Improve filtering of ``NamedExpr``, particularly within ``If`` nodes (#1233)

* Improve filtering of ``NamedExpr``, particularly within ``If`` nodes

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

3 files changed, 82 insertions, 3 deletions

diff --git a/ChangeLog b/ChangeLog
index 4c098507..5c617318 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -18,6 +18,12 @@ Release date: TBA
 
 * Added missing ``kind`` (for ``Const``) and ``conversion`` (for ``FormattedValue``) fields to repr.
 
+* Fix crash with assignment expressions, nested if expressions and filtering of statements
+
+  Closes PyCQA/pylint#5178
+
+* Fix incorrect filtering of assignment expressions statements
+
 
 What's New in astroid 2.8.4?
 ============================
diff --git a/astroid/nodes/node_classes.py b/astroid/nodes/node_classes.py
index b8b7c575..3e4ccd11 100644
--- a/astroid/nodes/node_classes.py
+++ b/astroid/nodes/node_classes.py
@@ -486,8 +486,28 @@ class LookupMixIn:
                 continue
 
             if isinstance(assign_type, NamedExpr):
-                _stmts = [node]
-                continue
+                # If the NamedExpr is in an if statement we do some basic control flow inference
+                if_parent = _get_if_statement_ancestor(assign_type)
+                if if_parent:
+                    # If the if statement is within another if statement we append the node
+                    # to possible statements
+                    if _get_if_statement_ancestor(if_parent):
+                        optional_assign = False
+                        _stmts.append(node)
+                        _stmt_parents.append(stmt.parent)
+                    # If the if statement is first-level and not within an orelse block
+                    # we know that it will be evaluated
+                    elif not if_parent.is_orelse:
+                        _stmts = [node]
+                        _stmt_parents = [stmt.parent]
+                    # Else we do not known enough about the control flow to be 100% certain
+                    # and we append to possible statements
+                    else:
+                        _stmts.append(node)
+                        _stmt_parents.append(stmt.parent)
+                else:
+                    _stmts = [node]
+                    _stmt_parents = [stmt.parent]
 
             # XXX comment various branches below!!!
             try:
@@ -534,7 +554,7 @@ class LookupMixIn:
             # An AssignName node overrides previous assignments if:
             #   1. node's statement always assigns
             #   2. node and self are in the same block (i.e., has the same parent as self)
-            if isinstance(node, AssignName):
+            if isinstance(node, (NamedExpr, AssignName)):
                 if isinstance(stmt, ExceptHandler):
                     # If node's statement is an ExceptHandler, then it is the variable
                     # bound to the caught exception. If self is not contained within
@@ -2798,6 +2818,9 @@ class If(mixins.MultiLineBlockMixin, mixins.BlockRangeMixIn, Statement):
         self.orelse: typing.List[NodeNG] = []
         """The contents of the ``else`` block."""
 
+        self.is_orelse: bool = False
+        """Whether the if-statement is the orelse-block of another if statement."""
+
         super().__init__(lineno=lineno, col_offset=col_offset, parent=parent)
 
     def postinit(
@@ -2819,6 +2842,8 @@ class If(mixins.MultiLineBlockMixin, mixins.BlockRangeMixIn, Statement):
             self.body = body
         if orelse is not None:
             self.orelse = orelse
+        if isinstance(self.parent, If) and self in self.parent.orelse:
+            self.is_orelse = True
 
     @decorators.cachedproperty
     def blockstart_tolineno(self):
@@ -4197,6 +4222,11 @@ class NamedExpr(mixins.AssignTypeMixin, NodeNG):
 
     _astroid_fields = ("target", "value")
 
+    optional_assign = True
+    """Whether this node optionally assigns a variable.
+
+    Since NamedExpr are not always called they do not always assign."""
+
     def __init__(
         self,
         lineno: Optional[int] = None,
@@ -4795,3 +4825,11 @@ def is_from_decorator(node):
         if isinstance(parent, Decorators):
             return True
     return False
+
+
+def _get_if_statement_ancestor(node: NodeNG) -> Optional[If]:
+    """Return the first parent node that is an If node (or None)"""
+    for parent in node.node_ancestors():
+        if isinstance(parent, If):
+            return parent
+    return None
diff --git a/tests/unittest_regrtest.py b/tests/unittest_regrtest.py
index 8cee979c..9938429a 100644
--- a/tests/unittest_regrtest.py
+++ b/tests/unittest_regrtest.py
@@ -22,8 +22,11 @@ import sys
 import textwrap
 import unittest
 
+import pytest
+
 from astroid import MANAGER, Instance, nodes, test_utils
 from astroid.builder import AstroidBuilder, extract_node
+from astroid.const import PY38_PLUS
 from astroid.exceptions import InferenceError
 from astroid.raw_building import build_module
 
@@ -156,6 +159,38 @@ def test():
         base = next(result._proxied.bases[0].infer())
         self.assertEqual(base.name, "int")
 
+    @pytest.mark.skipif(not PY38_PLUS, reason="needs assignment expressions")
+    def test_filter_stmts_nested_if(self) -> None:
+        builder = AstroidBuilder()
+        data = """
+def test(val):
+    variable = None
+
+    if val == 1:
+        variable = "value"
+        if variable := "value":
+            pass
+
+    elif val == 2:
+        variable = "value_two"
+        variable = "value_two"
+
+    return variable
+"""
+        module = builder.string_build(data, __name__, __file__)
+        test_func = module["test"]
+        result = list(test_func.infer_call_result(module))
+        assert len(result) == 3
+        assert isinstance(result[0], nodes.Const)
+        assert result[0].value is None
+        assert result[0].lineno == 3
+        assert isinstance(result[1], nodes.Const)
+        assert result[1].value == "value"
+        assert result[1].lineno == 7
+        assert isinstance(result[1], nodes.Const)
+        assert result[2].value == "value_two"
+        assert result[2].lineno == 12
+
     def test_ancestors_patching_class_recursion(self) -> None:
         node = AstroidBuilder().string_build(
             textwrap.dedent(