From 054dbe14c37b5a97e52bbf2174adaa8b0d154c8f Mon Sep 17 00:00:00 2001 From: "Gregory P. Smith" Date: Sat, 9 Oct 2021 12:22:14 -0700 Subject: [PATCH] bpo-45353: Remind sys.modules users to copy when iterating. This is true of all dictionaries in Python, but this one tends to catch people off guard as they don't realize when sys.modules might change out from underneath them as a hidden side effect of their code. Copying it first avoids the RuntimeError. An example when this happens in single threaded code are codecs being loaded which are an implicit time of use import that most need not think about. --- Doc/library/sys.rst | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst index ec12e02fb37d45..8b3c6fd7627317 100644 --- a/Doc/library/sys.rst +++ b/Doc/library/sys.rst @@ -1073,7 +1073,11 @@ always available. This is a dictionary that maps module names to modules which have already been loaded. This can be manipulated to force reloading of modules and other tricks. However, replacing the dictionary will not necessarily work as expected and - deleting essential items from the dictionary may cause Python to fail. + deleting essential items from the dictionary may cause Python to fail. If + you want to iterate over this global dictionary always use + ``sys.modules.copy()`` or ``tuple(sys.modules)`` to avoid exceptions as its + size may change during iteration as a side effect of code or activity in + other threads. .. data:: orig_argv