gh-153970: Fix CalledProcessError.__str__ crash when returncode is None

Lib/subprocess.py

@@ -151,8 +151,8 @@ def __str__(self):
                 return "Command %r died with unknown signal %d." % (
                         self.cmd, -self.returncode)
         else:
-            return "Command %r returned non-zero exit status %d." % (
-                    self.cmd, self.returncode)
+            return (f"Command {self.cmd!r} returned non-zero "
+                    f"exit status {self.returncode}.")
 
     @property
     def stdout(self):

Lib/test/test_subprocess.py

@@ -2449,6 +2449,10 @@ def test_CalledProcessError_str(self):
         err = subprocess.CalledProcessError(-9876543, "fake cmd")
         self.assertEqual(str(err), "Command 'fake cmd' died with unknown signal 9876543.")
 
+        # None return code (must not raise TypeError from __str__ itself)
+        err = subprocess.CalledProcessError(None, "fake cmd")
+        self.assertEqual(str(err), "Command 'fake cmd' returned non-zero exit status None.")
+
     def test_preexec(self):
         # DISCLAIMER: Setting environment variables is *not* a good use
         # of a preexec_fn.  This is merely a test.

Misc/NEWS.d/next/Library/2026-07-18-16-46-25.gh-issue-153970.Vn4pR8.rst

@@ -0,0 +1,2 @@
+Calling :func:`str` on a :exc:`subprocess.CalledProcessError` with a
+:attr:`!returncode` of ``None`` no longer raises :exc:`TypeError`.