[Python-ideas] [Python-Dev] If you shadow a module in the standard library that IDLE depends on, bad things happen

[Steven D'Aprano]

On Sun, Nov 01, 2015 at 08:41:03AM +0200, Serhiy Storchaka wrote:
> On 01.11.15 08:06, Steven D'Aprano wrote:
[...]
> >Scripts which deliberately or unintentionally shadow installed packages
> >will break from this change. I don't have a problem with this. You can't
> >fix harmful behaviour without breaking code that depends on that harmful
> >behaviour. Additionally, I expect that those who rely on the current
> >behaviour will be in a small minority, much fewer than those who will be
> >bitten by accidental shadowing into the indefinite future.
> 
> Unfortunately this is not such small minority
> 
> https://code.openhub.net/search?s=%22sys.path.pop(0)%22&p=0

The search results contain MANY duplicates. For example, in the first 
ten results, there are three duplicates of "common.py" from "kongove's 
autotest", and two duplicates of "common.py" from "Chromium OS".

The first hit does:

    sys.path.insert(0, path.dirname(__file__))
    import objects
    from objects import constants
    sys.path.pop(0)

which seems to be a very common pattern: insert something into the start 
of the path, then pop it out later. That's harmless, and won't be 
effected by shifting where "" in inserted. So I think that this search 
is not a good test for code that will be effected.

Besides, anyone who unconditionally pops the first item from sys.path is 
already on shakey ground. You should not assume that the first item will 
always be "", since it may have been changed before your code runs, e.g. 
by the PYTHONSTARTUP file, usercustomize, etc. Now, we shouldn't break 
people's code for no good reason, not even it it is already broken, but 
we have a good reason: having "" at the start of sys.path breaks code 
that inadvertently shadows other modules.

(And it may even be a security risk.)


-- 
Steve