Python3 reimporting __builtins__ - python-3.x

Question

Assume I delete all my builtins using __builtins__={} Is there a way to force the reimport of all base modules? I thought about using _frozen_importlib.BuiltinImporter, could that be done using that method?
The goal is to see whether it can be done and how to extricate yourself from such a situation. It has no use beyond simply learning esoteric parts of python. This is python3 btw.

Answer 1

Your question was proposed as a puzzle in the Python chat room, back in March 2018.
The following solution, given by user Aran-Fey, restores the builtins module afresh in the REPL:
__builtins__ = __loader__.create_module(__loader__.find_spec('builtins'))
A more difficult variant, additionally clearing the globals() namespace and thus denying access to __loader__, was also proposed (and solved) in the chat room.
Note that setting __builtins__ = {} only disables access to the builtin namespace within an interactive REPL, not within a script, and the lookup in __builtins__ is considered a CPython implementation detail.

Answer 2

One highly impractical method of accessing the builtins module when its name is inaccessible is described at the bottom of https://nedbatchelder.com/blog/201206/eval_really_is_dangerous.html.
Update: from a thread on Reddit about recovering cleared globals, a similar snippet that will get you the original builtins:
[
c for c in ().__class__.__base__.__subclasses__()
if c.__name__ == 'catch_warnings'
][0]()._module.__builtins__
By reassigning this object to __builtins__, the built-in objects become accessible again.
>>> __builtins__ = {}
>>> max
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'max' is not defined
>>> __builtins__ = [c for c in ().__class__.__base__.__subclasses__() if c.__name__ == 'catch_warnings'][0]()._module.__builtins__
>>> max
<built-in function max>
Quick explanation of the components of the statement:
() #tuple instance
().__class__ #the tuple class
().__class__.__base__ #tuple's base class, i.e. `object`
().__class__.__base__.__subclasses__() #every class that inherits from `object`
[c for c in ().__class__.__base__.__subclasses__() if c.__name__ == 'catch_warnings'][0] #the `warnings.catch_warnings` class
[c for c in ().__class__.__base__.__subclasses__() if c.__name__ == 'catch_warnings'][0]()._module #create a `catch_warnings` instance and access its `_module` attribute, which is the `warnings` module.
[c for c in ().__class__.__base__.__subclasses__() if c.__name__ == 'catch_warnings'][0]()._module.__builtins__ #`warnings.__builtins__` is the builtins module
__builtins__ = [c for c in ().__class__.__base__.__subclasses__() if c.__name__ == 'catch_warnings'][0]()._module.__builtins__ #assign result to `__builtins__`

Answer 3

As #Kevin noted in the comments, this seems to be different between the REPL and a script. In a script, reassigning __builtins__ seems to have no effect (Python 3.7.3):
# test.py
__builtins__ = {}
print(max([1,2,3]))
def really_weird():
__builtins__ = {"max": lambda x: min(x)}
print(max([1,2,3]))
really_weird()
# Output
3
3
max didn't change, even when __builtins__ reassigned the name (sort of).
In the REPL, you are correct that _frozen_importlib.BuiltinImporter can be used.
>>> import _frozen_importlib # This must be before reassigning __builtins___, since __builtins__ includes __import__
>>> __builtins__ = {}
>>> max([1,2,3])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'max' is not defined
>>> __builtins__ = _frozen_importlib.BuiltinImporter().load_module("builtins")
>>> max([1,2,3])
3
One thing to note: The module name is builtins, not __builtins__.
Also, as is noted in the comments, doing import builtins as b; ... __builtins__ = b or something similar also works.
If you don't want to do setup work, #Kevin's answer does the same without previous imports.