cpython - can you recover from reassigning __builtins__ in python?

Question

If I open up interactive mode and type:

__builtins__ = 0 # breaks everything

have I completely broken the session? If so, what is going on behind the scenes to assign __builtins__ to the builtin module that can't be handled by the interpreter? If not, how can I recover from this?

Just a few of my own attempts to fix it:

• Any attempt to import anything results in an error "ImportError __import__ not found"
• all functions I might use to do anything other than evaluate numerical expressions are broken
• There is another variable __package__ still accessible, but I don't know if/how it can be used.

Answer 1

You can usually get access to anything you need, even when __builtins__ has been removed. It's just a matter of digging far enough. For example:

Python 2.7.3 (default, Apr 10 2012, 23:31:26) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> __builtins__ = 0
>>> open
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'open' is not defined
>>> dir
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'dir' is not defined
>>> int
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'int' is not defined
>>> float
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'float' is not defined
>>>
>>> __builtins__ = [t for t in ().__class__.__bases__[0].__subclasses__() if 'warning' in t.__name__][0]()._module.__builtins__
>>>
>>> open
<built-in function open>
>>> int
<type 'int'>
>>> float
<type 'float'>
>>>

For an explanation of what the heck just happened here, read Eval really is dangerous, where similar techniques are used to demonstrate that you cannot safely execute untrusted Python code.