From 11f7edfd0eeba13f0affedf54735f889e4c23a24 Mon Sep 17 00:00:00 2001
From: Bas van Beek <43369155+BvB93@users.noreply.github.com>
Date: Mon, 19 Jul 2021 19:34:17 +0200
Subject: [PATCH] ENH: Delay assembling of the error message until `__str__` is
 called

Co-Authored-By: Eric Wieser <425260+eric-wieser@users.noreply.github.com>
---
 numpy/core/_exceptions.py            | 47 +++++++++-------------------
 numpy/core/tests/test__exceptions.py | 10 ------
 2 files changed, 15 insertions(+), 42 deletions(-)

diff --git a/numpy/core/_exceptions.py b/numpy/core/_exceptions.py
index 7056d6c84eaa..157d78412a8d 100644
--- a/numpy/core/_exceptions.py
+++ b/numpy/core/_exceptions.py
@@ -180,47 +180,30 @@ class AxisError(ValueError, IndexError):
 
     """
 
-    __slots__ = ("axis", "ndim")
+    __slots__ = ("axis", "ndim", "_msg")
 
     def __init__(self, axis, ndim=None, msg_prefix=None):
-        # single-argument form just delegates to base class
-        if ndim is None and msg_prefix is None:
-            msg = axis
+        if ndim is msg_prefix is None:
+            # single-argument form: directly set the error message
+            self._msg = axis
             self.axis = None
             self.ndim = None
-
-        # do the string formatting here, to save work in the C code
         else:
-            msg = ("axis {} is out of bounds for array of dimension {}"
-                   .format(axis, ndim))
-            if msg_prefix is not None:
-                msg = "{}: {}".format(msg_prefix, msg)
+            self._msg = msg_prefix
             self.axis = axis
             self.ndim = ndim
 
-        super().__init__(msg)
-
-    @classmethod
-    def _reconstruct(cls, axis, ndim, args):
-        """Auxiliary instance constructor used by `__reduce__`.
-
-        Directly assign all instance attributes without further sanitization.
-
-        This method avoids the `__init__` constructor in order to:
-        * Circumvent dealing with its axis-based overload.
-        * Avoid extracting the message prefix from ``self.args`` if applicable.
-
-        """
-        self = super().__new__(cls)
-        self.axis = axis
-        self.ndim = ndim
-        self.args = args
-        return self
+    def __str__(self):
+        axis = self.axis
+        ndim = self.ndim
 
-    def __reduce__(self):
-        """Return state information for `pickle`."""
-        cls = type(self)
-        return cls._reconstruct, (self.axis, self.ndim, self.args)
+        if axis is ndim is None:
+            return self._msg
+        else:
+            msg = f"axis {axis} is out of bounds for array of dimension {ndim}"
+            if self._msg is not None:
+                msg = f"{self._msg}: {msg}"
+            return msg
 
 
 @_display_as_base
diff --git a/numpy/core/tests/test__exceptions.py b/numpy/core/tests/test__exceptions.py
index 22ddcb06fdab..c87412aa4349 100644
--- a/numpy/core/tests/test__exceptions.py
+++ b/numpy/core/tests/test__exceptions.py
@@ -86,13 +86,3 @@ def test_pickling(self, args):
             attr1 = getattr(exc, name)
             attr2 = getattr(exc2, name)
             assert attr1 == attr2, name
-
-    def test_pickling_compat(self, args):
-        """Test that `AxisError` instances pickled prior to NumPy 1.22
-        can still be unpickled.
-        """
-        # Attach a `__reduce__` method equivalent to the one used prior to 1.22
-        exc = np.AxisError(*args)
-        exc.__reduce__ = super(np.AxisError, exc).__reduce__
-
-        assert pickle.loads(pickle.dumps(exc))