diff --git a/easybuild/easyblocks/p/python.py b/easybuild/easyblocks/p/python.py
index 3bbc7b8cb49..133b9dbf7dd 100644
--- a/easybuild/easyblocks/p/python.py
+++ b/easybuild/easyblocks/p/python.py
@@ -152,11 +152,12 @@ def det_installed_python_packages(names_only=True, python_cmd=None):
         '--disable-pip-version-check',
         '--format', 'json',
     ])
-    res = run_shell_cmd(cmd, fail_on_error=False, hidden=True)
+    # only check stdout, not stderr which might contain user facing warnings
+    # (on deprecation of Python 2.7, for example)
+    res = run_shell_cmd(cmd, split_stderr=True, fail_on_error=False, hidden=True)
     if res.exit_code:
         raise EasyBuildError(f'Failed to determine installed python packages: {res.output}')
 
-    # only check stdout, not stderr which might contain user facing warnings
     log.info(f'Got list of installed Python packages: {res.output}')
     pkgs = json.loads(res.output.strip())
     return [pkg['name'] for pkg in pkgs] if names_only else pkgs
diff --git a/test/easyblocks/easyblock_specific.py b/test/easyblocks/easyblock_specific.py
index fb229dcd412..f3cabe926da 100644
--- a/test/easyblocks/easyblock_specific.py
+++ b/test/easyblocks/easyblock_specific.py
@@ -276,14 +276,14 @@ def test_det_installed_python_packages(self):
         Test det_installed_python_packages function providyed by PythonPackage easyblock
         """
         pkg1 = None
-        res = pythonpackage.det_installed_python_packages(python_cmd=sys.executable)
+        res = python.det_installed_python_packages(python_cmd=sys.executable)
         # we can't make too much assumptions on which installed Python packages are found
         self.assertTrue(isinstance(res, list))
         if res:
             pkg1_name = res[0]
             self.assertTrue(isinstance(pkg1_name, str))
 
-        res_detailed = pythonpackage.det_installed_python_packages(python_cmd=sys.executable, names_only=False)
+        res_detailed = python.det_installed_python_packages(python_cmd=sys.executable, names_only=False)
         self.assertTrue(isinstance(res_detailed, list))
         if res_detailed:
             pkg1 = res_detailed[0]
@@ -294,6 +294,21 @@ def test_det_installed_python_packages(self):
             ver = pkg1['version']
             self.assertTrue(regex.match(ver), f"Pattern {regex.pattern} matches for pkg version: {ver}")
 
+        def mocked_run_shell_cmd_pip(cmd, **kwargs):
+            stderr = None
+            if "pip list" in cmd:
+                output = '[{"name": "example", "version": "1.2.3"}]'
+                stderr = "DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020"
+            else:
+                # unexpected command
+                return None
+
+            return RunShellCmdResult(cmd=cmd, exit_code=0, output=output, stderr=stderr, work_dir=None,
+                                     out_file=None, err_file=None, cmd_sh=None, thread_id=None, task_id=None)
+        python.run_shell_cmd = mocked_run_shell_cmd_pip
+        res = python.det_installed_python_packages(python_cmd=sys.executable)
+        self.assertEqual(res, ['example'])
+
     def test_det_py_install_scheme(self):
         """Test det_py_install_scheme function provided by PythonPackage easyblock."""
         res = pythonpackage.det_py_install_scheme(sys.executable)