understanding variable assignment in python

Question

I am noob and trying to understand Python.

For os.walk documentation says that it returns a tuple (dirpath, dirnames, filenames)

just of understanding I am trying to use it like below

import os
from os.path import join, getsize
file=[]
dir=[]
xroot,dir,file = os.walk('C:\Python27\mycode')

But it gives me error like : xroot,dir,file = os.walk('C:\Python27\mycode') ValueError: need more than 2 values to unpack

My question is why cant I assign it like above rather then it being part of loop (most example use that)?

There are multiple files in the directory (seems like two from your error message). When you use it in a loop, then you get (xroot,dir,file) for each file in the directory. — Jim Garrison
– Jim Garrison, Commented Jun 5, 2013 at 5:53

Blender · Accepted Answer · 2013-06-05 06:02:22Z

2

Your code attempts to unpack the generator returned by os.walk() into a three-tuple. This is fine, but the problem is that the generator yields only two items, which isn't going to work.

Each item in the generator is a three-tuple itself, which is what your for loop is really unpacking each iteration. A more verbose way of writing it would be:

for three_tuple in os.walk('C:\Python27\mycode'):
    xroot, dir, file = three_tuple

You might find it easier to actually turn that generator into a list:

>>> pprint.pprint(list(os.walk('.')))
[('.', ['foo'], ['main.py']),
 ('.\\foo', [], ['test.py', 'test.pyc', '__init__.py', '__init__.pyc'])]

As you can see, the result is an iterable (a list) where each element is a three-tuple, which can then be unpacked into a root folder, a list of folders and a list of files.

edited Jun 5, 2013 at 6:02

answered Jun 5, 2013 at 5:57

Blender

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John La Rooy · Accepted Answer · 2013-06-05 05:57:12Z

1

os.walk returns an iterator. The usual think to do is loop over it

for xroot, dir, file in os.walk('C:\Python27\mycode'):
    ...

but you can also just use xroot, dir, file = next(os.walk('C:\Python27\mycode')) to single step through

answered Jun 5, 2013 at 5:57

John La Rooy

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Srikar Appalaraju · Accepted Answer · 2013-06-05 05:58:23Z

1

os.walk does not return root,dir,file. it returns a generator object for the programmer to loop through. Quite possibly since a given path might have sub-directories, files etc.

>>> import os
>>> xroot,dir,file = os.walk('/tmp/') #this is wrong.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack
>>> os.walk('/tmp/')
<generator object walk at 0x109e5c820> #generator object returned, use it
>>> for xroot, dir, file in os.walk('/tmp/'):
...     print xroot, dir, file
... 
/tmp/ ['launched-IqEK']
/tmp/launch-IqbUEK [] ['foo']
/tmp/launch-ldsaxE [] ['bar']
>>>

answered Jun 5, 2013 at 5:58

Srikar Appalaraju

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1 Comment

theodox Over a year ago

The SO thread on generators: stackoverflow.com/questions/231767/…

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