bpo-42877: add the 'compact' param to TracebackException's __init__ (#24179)

Use it to reduce the time and memory taken up by several of traceback's module-level functions.

Doc/library/traceback.rst

@@ -212,11 +212,16 @@ The module also defines the following classes:
 :class:`TracebackException` objects are created from actual exceptions to
 capture data for later printing in a lightweight fashion.
 
-.. class:: TracebackException(exc_type, exc_value, exc_traceback, *, limit=None, lookup_lines=True, capture_locals=False)
+.. class:: TracebackException(exc_type, exc_value, exc_traceback, *, limit=None, lookup_lines=True, capture_locals=False, compact=False)
 
    Capture an exception for later rendering. *limit*, *lookup_lines* and
    *capture_locals* are as for the :class:`StackSummary` class.
 
+   If *compact* is true, only data that is required by :class:`TracebackException`'s
+   ``format`` method is saved in the class attributes. In particular, the
+   ``__context__`` field is calculated only if ``__cause__`` is ``None`` and
+   ``__suppress_context__`` is false.
+
    Note that when locals are captured, they are also shown in the traceback.
 
    .. attribute:: __cause__
@@ -294,6 +299,9 @@ capture data for later printing in a lightweight fashion.
       The message indicating which exception occurred is always the last
       string in the output.
 
+   .. versionchanged:: 3.10
+      Added the *compact* parameter.
+
 
 :class:`StackSummary` Objects
 -----------------------------

Lib/test/test_traceback.py

@@ -1173,6 +1173,46 @@ class TestTracebackException(unittest.TestCase):
         self.assertIn(
             "RecursionError: maximum recursion depth exceeded", res[-1])
 
+    def test_compact_with_cause(self):
+        try:
+            try:
+                1/0
+            finally:
+                cause = Exception("cause")
+                raise Exception("uh oh") from cause
+        except Exception:
+            exc_info = sys.exc_info()
+            exc = traceback.TracebackException(*exc_info, compact=True)
+            expected_stack = traceback.StackSummary.extract(
+                traceback.walk_tb(exc_info[2]))
+        exc_cause = traceback.TracebackException(Exception, cause, None)
+        self.assertEqual(exc_cause, exc.__cause__)
+        self.assertEqual(None, exc.__context__)
+        self.assertEqual(True, exc.__suppress_context__)
+        self.assertEqual(expected_stack, exc.stack)
+        self.assertEqual(exc_info[0], exc.exc_type)
+        self.assertEqual(str(exc_info[1]), str(exc))
+
+    def test_compact_no_cause(self):
+        try:
+            try:
+                1/0
+            finally:
+                exc_info_context = sys.exc_info()
+                exc_context = traceback.TracebackException(*exc_info_context)
+                raise Exception("uh oh")
+        except Exception:
+            exc_info = sys.exc_info()
+            exc = traceback.TracebackException(*exc_info, compact=True)
+            expected_stack = traceback.StackSummary.extract(
+                traceback.walk_tb(exc_info[2]))
+        self.assertEqual(None, exc.__cause__)
+        self.assertEqual(exc_context, exc.__context__)
+        self.assertEqual(False, exc.__suppress_context__)
+        self.assertEqual(expected_stack, exc.stack)
+        self.assertEqual(exc_info[0], exc.exc_type)
+        self.assertEqual(str(exc_info[1]), str(exc))
+
     def test_no_refs_to_exception_and_traceback_objects(self):
         try:
             1/0

Lib/traceback.py

@@ -110,8 +110,8 @@ def print_exception(exc, /, value=_sentinel, tb=_sentinel, limit=None, \
     value, tb = _parse_value_tb(exc, value, tb)
     if file is None:
         file = sys.stderr
-    for line in TracebackException(
-            type(value), value, tb, limit=limit).format(chain=chain):
+    te = TracebackException(type(value), value, tb, limit=limit, compact=True)
+    for line in te.format(chain=chain):
         print(line, file=file, end="")
 
 
@@ -126,8 +126,8 @@ def format_exception(exc, /, value=_sentinel, tb=_sentinel, limit=None, \
     printed as does print_exception().
     """
     value, tb = _parse_value_tb(exc, value, tb)
-    return list(TracebackException(
-        type(value), value, tb, limit=limit).format(chain=chain))
+    te = TracebackException(type(value), value, tb, limit=limit, compact=True)
+    return list(te.format(chain=chain))
 
 
 def format_exception_only(exc, /, value=_sentinel):
@@ -146,8 +146,8 @@ def format_exception_only(exc, /, value=_sentinel):
     """
     if value is _sentinel:
         value = exc
-    return list(TracebackException(
-        type(value), value, None).format_exception_only())
+    te = TracebackException(type(value), value, None, compact=True)
+    return list(te.format_exception_only())
 
 
 # -- not official API but folk probably use these two functions.
@@ -476,7 +476,8 @@ class TracebackException:
     """
 
     def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
-            lookup_lines=True, capture_locals=False, _seen=None):
+            lookup_lines=True, capture_locals=False, compact=False,
+            _seen=None):
         # NB: we need to accept exc_traceback, exc_value, exc_traceback to
         # permit backwards compat with the existing API, otherwise we
         # need stub thunk objects just to glue it together.
@@ -485,6 +486,7 @@ class TracebackException:
         if _seen is None:
             _seen = set()
         _seen.add(id(exc_value))
+
         # TODO: locals.
         self.stack = StackSummary.extract(
             walk_tb(exc_traceback), limit=limit, lookup_lines=lookup_lines,
@@ -504,7 +506,7 @@ class TracebackException:
         if lookup_lines:
             self._load_lines()
         self.__suppress_context__ = \
-            exc_value.__suppress_context__ if exc_value else False
+            exc_value.__suppress_context__ if exc_value is not None else False
 
         # Convert __cause__ and __context__ to `TracebackExceptions`s, use a
         # queue to avoid recursion (only the top-level call gets _seen == None)
@@ -524,8 +526,13 @@ class TracebackException:
                         _seen=_seen)
                 else:
                     cause = None
+
+                if compact:
+                    need_context = cause is None and not e.__suppress_context__
+                else:
+                    need_context = True
                 if (e and e.__context__ is not None
-                    and id(e.__context__) not in _seen):
+                    and need_context and id(e.__context__) not in _seen):
                     context = TracebackException(
                         type(e.__context__),
                         e.__context__,

Misc/NEWS.d/next/Library/2021-01-13-12-55-41.bpo-42877.Fi1zEG.rst

@@ -0,0 +1,4 @@
+Added the ``compact`` parameter to the constructor of
+:class:`traceback.TracebackException` to reduce time and memory
+for use cases that only need to call :func:`TracebackException.format`
+and :func:`TracebackException.format_exception_only`.