cue: add Value.lookup

Implement lookup.

Updates #3100.
Updates #3074.

Signed-off-by: Aram Hăvărneanu <[email protected]>
Change-Id: I8b8c83cd244e93730eda7826bbb222a0ec496650
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue-py/+/1194802
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

cue/value.py

@@ -79,6 +79,18 @@ def unify(self, other: 'Value') -> 'Value':
         v = libcue.unify(self._val, other._val)
         return Value(self._ctx, v)
 
+    def lookup(self, path: str) -> 'Value':
+        """
+        Return the CUE value at path.
+
+        Args:
+            path: CUE path relative to self.
+
+        Returns:
+            Value: the value reached at path, starting from self.
+        """
+        return _lookup(self, path)
+
     def to_int(self) -> int:
         """
         Convert CUE value to integer.
@@ -278,3 +290,12 @@ def _to_json(val: Value) -> str:
     s = dec.decode("utf-8")
     libcue.libc_free(buf_ptr[0])
     return s
+
+def _lookup(val: Value, path: str) -> Value:
+    val_ptr = libcue.ffi.new("cue_value*")
+    path_ptr = libcue.ffi.new("char[]", path.encode("utf-8"))
+
+    err = libcue.lookup_string(val._val, path_ptr, val_ptr)
+    if err != 0:
+        raise Error(err)
+    return Value(val._ctx, val_ptr[0])

tests/test_value.py

@@ -139,3 +139,13 @@ def test_decode_error():
 
     with pytest.raises(cue.Error):
         ctx.compile("false").to_unsigned()
+
+def test_lookup():
+    ctx = cue.Context()
+
+    val = ctx.compile("x: true")
+    assert val.lookup("x").to_bool() == True
+
+    val = ctx.compile(r'x: y: { a: 1, b: "hello"}')
+    assert val.lookup("x").lookup("y").lookup("b").to_str() == "hello"
+    assert val.lookup("x.y.a").to_int() == 1