From 6c2c06eb8d0a284270566e20215e95e19b1859e6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=A9r=C3=A9mie=20du=20Boisberranger?=
 <jeremie@probabl.ai>
Date: Mon, 15 Apr 2024 19:53:00 +0200
Subject: [PATCH 1/3] numpy's behavior of element-wise comparison with strings
 may be unexpected

---
 sklearn/calibration.py            |  4 +---
 sklearn/tests/test_calibration.py | 11 +++++++++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/sklearn/calibration.py b/sklearn/calibration.py
index b02236eb600fe..40d3e5363a7e0 100644
--- a/sklearn/calibration.py
+++ b/sklearn/calibration.py
@@ -388,9 +388,7 @@ def fit(self, X, y, sample_weight=None, **fit_params):
                 n_folds = self.cv.n_splits
             else:
                 n_folds = None
-            if n_folds and np.any(
-                [np.sum(y == class_) < n_folds for class_ in self.classes_]
-            ):
+            if n_folds and np.any(np.unique(y, return_counts=True)[1] < n_folds):
                 raise ValueError(
                     f"Requesting {n_folds}-fold "
                     "cross-validation but provided less than "
diff --git a/sklearn/tests/test_calibration.py b/sklearn/tests/test_calibration.py
index eb7204b366729..833ef2ea7e558 100644
--- a/sklearn/tests/test_calibration.py
+++ b/sklearn/tests/test_calibration.py
@@ -1088,3 +1088,14 @@ def predict_proba(self, X):
     calibrator = CalibratedClassifierCV(model)
     # Does not raise an error
     calibrator.fit(*data)
+
+
+def test_error_less_class_samples_than_folds():
+    """Check that CalibratedClassifierCV works with string targets.
+
+    non-regression test for issue #28841.
+    """
+    X = np.random.normal(size=(20, 3))
+    y = ["a"] * 10 + ["b"] * 10
+
+    CalibratedClassifierCV(cv=3).fit(X, y)

From d740430dcdd67182068a598f6e6baba6a4da0996 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=A9r=C3=A9mie=20du=20Boisberranger?=
 <jeremie@probabl.ai>
Date: Mon, 15 Apr 2024 20:53:04 +0200
Subject: [PATCH 2/3] changelog entry

---
 doc/whats_new/v1.5.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/doc/whats_new/v1.5.rst b/doc/whats_new/v1.5.rst
index 1fc651e303e56..d075f4049d3ac 100644
--- a/doc/whats_new/v1.5.rst
+++ b/doc/whats_new/v1.5.rst
@@ -127,6 +127,13 @@ Changelog
     :pr:`123456` by :user:`Joe Bloggs <joeongithub>`.
     where 123455 is the *pull request* number, not the issue number.
 
+:mod:`sklearn.calibration`
+.........................
+
+- |Fix| Fixed a regression in :class:`calibration.CalibratedClassifierCV` where
+  an error was wrongly raised with string targets. 
+  :pr:`28843` by :user:`Jérémie du Boisberranger <jeremiedbb>`.
+
 :mod:`sklearn.cluster`
 ......................
 

From 9130934fa90c36841cb2c24779932ef9b0e38524 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=A9r=C3=A9mie=20du=20Boisberranger?=
 <jeremie@probabl.ai>
Date: Mon, 15 Apr 2024 22:36:49 +0200
Subject: [PATCH 3/3] Update doc/whats_new/v1.5.rst

---
 doc/whats_new/v1.5.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/whats_new/v1.5.rst b/doc/whats_new/v1.5.rst
index d075f4049d3ac..adbd64f2f4477 100644
--- a/doc/whats_new/v1.5.rst
+++ b/doc/whats_new/v1.5.rst
@@ -128,7 +128,7 @@ Changelog
     where 123455 is the *pull request* number, not the issue number.
 
 :mod:`sklearn.calibration`
-.........................
+..........................
 
 - |Fix| Fixed a regression in :class:`calibration.CalibratedClassifierCV` where
   an error was wrongly raised with string targets.