Backport PR #9947: Add set_trace to core debugger.

This PR adds a `set_trace` method to `IPython.core.debugger` to mirror the functionality of `IPython.terminal.debugger`. See  9940 for a discussion.

Signed-off-by: Thomas Kluyver <[e-mail]>

IPython/core/debugger.py

@@ -621,3 +621,12 b' class Pdb(OldPdb, object):'
                 self.print_stack_trace()
 
         do_w = do_where
+
+
+def set_trace(frame=None):
+    """
+    Start debugging from `frame`.
+
+    If frame is not specified, debugging starts from caller's frame.
+    """
+    Pdb().set_trace(frame or sys._getframe().f_back)

IPython/terminal/debugger.py

@@ -7,6 +7,8 b' from prompt_toolkit.token import Token'
 from prompt_toolkit.shortcuts import create_prompt_application
 from prompt_toolkit.interface import CommandLineInterface
 from prompt_toolkit.enums import EditingMode
+import sys
+
 
 class TerminalPdb(Pdb):
     def __init__(self, *args, **kwargs):
@@ -72,6 +74,12 b' class TerminalPdb(Pdb):'
         except Exception:
             raise
 
-def set_trace():
-    TerminalPdb().set_trace()
+
+def set_trace(frame=None):
+    """
+    Start debugging from `frame`.
+
+    If frame is not specified, debugging starts from caller's frame.
+    """
+    TerminalPdb().set_trace(frame or sys._getframe().f_back)