Only emit pip constraint deprecation warning once

src/pip/_internal/build_env.py

@@ -31,6 +31,9 @@
 
 logger = logging.getLogger(__name__)
 
+# Global flag to track if deprecation warning has been shown
+_DEPRECATION_WARNING_SHOWN = False
+
 
 class ExtraEnviron(TypedDict, total=False):
     extra_environ: dict[str, str]
@@ -123,15 +126,25 @@ def _deprecation_constraint_check(self) -> None:
         Check for deprecation warning: PIP_CONSTRAINT affecting build environments.
 
         This warns when build-constraint feature is NOT enabled and PIP_CONSTRAINT
-        is not empty.
+        is not empty, but only shows the warning once per process.
         """
+        global _DEPRECATION_WARNING_SHOWN
+
         if self._build_constraint_feature_enabled:
             return
 
+        if _DEPRECATION_WARNING_SHOWN:
+            return
+
         pip_constraint = os.environ.get("PIP_CONSTRAINT")
         if not pip_constraint or not pip_constraint.strip():
             return
 
+        # Don't warn if we're in a build environment that ignores constraints
+        if os.environ.get("_PIP_IN_BUILD_IGNORE_CONSTRAINTS") == "1":
+            return
+
+        _DEPRECATION_WARNING_SHOWN = True
         deprecated(
             reason=(
                 "Setting PIP_CONSTRAINT will not affect "

tests/unit/test_build_constraints.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import os
+import warnings
 from pathlib import Path
 from unittest import mock
 
@@ -17,6 +18,12 @@
 class TestSubprocessBuildEnvironmentInstaller:
     """Test SubprocessBuildEnvironmentInstaller build constraints functionality."""
 
+    def setup_method(self) -> None:
+        """Reset the global deprecation warning flag before each test."""
+        import pip._internal.build_env
+
+        pip._internal.build_env._DEPRECATION_WARNING_SHOWN = False
+
     @mock.patch.dict(os.environ, {}, clear=True)
     def test_deprecation_check_no_pip_constraint(self) -> None:
         """Test no deprecation warning when PIP_CONSTRAINT is not set."""
@@ -112,3 +119,37 @@ def test_install_calls_deprecation_check(
 
         # Verify that call_subprocess was called (install proceeded after warning)
         mock_call_subprocess.assert_called_once()
+
+    @mock.patch.dict(os.environ, {"PIP_CONSTRAINT": "constraints.txt"})
+    def test_deprecation_check_warning_shown_only_once(self) -> None:
+        """Test deprecation warning is shown only once per process."""
+        finder = make_test_finder()
+        installer = SubprocessBuildEnvironmentInstaller(
+            finder,
+            build_constraint_feature_enabled=False,
+        )
+
+        with pytest.warns(PipDeprecationWarning):
+            installer._deprecation_constraint_check()
+
+        with warnings.catch_warnings(record=True) as warning_list:
+            warnings.simplefilter("always")
+            installer._deprecation_constraint_check()
+        assert len(warning_list) == 0
+
+    @mock.patch.dict(
+        os.environ,
+        {"PIP_CONSTRAINT": "constraints.txt", "_PIP_IN_BUILD_IGNORE_CONSTRAINTS": "1"},
+    )
+    def test_deprecation_check_no_warning_when_ignoring_constraints(self) -> None:
+        """Test no deprecation warning when _PIP_IN_BUILD_IGNORE_CONSTRAINTS is set."""
+        finder = make_test_finder()
+        installer = SubprocessBuildEnvironmentInstaller(
+            finder,
+            build_constraint_feature_enabled=False,
+        )
+
+        with warnings.catch_warnings(record=True) as warning_list:
+            warnings.simplefilter("always")
+            installer._deprecation_constraint_check()
+        assert len(warning_list) == 0