Implement XPath function sum().

tests/test_model.py

@@ -7,7 +7,7 @@ from yangson.datatype import YangTypeError
 from yangson.instance import MinElements, NonexistentInstance
 from yangson.instvalue import ArrayValue
 from yangson.context import Context, BadPath, BadPrefName
-from yangson.xpath import InvalidXPath, NotSupported, XPathParser
+from yangson.xpath import InvalidXPath, NotSupported, XPathParser, XPathTypeError
 
 tree = """+--rw test:contA
 |  +--rw leafA?
@@ -387,6 +387,11 @@ def test_xpath(instance):
     xptest("string(number('foo'))", "NaN")
     xptest("number(true()) = 1")
     xptest("number(false()) = 0")
+    xptest("sum(leafA | leafB)", 77, conta)
+    xptest("string(sum(//leafE))", "NaN")
+    xptest("sum(//leafF)", 2)
+    with pytest.raises(XPathTypeError):
+        xptest("sum(42)")
 
 def test_instance_paths(data_model, instance):
     rid1 = data_model.parse_resource_id("/test:contA/testb:leafN")

yangson/xpath.py

@@ -578,6 +578,17 @@ class FuncSubstringBefore(BinaryExpr):
         ind = lres.find(rres)
         return lres[:ind] if ind >= 0 else ""
 
+class FuncSum(UnaryExpr):
+
+    def _eval(self, xctx: XPathContext) -> float:
+        ns = self.expr._eval(xctx)
+        if not isinstance(ns, NodeSet):
+            raise XPathTypeError(ns)
+        try:
+            return float(sum([n.value for n in ns]))
+        except TypeError:
+            return float('nan')
+
 class FuncTranslate(BinaryExpr):
 
     def __init__(self, s1: Expr, s2: Expr, s3: Expr) -> None:
@@ -933,6 +944,9 @@ class XPathParser(Parser):
     def _func_substring_before(self) -> FuncSubstringBefore:
         return FuncSubstringBefore(*self._two_args())
 
+    def _func_sum(self) -> FuncSum:
+        return FuncSum(self.parse())
+
     def _func_translate(self) -> FuncTranslate:
         s1, s2 = self._two_args()
         self.char(",")