Avoid instantiating IPython on import

Fixes #223.

ipdb/__main__.py

@@ -22,34 +22,36 @@
     import ConfigParser as configparser
 
 
-shell = get_ipython()
-if shell is None:
-    # Not inside IPython
-    # Build a terminal app in order to force ipython to load the
-    # configuration
-    ipapp = TerminalIPythonApp()
-    # Avoid output (banner, prints)
-    ipapp.interact = False
-    ipapp.initialize(['--no-term-title'])
-    shell = ipapp.shell
-else:
-    # Running inside IPython
-
-    # Detect if embed shell or not and display a message
-    if isinstance(shell, InteractiveShellEmbed):
-        sys.stderr.write(
-            "\nYou are currently into an embedded ipython shell,\n"
-            "the configuration will not be loaded.\n\n"
-        )
+def _get_debugger_cls():
+    shell = get_ipython()
+    if shell is None:
+        # Not inside IPython
+        # Build a terminal app in order to force ipython to load the
+        # configuration
+        ipapp = TerminalIPythonApp()
+        # Avoid output (banner, prints)
+        ipapp.interact = False
+        ipapp.initialize(["--no-term-title"])
+        shell = ipapp.shell
+    else:
+        # Running inside IPython
+
+        # Detect if embed shell or not and display a message
+        if isinstance(shell, InteractiveShellEmbed):
+            sys.stderr.write(
+                "\nYou are currently into an embedded ipython shell,\n"
+                "the configuration will not be loaded.\n\n"
+            )
 
-# Let IPython decide about which debugger class to use
-# This is especially important for tools that fiddle with stdout
-debugger_cls = shell.debugger_cls
+    # Let IPython decide about which debugger class to use
+    # This is especially important for tools that fiddle with stdout
+    return shell.debugger_cls
 
 
 def _init_pdb(context=None, commands=[]):
     if context is None:
         context = os.getenv("IPDB_CONTEXT_SIZE", get_context_from_config())
+    debugger_cls = _get_debugger_cls()
     try:
         p = debugger_cls(context=context)
     except TypeError:

tests/test_opts.py

@@ -14,34 +14,34 @@
 from ipdb.__main__ import main
 
 
-@patch('ipdb.__main__.debugger_cls')
+@patch('ipdb.__main__._get_debugger_cls')
 class OptsTest(unittest.TestCase):
     def set_argv(self, *argv):
         argv_patch = patch('ipdb.__main__.sys.argv', argv)
         argv_patch.start()
         self.addCleanup(argv_patch.stop)
 
     @patch('ipdb.__main__.sys.version_info', (3, 7))
-    def test_debug_module_script(self, debugger_cls):
+    def test_debug_module_script(self, get_debugger_cls):
         module_name = 'my_buggy_module'
         self.set_argv('ipdb', '-m', module_name)
 
         main()
 
-        debugger = debugger_cls.return_value
+        debugger = get_debugger_cls.return_value.return_value
         debugger._runmodule.assert_called_once_with(module_name)
 
     @patch('ipdb.__main__.os.path.exists')
-    def test_debug_script(self, exists, debugger_cls):
+    def test_debug_script(self, exists, get_debugger_cls):
         script_name = 'my_buggy_script'
         self.set_argv('ipdb', script_name)
 
         main()
 
-        debugger = debugger_cls.return_value
+        debugger = get_debugger_cls.return_value.return_value
         debugger._runscript.assert_called_once_with(script_name)
 
-    def test_option_m_fallback_on_py36(self, debugger_cls):
+    def test_option_m_fallback_on_py36(self, get_debugger_cls):
         self.set_argv('ipdb', '-m', 'my.module')
         with patch('ipdb.__main__.sys.version_info', (3, 6)):
             with self.assertRaises(GetoptError):