Replace module level mutable containers with immutable containers #139003
Description
There are many module level containers with constants. By replacing the mutable containers with immutable variants (e.g. replace a set with a frozenset or a list with a tuple) we improve performance (especially in the free-threaded build, see for example #138429) and avoid accidental modification of these containers.
The number of module level lists, dicts and sets on current main is:
number of mutable module level containers by type:
<class 'dict'>: 266
<class 'list'>: 150
<class 'set'>: 63
<class 'collections.defaultdict'>: 2
<class '_strptime.TimeRE'>: 1
<class 'email._encoded_words._QByteMap'>: 1
Script to list all the module level mutable containers
import sys
import importlib
import pkgutil
from collections import Counter
blacklist = ('idlelib.idle')
excluded_submodule_names = ('__main__')
search_submodules = 2
mutable_containers = (list, dict, set)
def list_container_types(module, mcc):
print_module = False
for a in dir(module):
if a in ('__all__', '__path__', '__builtins__', '__annotations__', '__conditional_annotations__'):
# why is __all__ a list and not a tuple?
continue
# if not a.startswith('_'):
# continue
attr = getattr(module, a)
tp = type(attr)
if issubclass(tp, mutable_containers):
if not print_module:
print(f'{module}:')
print_module = True
print(f' {a}: {tp}')
mcc.update([tp])
def search_modules(module_names, search_submodules: int, mcc):
for name in module_names:
if name in blacklist:
continue
try:
module = importlib.import_module(name)
except:
print(f'{name}: error on import')
module = None
list_container_types(module, mcc)
if search_submodules:
try:
sub_names = list(z.name for z in pkgutil.iter_modules(module.__path__))
except Exception as ex:
sub_names = []
mm = [name + '.' + sub_name for sub_name in sub_names if sub_name not in excluded_submodule_names]
search_modules(mm, search_submodules - 1, mcc)
mcc = Counter()
module_names = sorted(list(sys.builtin_module_names)) + sorted(list(sys.stdlib_module_names))
search_modules(module_names, search_submodules=2, mcc = mcc)
print()
print('number of module level containers by type:')
for key, value in mcc.items():
print(f'{key}: {value}')
Not all the mutable containers can be replaced by immutable containers. Some of them need to be mutable (e.g. copyreg.dispatch_table). And some of them are part of the public API and we might not want to change the type only for performance reasons.
Example candidates: _pydatetime._DAYNAMES (would improve performance of date.cdate), token.EXACT_TOKEN_TYPES, xml.etree.ElementTree.HTML_EMPTY