Support PEP 810 lazy imports (Python 3.15)

src/debugpy/__main__.py

@@ -63,7 +63,8 @@
 
         # Do not use dirname() to walk up - this can be a relative path, e.g. ".".
         sys.path[0] = sys.path[0] + "/../"
-        import debugpy  # noqa
+        # Not an import statement: PEP 810 lazy imports could defer it past the del below.
+        __import__("debugpy")
         del sys.path[0]
 
     from debugpy.server import cli

src/debugpy/_vendored/__init__.py

@@ -106,6 +106,20 @@ def check_modules(project, match, root=None):
     return unvendored, extensions
 
 
+@contextlib.contextmanager
+def _eager_imports():
+    """A context manager under which plain imports are not deferred by PEP 810."""
+    get_lazy_imports = getattr(sys, "get_lazy_imports", None)
+    if get_lazy_imports is None or get_lazy_imports() != "all":
+        yield
+        return
+    sys.set_lazy_imports("normal")
+    try:
+        yield
+    finally:
+        sys.set_lazy_imports("all")
+
+
 @contextlib.contextmanager
 def vendored(project, root=None):
     """A context manager under which the vendored project will be imported."""
@@ -114,7 +128,10 @@ def vendored(project, root=None):
     # Add the vendored project directory, so that it gets tried first.
     sys.path.insert(0, root)
     try:
-        yield root
+        # An import resolved after the directory is removed from sys.path would
+        # fail, so everything must be loaded while it is still there.
+        with _eager_imports():
+            yield root
     finally:
         sys.path.remove(root)
 

src/debugpy/server/__init__.py

@@ -3,5 +3,6 @@
 # for license information.
 
 # "force_pydevd" must be imported first to ensure (via side effects)
-# that the debugpy-vendored copy of pydevd gets used.
-import debugpy._vendored.force_pydevd  # noqa
+# that the debugpy-vendored copy of pydevd gets used. A plain import
+# statement would never run it when PEP 810 lazy imports are enabled globally.
+__import__("debugpy._vendored.force_pydevd")

src/debugpy/server/cli.py

@@ -455,6 +455,8 @@ def attach_to_pid():
     }
     setup = encode(json.dumps(setup))
 
+    # __import__ rather than an import statement, which PEP 810 lazy imports could
+    # defer past `del sys.path[0]`. The name is encoded because pydevd rejects quotes.
     python_code = """
 import codecs;
 import json;
@@ -466,15 +468,17 @@ def attach_to_pid():
 setup = json.loads(decode({setup}));
 
 sys.path.insert(0, script_dir);
-import attach_pid_injected;
+attach_pid_injected = __import__(decode({module}));
 del sys.path[0];
 
 attach_pid_injected.attach(setup);
 """
     python_code = (
         python_code.replace("\r", "")
         .replace("\n", "")
-        .format(script_dir=script_dir, setup=setup)
+        .format(
+            script_dir=script_dir, setup=setup, module=encode("attach_pid_injected")
+        )
     )
 
     # attempt pep 768 style code injection

tests/debugpy/test_lazy_imports.py

@@ -0,0 +1,49 @@
+# Copyright (c) Microsoft Corporation. All rights reserved.
+# Licensed under the MIT License. See LICENSE in the project root
+# for license information.
+
+import sys
+
+import pytest
+
+from tests import debug
+from tests.debug import runners, targets
+from tests.patterns import some
+
+pytestmark = pytest.mark.skipif(
+    not hasattr(sys, "set_lazy_imports"),
+    reason="PEP 810 lazy imports require Python 3.15+",
+)
+
+
+@pytest.mark.parametrize("run", runners.all)
+@pytest.mark.parametrize("target", targets.all_named)
+def test_lazy_imports_all(pyfile, target, run):
+    @pyfile
+    def code_to_debug():
+        import sys
+
+        import debuggee
+        from debuggee import backchannel
+
+        debuggee.setup()
+        backchannel.send(sys.get_lazy_imports())  # @bp
+
+    with debug.Session() as session:
+        session.spawn_adapter.env["PYTHON_LAZY_IMPORTS"] = "all"
+        env = (
+            session.config.env
+            if run.request == "launch"
+            else session.spawn_debuggee.env
+        )
+        env["PYTHON_LAZY_IMPORTS"] = "all"
+
+        backchannel = session.open_backchannel()
+        with run(session, target(code_to_debug)):
+            session.set_breakpoints(code_to_debug, all)
+
+        session.wait_for_stop(
+            "breakpoint", expected_frames=[some.dap.frame(code_to_debug, line="bp")]
+        )
+        session.request_continue()
+        assert backchannel.receive() == "all"