cue: add Value.default

Implement extraction of default values from CUE values.

Updates #3100.
Updates #3074.

Signed-off-by: Aram Hăvărneanu <[email protected]>
Change-Id: Ia4e3ea639658018c38e1c510cba86fec83274aa5
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue-py/+/1194803
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

cue/value.py

@@ -16,7 +16,7 @@
 Perform operations on CUE values.
 """
 
-from typing import Any, final
+from typing import Any, Optional, final
 from cue.error import Error
 import libcue
 
@@ -215,6 +215,15 @@ def to_json(self) -> str:
 
         return _to_json(self)
 
+    def default(self) -> Optional['Value']:
+        """
+        Return default value.
+
+        Returns:
+            Optional[Value]: the default value, if it exists, or None otherwise.
+        """
+        return _default(self)
+
 def _to_int(val: Value) -> int:
     ptr = libcue.ffi.new("int64_t*")
     err = libcue.dec_int64(val._val, ptr)
@@ -299,3 +308,10 @@ def _lookup(val: Value, path: str) -> Value:
     if err != 0:
         raise Error(err)
     return Value(val._ctx, val_ptr[0])
+
+def _default(val: Value) -> Optional[Value]:
+    ok_ptr = libcue.ffi.new("bool*")
+    res = libcue.default(val._val, ok_ptr)
+    if ok_ptr[0] == 1:
+        return Value(val._ctx, res)
+    return None

tests/test_value.py

@@ -149,3 +149,21 @@ def test_lookup():
     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
+
+def test_default():
+    ctx = cue.Context()
+
+    v = ctx.compile("int")
+    assert v.default() == None
+
+    v = ctx.compile("1")
+    assert v.default() == None
+
+    v = ctx.compile("int | *1")
+    assert v.default() == ctx.to_value(1)
+
+    v = ctx.compile(r'string | *"hello"')
+    assert v.default() == ctx.to_value("hello")
+
+    v = ctx.compile(r'(int | *1) & 2')
+    assert v.default() == None