bpo-43487: Add unicode dundermethod fixer to 2to3

Lib/lib2to3/fixes/fix_unicode_dundermethods.py

@@ -0,0 +1,38 @@
+"""Fixer to convert name of __unicode__ class methods to __str__, unless it exists already.
+"""
+# Author: Bart Broere
+
+
+from lib2to3.fixer_base import BaseFix
+from lib2to3.pytree import Leaf
+
+
+class FixUnicodeDundermethods(BaseFix):
+    def match(self, node):
+        if isinstance(node, Leaf):
+            try:
+                if node.value == "__unicode__" and node.prev_sibling.value == "def":
+                    return True
+            except AttributeError:
+                return False
+        return False
+
+    def transform(self, node, results):
+        try:
+            for element in node.parent.parent.parent.leaves():
+                if element.value == "class":
+                    if not self._method_exists(node.parent.parent.parent, "__str__"):
+                        node.value = "__str__"
+                    return node
+                break
+        except AttributeError:
+            return None
+        return None
+
+    def _method_exists(self, class_node, method_name):
+        for element in class_node.leaves():
+            for element_j in element.leaves():
+                if element_j.value == "def":
+                    if element_j.next_sibling.value == method_name:
+                        return True
+        return False

Misc/NEWS.d/next/Library/2021-03-18-18-13-08.bpo-43487.WV817A.rst

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+Add a fixer to the 2to3 library that converts ``__unicode__`` methods
+to ``__str__``, if this can be done safely.