Python：暂时更改变量范围规则

Question

I was wondering if there is a trick to temporarily change the variable scoping rules in Python. For example, is it to possible to temporarily turn the " LEGB " rule into "LB", for debugging purposes?

The reason I'm asking is this: every now and then - especially when running stuff in (larger) ipython notebooks - a typo like the one below, doesn't become a NameError and can result in weird bugs:

def dostuff():
    for i in range(3):
        print j# typo

j=0
dostuff()

This can partially be addressed with static code checkers, but I thought there might be a neat trick to do this within a function?!

Thanks, Andreas

PS: Yes, polluting namespaces makes the above more likely.

Answer 1

The scoping rules of python are built into the language. You can't change/disable them via any clever tricks that I can think of...

There are a few "best practices" that can help -- particularly when working in scripts. One piece of advice is to pack everything into a main function and call that (which helps with namespace pollution that you mentioned):

def main():
    j = 0
    dostuff()

if __name__ == '__main__':
    main()

But this probably doesn't help you too much when typing things out in the REPL.

---

EDIT: While you can't change the scoping rules of the language, you can clear out the module's globals, call the function and then reset the globals after the function call. Here's a context manager that demonstrates that sort of usage:

>>> class CopyRestoreDict(object):
...   def __init__(self, dct):
...     self._dict = dct
...     self._copy = {}
...   def __enter__(self):
...     self._copy.update(self._dict)
...     self._dict.clear()
...   def __exit__(self, *args):
...     self._dict.update(self._copy)
...     self._copy.clear()
... 
>>> def foo():
...   print j
... 
>>> j = 1
>>> foo()
1
>>> with CopyRestoreDict(globals()):
...   foo()
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
NameError: name 'foo' is not defined
>>> j
1