Python - 过滤（lambda）元组索引错误列表

Question

I'm trying to filter a tuple as follows...some code not included for brevity:

    1) print >> sys.stderr, "audio", audio
    2) print >> sys.stderr, "audio[0]", audio[0]
    3) print >> sys.stderr, "audio[1]", audio[1]
    4) audio_lang = filter(lambda a: a[1]==LANG, audio)

It is being passed a tuple with 2 elements, the run is as follows:

 D:\Staging\Test>cleanMKV.py .
 audio [('fre',), ('eng',)]
 audio[0] ('fre',)
 audio[1] ('eng',)
 Traceback (most recent call last):
 File "D:\Staging\Test\cleanMKV.py",
 audio_lang = filter(lambda a: a[1]
 File "D:\Staging\Test\cleanMKV.py",
 audio_lang = filter(lambda a: a[1]
 IndexError: tuple index out of range

The tuple was created properly with RE and I'm at a point I want to filter as shown on line 4. It is trying to reference the audio a[1] in audio.

Any help appreciated.

Answer 1

First, you define

audio = [('fre',), ('eng',)]

which is actually a list of two elements ('fre',) and ('eng',) , which are both tuples containing only a single element, which are 'fre' and 'eng' respectively.

Now, if you do

audio_lang = filter(lambda a: a[1]==LANG, audio)

you are saying to keep only elements of audio, where the second elements equals to LANG . But as your list elements ('fre',) and ('eng',) are tuples of length 1, you are getting

IndexError: tuple index out of range

because you are trying to access the second elements, which do not exist.

You could do

audio_lang = filter(lambda a: a[0]==LANG, audio)

that is access the first element or redefine audio and do

audio = ['fre', 'eng']
audio_lang = filter(lambda a: a == LANG, audio)

But I do not understand, what are you trying to accomplish, so this might not be what you want. But hopefully this explains the source of the error you are having.

Answer 2

a is a tuple but audio is a list. There is only one item in each of the two tuples that make up audio . Therefore, evaluating either audio[0][1] or audio[1][1] will return tuple index out of range .

>>> audio = [('fre',),('eng',)]

>>> audio
[('fre',), ('eng',)]
>>> audio[0]
('fre',)
>>> audio[0][0]
'fre'
>>> audio[0][1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: tuple index out of range

... but:

>>> audio = [('fre',1),('eng',2)]
>>> audio[0][1]
1