, context) or stream
         fallback = stream_tell(stream)
         stream_seek(stream, offset, 2 if offset < 0 else 0)
         buildret = self.subcon._build(obj, stream, context, path)

docs/transition29.rst

@@ -79,7 +79,7 @@ EmbeddedSwitch added, in Conditional (`see tutorial page <https://construct.read
 
 StopIf raises `StopFieldError` instead of `StopIteration` (`see API page <https://construct.readthedocs.io/en/latest/api/conditional.html#construct.StopIf>`_)
 
-Pointer changed size to 0, can be parsed lazily
+Pointer changed size to 0, can be parsed lazily, can also select a stream from context entry
 
 PrefixedArray parameter `lengthfield` renamed to `countfield` (`see API page <https://construct.readthedocs.io/en/latest/api/tunneling.html#construct.PrefixedArray>`_)
 

tests/test_core.py

@@ -876,6 +876,12 @@ def test_pointer():
     common(Pointer(2,             Byte), b"\x00\x00\x07", 7, 0)
     common(Pointer(lambda ctx: 2, Byte), b"\x00\x00\x07", 7, 0)
 
+    d = Struct(
+        'inner' / Struct(),
+        'x' / Pointer(0, Byte, stream=this.inner._io),
+    )
+    d.parse(bytes(20)) == 0
+
 def test_peek():
     d = Peek(Int8ub)
     assert d.parse(b"\x01") == 1