os.listdir unwanted behaviour

Peter Otten __peter__ at web.de
Tue Sep 29 09:15:30 CEST 2009

Chris Adamson wrote:

> I am writing code that cycles through files in a directory and for each
> file it writes out another file with info in it. It appears that as I am
> iterating through the list returned by os.listdir it is being updated
> with the new files that are being added to the directory. This occurs
> even if I reassign the list to another variable.

My guess is that this has nothing to do with os.listdir():

>>> import os
>>> files = os.listdir(".")
>>> files
['b', 'a']
>>> os.system("touch c")
>>> files
['b', 'a'] # look Ma, no automatic updates!
>>> os.listdir(".")
['b', 'c', 'a']

It is normal Python behaviour that assignment doesn't copy a list; it just 
creates another reference:

>>> a = [1]
>>> b = a
>>> id(a) == id(b) 
>>> b.append(2)
>>> a
[1, 2]

Use slicing to make an actual copy:

>>> b = a[:] # b = list(a) would work, too
>>> id(a) == id(b)
>>> b.append(3)
>>> a
[1, 2]
>>> b
[1, 2, 3]

> Here is my code:

No, it's not. If you post a simplified version it is crucial that you don't 
remove the parts that actually cause the undesired behaviour. In your case 
there has to be a mutating operation on the list like append() or extend().


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