Add support for Python 3.8's `NamedExpr` nodes, which is part of assignment expressions.

Close #674

6 files changed, 69 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 8061d6b6..251794d4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,10 @@ What's New in astroid 2.3.0?
 ============================
 Release Date: TBA
 
+* Add support for Python 3.8's `NamedExpr` nodes, which is part of assignment expressions.
+
+  Close #674
+
 * Added support for inferring `IfExp` nodes.
 
 * Instances of exceptions are inferred as such when inferring in non-exception context
diff --git a/astroid/as_string.py b/astroid/as_string.py
index 7042272f..9d92b3cf 100644
--- a/astroid/as_string.py
+++ b/astroid/as_string.py
@@ -611,6 +611,12 @@ class AsStringVisitor3(AsStringVisitor):
             super(AsStringVisitor3, self).visit_comprehension(node),
         )
 
+    def visit_namedexpr(self, node):
+        """Return an assignment expression node as string"""
+        target = node.target.accept(self)
+        value = node.value.accept(self)
+        return "%s := %s" % (target, value)
+
 
 def _import_string(names):
     """return a list of (name, asname) formatted as a string"""
diff --git a/astroid/node_classes.py b/astroid/node_classes.py
index 2408e568..d3d7634d 100644
--- a/astroid/node_classes.py
+++ b/astroid/node_classes.py
@@ -4623,6 +4623,31 @@ class JoinedStr(NodeNG):
         yield from self.values
 
 
+class NamedExpr(mixins.AssignTypeMixin, NodeNG):
+    """Represents the assignment from the assignment expression
+
+    >>> module = astroid.parse('if a := 1: pass')
+    >>> module.body[0].test
+    <NamedExpr l.1 at 0x7f23b2e4ed30>
+    """
+
+    _astroid_fields = ("target", "value")
+    target = None
+    """The assignment target
+
+    :type: Name
+    """
+    value = None
+    """The value that gets assigned in the expression
+
+    :type: NodeNG
+    """
+
+    def postinit(self, target, value):
+        self.target = target
+        self.value = value
+
+
 class Unknown(mixins.AssignTypeMixin, NodeNG):
     """This node represents a node in a constructed AST where
     introspection is not possible.  At the moment, it's only used in
diff --git a/astroid/nodes.py b/astroid/nodes.py
index 20672593..bf6911ac 100644
--- a/astroid/nodes.py
+++ b/astroid/nodes.py
@@ -59,6 +59,7 @@ from astroid.node_classes import (
     Keyword,
     List,
     Name,
+    NamedExpr,
     Nonlocal,
     Pass,
     Print,
@@ -149,6 +150,7 @@ ALL_NODE_CLASSES = (
     List,
     ListComp,
     Name,
+    NamedExpr,
     Nonlocal,
     Module,
     Pass,
diff --git a/astroid/rebuilder.py b/astroid/rebuilder.py
index c18893d8..a60785c4 100644
--- a/astroid/rebuilder.py
+++ b/astroid/rebuilder.py
@@ -1063,6 +1063,13 @@ class TreeRebuilder3(TreeRebuilder):
         )
         return newnode
 
+    def visit_namedexpr(self, node, parent):
+        newnode = nodes.NamedExpr(node.lineno, node.col_offset, parent)
+        newnode.postinit(
+            self.visit(node.target, newnode), self.visit(node.value, newnode)
+        )
+        return newnode
+
 
 if sys.version_info >= (3, 0):
     TreeRebuilder = TreeRebuilder3
diff --git a/astroid/tests/unittest_nodes.py b/astroid/tests/unittest_nodes.py
index 800eef3a..96a7f830 100644
--- a/astroid/tests/unittest_nodes.py
+++ b/astroid/tests/unittest_nodes.py
@@ -1134,5 +1134,30 @@ def test_f_string_correct_line_numbering():
     assert node.last_child().last_child().lineno == 5
 
 
+@pytest.mark.skipif(
+    sys.version_info[:2] < (3, 8), reason="needs assignment expressions"
+)
+def test_assignment_expression():
+    code = """
+    if __(a := 1):
+        pass
+    if __(b := test):
+        pass
+    """
+    first, second = astroid.extract_node(code)
+
+    assert isinstance(first.target, nodes.AssignName)
+    assert first.target.name == "a"
+    assert isinstance(first.value, nodes.Const)
+    assert first.value.value == 1
+    assert first.as_string() == "a := 1"
+
+    assert isinstance(second.target, nodes.AssignName)
+    assert second.target.name == "b"
+    assert isinstance(second.value, nodes.Name)
+    assert second.value.name == "test"
+    assert second.as_string() == "b := test"
+
+
 if __name__ == "__main__":
     unittest.main()