horrible.py

>>> class CustomGlobals(dict):
...   def __getitem__(self, key):
...     if key == 'x':
...       return get_x()
...     return super().__getitem__(key)
...
>>> def get_x():
...   print('get_x called!')
...   return 1+1
...
>>> eval('print("x + 1 =", x + 1)', CustomGlobals())
get_x called!
x + 1 = 3