Clearing globals in CPython

Peter Otten __peter__ at web.de
Wed Oct 1 19:24:05 CEST 2014


Steven D'Aprano wrote:

> Out of curiosity, I ran:
> 
> globals().clear()
> 
> in the interactive interpreter. It broke much more than I expected!
> Built-ins were no longer available, and import stopped working.
> 
> I expected that global variables would be all lost, but built-ins would
> remain, since they don't live in the global namespace. I was wrong:
> 
>>>> globals().clear()
>>>> x = len([])
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> NameError: name 'len' is not defined
> 
> The reason, I think, is that CPython has a special __builtins__ global
> variable that the interpreter uses to access built-in functions. What
> *appears* to be happening is that if that __builtins__ global is missing,
> CPython can not access the built-ins.
> 
> (Supporting this interpretation: IronPython, like CPython, has a
> __builtins__ global, and behaves the same when the globals() are cleared.
> Jython, which does not have a __builtins__ global, does not.)
> 
> Well that's okay, I thought to myself, I'll just import the built-in
> functions:
> 
>>>> from builtins import len  # use '__builtin__' in Python 2
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> ImportError: __import__ not found
> 
> Oops.
> 
> So, with no built-ins available, import no longer works. That makes things
> rather tricky.
> 
> Obviously the easiest way to recover is to exit the current session and
> restart it, but as a challenge, can we recover from this state?

$ python3
Python 3.4.0 (default, Apr 11 2014, 13:05:11) 
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> globals().clear()
>>> import that
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: __import__ not found
>>> object = 1 .__class__.__bases__[0]
>>> Quitter = [c for c in object.__subclasses__() if c.__name__ == 
"Quitter"][0]
>>> __builtins__ = Quitter.__call__.__globals__["__builtins__"]
>>> import this
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