Asking for Help: Why don't my list and generator comprehensions work?

lwickjr: [2008-Jul-09]

Python 2.5.2 (r252:60911, Feb 21 2008, 13:11:45) [MSC v.1310 32 bit (Intel)] on win32
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IDLE 1.2.2      ==== No Subprocess ====
>>> import os
>>> dir()
['__builtins__', '__doc__', '__file__', '__name__', 'idlelib', 'os']
>>> Source = ('all', 'F:\\Documents\\Pictures\\New\\06')
>>> [[os.path.join(S, t) for t in os.listdir(S)] for S in Source[1:]]
>>> dir()
['S', 'Source', '__builtins__', '__doc__', '__file__', '__name__', 'idlelib', 'os', 't']
>>> del S, t
>>> [(os.path.join(S, t) for t in os.listdir(S)) for S in Source[1:]]
[<generator object at 0x010B5AA8>]
>>> dir()
['S', 'Source', '__builtins__', '__doc__', '__file__', '__name__', 'idlelib', 'os']
>>> del S
>>> dir()
['Source', '__builtins__', '__doc__', '__file__', '__name__', 'idlelib', 'os']
>>> [os.path.join(S, t) for t in os.listdir(S) for S in Source[1:]]
Traceback (most recent call last):
  File "<pyshell#10>", line 1, in <module>
    [os.path.join(S, t) for t in os.listdir(S) for S in Source[1:]]
NameError: name 'S' is not defined

The first example generates nested lists, one sublist per directory in Source.

The second example generates a list of generator objects, one per directory in Source.

The third example generates an undefined variable error.

What is wrong with the third example? If I pre-define S = Source[1], it works correctly, generating a flat list, while if I predefine S equal to something else, it uses that value in violation of the semantics of list comprehensions as I understand them. Is something wrong with my list comprehension, with Python 2.5.2 itself, or with my understanding of the relavent semantics?

My answer: In case someone reads this, the third example has got "for" loops in the wrong order. The first for loop runs first, and therefore it should be written as [os.path.join(S, t) for S in Source[1:] for t in os.listdir(S)].

Answer from PaulBoddie:

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Asking for Help/Why don't my list and generator comprehensions work? (last edited 2009-11-05 23:06:27 by PaulBoddie)

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