gh-160125: Support package.module.function for pdb breakpoint command

Doc/library/pdb.rst

@@ -458,7 +458,8 @@ can be overridden by the local file.
 
    With a *function* argument, set a break at the first executable statement within
    that function. *function* can be any expression that evaluates to a function
-   in the current namespace.
+   in the current namespace, or in the form ``package.module.function`` if
+   ``package.module`` is importable.
 
    If a second argument is present, it is an expression which must evaluate to
    true before the breakpoint is honored.

Lib/pdb.py

@@ -1688,10 +1688,10 @@ def lineinfo(self, identifier):
         else:
             # More than one part.
             # First is module, second is method/class
-            f = self.lookupmodule(parts[0])
+            f = self.lookupmodule('.'.join(parts[:-1]))
             if f:
                 fname = f
-                item = parts[1]
+                item = parts[-1]
             else:
                 return failed
         answer = find_function(item, self.canonic(fname))

Lib/test/test_pdb.py

@@ -514,6 +514,36 @@ def test_pdb_breakpoint_with_filename():
     (Pdb) continue
     """
 
+def test_pdb_breakpoint_in_package():
+    """Breakpoints for package.module.function should work even if the module is not yet imported.
+
+    >>> def test_function():
+    ...     import pdb; pdb.Pdb(nosigint=True, readrc=False).set_trace()
+    ...     import multiprocessing.util  # Now import the module
+    ...     multiprocessing.util.is_abstract_socket_namespace(None)  # We should break here
+
+    >>> with PdbTestInput([  # doctest: +NORMALIZE_WHITESPACE +ELLIPSIS
+    ...     'multiprocessing.util.is_abstract_socket_namespace',  # Make sure the module is not imported yet
+    ...     'break multiprocessing.util.is_abstract_socket_namespace',  # We can still set a bp on it
+    ...     'continue',  # will stop at the function call
+    ...     'clear 1',
+    ...     'continue',
+    ... ]):
+    ...    test_function()
+    > <doctest test.test_pdb.test_pdb_breakpoint_in_package[0]>(2)test_function()
+    -> import pdb; pdb.Pdb(nosigint=True, readrc=False).set_trace()
+    (Pdb) multiprocessing.util.is_abstract_socket_namespace
+    *** NameError: name 'multiprocessing' is not defined. Did you forget to import 'multiprocessing'?
+    (Pdb) break multiprocessing.util.is_abstract_socket_namespace
+    Breakpoint 1 at .../multiprocessing/util.py:...
+    (Pdb) continue
+    > .../multiprocessing/util.py(...)is_abstract_socket_namespace()
+    -> if not address:
+    (Pdb) clear 1
+    Deleted breakpoint 1 at .../multiprocessing/util.py:...
+    (Pdb) continue
+    """
+
 def test_pdb_breakpoint_on_disabled_line():
     """New breakpoint on once disabled line should work
 

Misc/NEWS.d/next/Library/2026-10-21-15-10-00.gh-issue-160125.pdbpkg.rst

@@ -0,0 +1,3 @@
+Support setting breakpoints with the :mod:`pdb` ``break`` and ``tbreak``
+commands using ``package.module.function`` names when the module has not been
+imported in the current namespace.