Closed
Description
Bug report
Bug description:
Discovered while working on #117178.
PEP 726 notes that the existing way for a module to override default methods is:
# mod.py
import sys
from types import ModuleType
CONSTANT = 3.14
class ImmutableModule(ModuleType):
def __setattr__(name, value):
raise AttributeError('Read-only attribute!')
def __delattr__(name):
raise AttributeError('Read-only attribute!')
sys.modules[__name__].__class__ = ImmutableModuleIf I want to load this module lazily, I can use this MRE (adapted from #117178):
import sys
import importlib
import importlib.util
def lazy_import(name):
spec = importlib.util.find_spec(name)
loader = importlib.util.LazyLoader(spec.loader)
spec.loader = loader
module = importlib.util.module_from_spec(spec)
sys.modules[name] = module
loader.exec_module(module)
return module
# Eagerly load the module
eager_module = importlib.import_module(sys.argv[1])
print(f"Eagerly loaded: {type(eager_module)}")
del sys.modules[sys.argv[1]]
# Lazy-load the module...
lazy_module = lazy_import(sys.argv[1])
print(f"Lazy-loaded: {type(lazy_module)}")
# ... and then trigger load by listing its contents
print(dir(lazy_module)[:5])
print(f"Fully loaded: {type(lazy_module)}")This currently (with the fix in #117179) produces:
❯ ./python repro.py mod
Eagerly loaded: <class 'mod.ImmutableModule'>
Lazy-loaded: <class 'importlib.util._LazyModule'>
Traceback (most recent call last):
File "/home/chris/Projects/cpython/repro.py", line 23, in <module>
print(dir(lazy_module)[:5])
~~~^^^^^^^^^^^^^
File "/home/chris/Projects/cpython/Lib/importlib/util.py", line 223, in __getattribute__
self.__class__ = types.ModuleType
^^^^^^^^^^^^^^
File "/home/chris/Projects/cpython/mod.py", line 8, in __setattr__
raise AttributeError('Read-only attribute!')
AttributeError: Read-only attribute!. Did you mean: '__doc__'?Making the following change:
- self.__class__ = types.ModuleType
+ object.__setattr__(self, '__class__', types.ModuleType)Results in:
❯ ./python repro.py mod
Eagerly loaded: <class 'mod.ImmutableModule'>
Lazy-loaded: <class 'importlib.util._LazyModule'>
['CONSTANT', 'ImmutableModule', 'ModuleType', '__builtins__', '__cached__']
Fully loaded: <class 'module'>If instead we change to:
- self.__class__ = types.ModuleType
+ if isinstance(self, _LazyModule):
+ object.__setattr__(self, '__class__', types.ModuleType)We get:
❯ ./python repro.py mod
Eagerly loaded: <class 'mod.ImmutableModule'>
Lazy-loaded: <class 'importlib.util._LazyModule'>
['CONSTANT', 'ImmutableModule', 'ModuleType', '__builtins__', '__cached__']
Fully loaded: <class 'mod.ImmutableModule'>I believe making this change is the best way to respect the intentions of the module writer.
I've tagged this as a bug because these things have not worked together, but it might be more of a feature for them to start to, as neither is exactly standard practice.
CPython versions tested on:
3.11, 3.13, CPython main branch
Operating systems tested on:
Linux