Unexpected side-effects of assigning to sys.modules[__name__]

[Steven D'Aprano]

Given this module:

#funny.py
import sys
print "Before:"
print " __name__ =", __name__
print " sys.modules[__name__] =", sys.modules[__name__]
sys.modules[__name__] = 123
print "After:"
print " __name__ =", __name__
print " sys =", sys


when I run it I get these results:


[steve@sylar python]$ python2.6 funny.py
Before:
__name__ = __main__
sys.modules[__name__] = <module '__main__' from 'funny.py'>
After:
__name__ = None
sys = None



I'm completely perplexed by this behaviour. sys.modules() seems to be a
regular dict, at least according to type(), and yet assigning to an item
of it seems to have unexpected, and rather weird, side-effects.

What am I missing?

 

[Jean-Michel Pichavant]

Given this module:

#funny.py
import sys
print "Before:"
print " __name__ =", __name__
print " sys.modules[__name__] =", sys.modules[__name__]
sys.modules[__name__] = 123
print "After:"
print " __name__ =", __name__
print " sys =", sys


when I run it I get these results:


[steve@sylar python]$ python2.6 funny.py
Before:
__name__ = __main__
sys.modules[__name__] = <module '__main__' from 'funny.py'>
After:
__name__ = None
sys = None



I'm completely perplexed by this behaviour. sys.modules() seems to be a
regular dict, at least according to type(), and yet assigning to an item
of it seems to have unexpected, and rather weird, side-effects.

What am I missing?

Maybe when you assign 123 to sys.modules[__name__], you've removed the
last reference on <module '__main__' from 'funny.py'> and it is
garbaged. You are then loosing all your initial namespace.


try this one:
#funny.py
import sys
print "Before:"
print " __name__ =", __name__
print " sys.modules[__name__] =", sys.modules[__name__]
foo = sys.modules[__name__] # backup ref for the garbage collector
sys.modules[__name__] = 123
print "After:"
print " __name__ =", __name__
print " sys =", sys

Jean-Michel

 

[Steven D'Aprano]

I'm completely perplexed by this behaviour. sys.modules() seems to be
a regular dict, at least according to type(), and yet assigning to an
item of it seems to have unexpected, and rather weird, side-effects.

What am I missing?

Maybe when you assign 123 to sys.modules[__name__], you've removed the
last reference on <module '__main__' from 'funny.py'> and it is
garbaged. You are then loosing all your initial namespace.

By Jove, I think you've got it! How obvious in hindsight. Thank you.