From 883b65d0a9a326421cf2d7c5522f9bfbb7885ecb Mon Sep 17 00:00:00 2001
From: Eugene Toder <eugene.toder@beacon.io>
Date: Fri, 23 Oct 2020 00:07:53 -0400
Subject: [PATCH] bpo-42125: linecache: get module name from __spec__ if
 available

This allows getting source code for the __main__ module when a custom
loader is used.
---
 Lib/linecache.py                              | 12 +++---
 Lib/test/test_linecache.py                    | 38 +++++++++++++++++++
 .../2020-12-15-22-30-49.bpo-42125.UGyseY.rst  |  2 +
 3 files changed, 45 insertions(+), 7 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-12-15-22-30-49.bpo-42125.UGyseY.rst

diff --git a/Lib/linecache.py b/Lib/linecache.py
index 513b17e999880b5..ac930e4d1090438 100644
--- a/Lib/linecache.py
+++ b/Lib/linecache.py
@@ -166,13 +166,11 @@ def lazycache(filename, module_globals):
         return False
     # Try for a __loader__, if available
     if module_globals and '__name__' in module_globals:
-        name = module_globals['__name__']
-        if (loader := module_globals.get('__loader__')) is None:
-            if spec := module_globals.get('__spec__'):
-                try:
-                    loader = spec.loader
-                except AttributeError:
-                    pass
+        spec = module_globals.get('__spec__')
+        name = getattr(spec, 'name', None) or module_globals['__name__']
+        loader = getattr(spec, 'loader', None)
+        if loader is None:
+            loader = module_globals.get('__loader__')
         get_source = getattr(loader, 'get_source', None)
 
         if name and get_source:
diff --git a/Lib/test/test_linecache.py b/Lib/test/test_linecache.py
index cfc6ba89e774c06..7468fafc1124e40 100644
--- a/Lib/test/test_linecache.py
+++ b/Lib/test/test_linecache.py
@@ -5,6 +5,7 @@
 import os.path
 import tempfile
 import tokenize
+from importlib.machinery import ModuleSpec
 from test import support
 from test.support import os_helper
 
@@ -97,6 +98,16 @@ class BadUnicode(GetLineTestsBadData, unittest.TestCase):
     file_byte_string = b'\x80abc'
 
 
+class FakeLoader:
+    def get_source(self, fullname):
+        return f'source for {fullname}'
+
+
+class NoSourceLoader:
+    def get_source(self, fullname):
+        return None
+
+
 class LineCacheTests(unittest.TestCase):
 
     def test_getline(self):
@@ -238,6 +249,33 @@ def raise_memoryerror(*args, **kwargs):
         self.assertEqual(lines3, [])
         self.assertEqual(linecache.getlines(FILENAME), lines)
 
+    def test_loader(self):
+        filename = 'scheme://path'
+
+        for loader in (None, object(), NoSourceLoader()):
+            linecache.clearcache()
+            module_globals = {'__name__': 'a.b.c', '__loader__': loader}
+            self.assertEqual(linecache.getlines(filename, module_globals), [])
+
+        linecache.clearcache()
+        module_globals = {'__name__': 'a.b.c', '__loader__': FakeLoader()}
+        self.assertEqual(linecache.getlines(filename, module_globals),
+                         ['source for a.b.c\n'])
+
+        for spec in (None, object(), ModuleSpec('', FakeLoader())):
+            linecache.clearcache()
+            module_globals = {'__name__': 'a.b.c', '__loader__': FakeLoader(),
+                              '__spec__': spec}
+            self.assertEqual(linecache.getlines(filename, module_globals),
+                             ['source for a.b.c\n'])
+
+        linecache.clearcache()
+        spec = ModuleSpec('x.y.z', FakeLoader())
+        module_globals = {'__name__': 'a.b.c', '__loader__': spec.loader,
+                          '__spec__': spec}
+        self.assertEqual(linecache.getlines(filename, module_globals),
+                         ['source for x.y.z\n'])
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2020-12-15-22-30-49.bpo-42125.UGyseY.rst b/Misc/NEWS.d/next/Library/2020-12-15-22-30-49.bpo-42125.UGyseY.rst
new file mode 100644
index 000000000000000..6a3cb9b10cfd3b6
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-12-15-22-30-49.bpo-42125.UGyseY.rst
@@ -0,0 +1,2 @@
+linecache: get module name from `__spec__` if available. This allows getting
+source code for the `__main__` module when a custom loader is used.