Python import and module scoping (specifically sys.ps1) -

why following not work:

from sys import ps1 ps1 = 'something else ' 

but does?

import sys sys.ps1 = 'something else ' 

if run simple test

import sys sys import ps1 ps1 = 'something else' sys.ps1 = 'something else' 

the first assignment doesn't work, second does. id() both ps1 , sys.ps1 same, should refer same thing, right?

the short version is: assignment doesn't mutate in python, rebinds names. rebinding different name, happens bound reference same string sys.ps1 doesn't affect sys.ps1 in way.

let's go through step step:

from sys import ps1 

this imports sys (but doesn't bind name sys in current globals), binds name ps1 in current globals same object sys's ps1.

ps1 = 'something else ' 

this rebinds name ps1 in current globals same object literal 'something else'. there's no way possibly affect sys module.

import sys 

this imports sys, , binds name sys in current globals it.

sys.ps1 = 'something else ' 

this looks name sys in current globals, getting reference sys module, rebinds name ps1 in module same object literal 'something else '.

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or, putting in pseudo-python terms, instead of in english…

your first code sample this:

tempspace._sys = __import__('sys') globals.ps1 = tempspace._sys.ps1 del tempspace._sys globals.ps1 = 'something else ' 

your second this:

globals.sys = sys_modules.sys = __import__('sys') globals.sys.ps1 = 'something else ' 

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here transcript of described in comments.

python 2.7.2 (default, jun 20 2012, 16:23:33)  [gcc 4.2.1 compatible apple clang 4.0 (tags/apple/clang-418.0.60)] on darwin type "help", "copyright", "credits" or "license" more information. >>> import sys >>> sys import ps1 >>> id(sys.ps1) 4360831168 >>> id(ps1) 4360831168 >>> sys.ps1 = 'something else ' else ps1 '>>> ' else  

sys.ps1 , ps1 have same id, because 2 different names reference same object.

and changing sys.ps1 not change ps1.