How to Reload a Python Module? Clearing Variable Info

Question

I've been storing variable information inside the builtins module and I want to clear all the variable information that is stored inside of it. I can't figure out how to do it though.

import importlib
import builtins
print("add" in dir(builtins))
builtins.add = "x"
print("add" in dir(builtins))
importlib.reload(builtins)
print("add" in dir(builtins))

This doesn't seem to work.

"I've been storing variable information inside the builtins module" - jesus crap, why? — user2357112
– user2357112, Commented Mar 1, 2016 at 3:58
builtins is not a normal module. It's loaded as part of the core interpreter; I doubt reload actually does anything meaningful for it. — ShadowRanger
– ShadowRanger, Commented Mar 1, 2016 at 3:59
BTW, per the docs: "It is generally not very useful to reload built-in or dynamically loaded modules. Reloading sys, __main__, builtins and other key modules is not recommended. In many cases extension modules are not designed to be initialized more than once, and may fail in arbitrary ways when reloaded." — ShadowRanger
– ShadowRanger, Commented Mar 1, 2016 at 4:04
@user2357112 Yea, I realize what I am doing is not conventional. I am supplying a builtins module to a dictionary that is being passed into the global_env parameter of an exec function. Code from the exec function stores its global variables inside of builtins module naturally and I want a clean copy of the builtins module. — btomtom5
– btomtom5, Commented Mar 1, 2016 at 4:05
@btomtom5: Okay, you're not making this seem any more sane. exec-ed code that stores to builtins on purpose? WTF?!? If it's just assigning values to its own globals, just create a new dict and then pass a reference to it to exec to populate, don't pollute the namespace of every Python module to do something terrible. — ShadowRanger
– ShadowRanger, Commented Mar 1, 2016 at 4:08

user2357112 · Accepted Answer · 2016-03-01 05:00:50Z

1

reload does not clear a module's __dict__ before reloading it, so reloading a module doesn't affect any names you add to it that weren't originally part of the module.

It is essentially always better to avoid messing with the builtins module than to try to restore it after messing with it. That said, if you really want to do this, save a copy of builtins.__dict__ before messing with it so you can restore from the backup later:

# Before screwing with builtins:
builtins_backup = builtins.__dict__.copy()

# After screwing with builtins:
builtins.__dict__.clear()
builtins.__dict__.update(builtins_backup)

That's not going to be threadsafe if another thread comes in between the clear and the update, but trying to reload builtins is hardly in safe territory anyway.

answered Mar 1, 2016 at 5:00

user2357112

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1 Comment

btomtom5 Over a year ago

aah this makes sense. I appreciate this a lot! That being said, this does seem incredibly unstable and seems like bad practice as you suggested. I will use this to front functionality for now before I come up with a more stable solution.

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