example of triggering restricted mode by touching __builtins__ (run this in the repl)

restricted-mode.py

def f(x):
  print 'func_code is a restricted attribute', x.func_code

f(f) # this works

# overwrite __builtins__

old_builtins = __builtins__
new_builtins = type('module', (type(__builtins__),), {})('__builtins__')

for attr, value in ((attr, getattr(old_builtins, attr)) for attr in dir(old_builtins)):
  setattr(new_builtins, attr, value)

__builtins__ = new_builtins

f(f) # this fails

# Traceback (most recent call last):
#  File "<stdin>", line 1, in <module>
#  File "<stdin>", line 2, in f
# RuntimeError: function attributes not accessible in restricted mode