From 832aa16a6771b35aa1ea9e19911e5a6552603b88 Mon Sep 17 00:00:00 2001
From: Tarun <sdtarun2@gmail.com>
Date: Wed, 1 Apr 2026 01:13:03 +0530
Subject: [PATCH 1/2] [BUG] fix bare print on non-convergence and malformed
 ValueError format

- Replace `print("did not converge")` with `warnings.warn()` so that
  non-convergence is surfaced through Python's warning system instead of
  printing to stdout. This allows users to filter/capture the message
  programmatically.

- Fix string formatting bug in `_estimate_GCV_UBRE` where `format(gamma)`
  was passed as a separate argument to `ValueError` instead of being
  interpolated into the message string, resulting in a confusing tuple
  error message like `('msg {}', '0.5')`.
---
 pygam/pygam.py                  |  9 ++++++---
 pygam/tests/test_GAM_methods.py | 25 +++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/pygam/pygam.py b/pygam/pygam.py
index 6f86ae43..6648691f 100644
--- a/pygam/pygam.py
+++ b/pygam/pygam.py
@@ -818,7 +818,11 @@ def _pirls(self, X, Y, weights):
         if diff < self.tol:
             return
 
-        print("did not converge")
+        warnings.warn(
+            "PIRLS did not converge."
+            " Try increasing max_iter or decreasing tol.",
+            stacklevel=2,
+        )
         return
 
     def _on_loop_start(self, variables):
@@ -1194,8 +1198,7 @@ def _estimate_GCV_UBRE(
         """
         if gamma < 1:
             raise ValueError(
-                "gamma scaling should be greater than 1, but found gamma = {}",
-                format(gamma),
+                f"gamma scaling should be greater than 1, but found gamma = {gamma}"
             )
 
         if modelmat is None:
diff --git a/pygam/tests/test_GAM_methods.py b/pygam/tests/test_GAM_methods.py
index ee29de14..c9273776 100644
--- a/pygam/tests/test_GAM_methods.py
+++ b/pygam/tests/test_GAM_methods.py
@@ -467,6 +467,31 @@ def test_fit_quantile_NOT_close_enough(head_circumference_X_y):
     assert np.abs(ratio - quantile) > tol
 
 
+def test_non_convergence_emits_warning(mcycle_X_y):
+    """Non-convergence should emit a warning, not print to stdout."""
+    X, y = mcycle_X_y
+    gam = LinearGAM(max_iter=1, tol=1e-100)
+    import warnings
+
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        gam.fit(X, y)
+        convergence_warnings = [
+            x for x in w if "did not converge" in str(x.message)
+        ]
+        assert len(convergence_warnings) == 1
+
+
+def test_GCV_UBRE_gamma_error_message(mcycle_X_y, mcycle_gam):
+    """ValueError for gamma < 1 should have a properly formatted message."""
+    with pytest.raises(ValueError, match=r"gamma.*0\.5"):
+        mcycle_gam._estimate_GCV_UBRE(
+            y=mcycle_X_y[1],
+            modelmat=mcycle_gam._modelmat(mcycle_X_y[0]),
+            gamma=0.5,
+        )
+
+
 def test_fit_quantile_raises_ValueError(head_circumference_X_y):
     """see that we DO NOT get fit on bad argument requests"""
     X, y = head_circumference_X_y

From 3ce140aa475cbb3cd4c6a0c3e8aabac8a3cbed84 Mon Sep 17 00:00:00 2001
From: Tarun <sdtarun2@gmail.com>
Date: Wed, 1 Apr 2026 13:39:30 +0530
Subject: [PATCH 2/2] [MNT] fix ruff-format: join strings and inline list
 comprehension

---
 pygam/pygam.py                  | 3 +--
 pygam/tests/test_GAM_methods.py | 4 +---
 2 files changed, 2 insertions(+), 5 deletions(-)

diff --git a/pygam/pygam.py b/pygam/pygam.py
index 6648691f..8e02899d 100644
--- a/pygam/pygam.py
+++ b/pygam/pygam.py
@@ -819,8 +819,7 @@ def _pirls(self, X, Y, weights):
             return
 
         warnings.warn(
-            "PIRLS did not converge."
-            " Try increasing max_iter or decreasing tol.",
+            "PIRLS did not converge. Try increasing max_iter or decreasing tol.",
             stacklevel=2,
         )
         return
diff --git a/pygam/tests/test_GAM_methods.py b/pygam/tests/test_GAM_methods.py
index c9273776..697cd606 100644
--- a/pygam/tests/test_GAM_methods.py
+++ b/pygam/tests/test_GAM_methods.py
@@ -476,9 +476,7 @@ def test_non_convergence_emits_warning(mcycle_X_y):
     with warnings.catch_warnings(record=True) as w:
         warnings.simplefilter("always")
         gam.fit(X, y)
-        convergence_warnings = [
-            x for x in w if "did not converge" in str(x.message)
-        ]
+        convergence_warnings = [x for x in w if "did not converge" in str(x.message)]
         assert len(convergence_warnings) == 1