bpo-35931: Gracefully handle SyntaxError in pdb debug command (GH-11782)

Previously, `debug print(` would cause the interpreter to exit on a SyntaxError whereas `print(` would properly display the error and return to the pdb prompt.

This patch fixes this by pre-compiling the code before passing it to `Pdb.run`.


https://bugs.python.org/issue35931

Lib/pdb.py

@@ -1093,10 +1093,16 @@ class Pdb(bdb.Bdb, cmd.Cmd):
         sys.settrace(None)
         globals = self.curframe.f_globals
         locals = self.curframe_locals
+        try:
+            code = compile(arg, "<string>", "exec")
+        except SyntaxError:
+            exc_info = sys.exc_info()[:2]
+            self.error(traceback.format_exception_only(*exc_info)[-1].strip())
+            return
         p = Pdb(self.completekey, self.stdin, self.stdout)
         p.prompt = "(%s) " % self.prompt.strip()
         self.message("ENTERING RECURSIVE DEBUGGER")
-        sys.call_tracing(p.run, (arg, globals, locals))
+        sys.call_tracing(p.run, (code, globals, locals))
         self.message("LEAVING RECURSIVE DEBUGGER")
         sys.settrace(self.trace_dispatch)
         self.lastcmd = p.lastcmd

Lib/test/test_pdb.py

@@ -1486,6 +1486,14 @@ class PdbTestCase(unittest.TestCase):
         stdout, _ = self._run_pdb(['-m', self.module_name + '.runme'], commands)
         self.assertTrue(any("VAR from module" in l for l in stdout.splitlines()), stdout)
 
+    def test_syntaxerror_in_command(self):
+        commands = "print(\ndebug print("
+        stdout, _ = self.run_pdb_script("", commands)
+        self.assertEqual(stdout.splitlines()[1:], [
+            '(Pdb) *** SyntaxError: unexpected EOF while parsing',
+            '(Pdb) *** SyntaxError: unexpected EOF while parsing',
+            '(Pdb) ',
+        ])
 
 def load_tests(*args):
     from test import test_pdb

Misc/NEWS.d/next/Library/2019-02-07-16-22-50.bpo-35931._63i7B.rst

@@ -0,0 +1 @@
+The :mod:`pdb` ``debug`` command now gracefully handles syntax errors.