Author: Armin Rigo <>
Changeset: r87177:2b6a25a67d6b
Date: 2016-09-17 10:48 +0200

Log:    Document sys.getsizeof()

diff --git a/pypy/doc/cpython_differences.rst b/pypy/doc/cpython_differences.rst
--- a/pypy/doc/cpython_differences.rst
+++ b/pypy/doc/cpython_differences.rst
@@ -449,6 +449,27 @@
   support (see ``multiline_input()``).  On the other hand,
   ``parse_and_bind()`` calls are ignored (issue `#2072`_).
+* ``sys.getsizeof()`` always raises ``TypeError``.  This is because a
+  memory profiler using this function is most likely to give results
+  inconsistent with reality on PyPy.  It would be possible to have
+  ``sys.getsizeof()`` return a number (with enough work), but that may
+  or may not represent how much memory the object uses.  It doesn't even
+  make really sense to ask how much *one* object uses, in isolation with
+  the rest of the system.  For example, instances have maps, which are
+  often shared across many instances; in this case the maps would
+  probably be ignored by an implementation of ``sys.getsizeof()``, but
+  their overhead is important in some cases if they are many instances
+  with unique maps.  Conversely, equal strings may share their internal
+  string data even if they are different objects---or empty containers
+  may share parts of their internals as long as they are empty.  Even
+  stranger, some lists create objects as you read them; if you try to
+  estimate the size in memory of ``range(10**6)`` as the sum of all
+  items' size, that operation will by itself create one million integer
+  objects that never existed in the first place.  Note that some of
+  these concerns also exist on CPython, just less so.  For this reason
+  we explicitly don't implement ``sys.getsizeof()``.
 .. _`is ignored in PyPy`:
 .. _`little point`:
 .. _`#2072`:
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