CODEWITHSUNDEEP
python
James T
James T

Reputation: 55

Nested list comprehension with split() in python

Long story short, I am trying to work with some crazy data I collected yesterday. A quick nested for loop would do this real easy but right now, I just want to get my nested list comprehension to work. There are a bunch of post on this subject and I am not sure whether it is just me or the because of Friday evening, I can't get it to work. My list comphrension is in format:

[ [str(j[0], sanitize(j[1]) for j in i.split(',')] for i in b]

where b is ['string, float\t\n', 'string, float\t\n'] or b might be a file object or whatever, doesn't matter a heck lot.

I could simply do something like

for i in b:
    out.append(str(j[0]), sanitize(j[1])

But, it should be possible by list comprehension. IT SHOULD!!! And, somehow, I am not getting it right.

Now, on to what I have done so far...

>>> b
['08:54:55, 0.031250\n', '08:55:57, 0.031250\n']

>>> [i.split(',') for i in b]
[['08:54:55', ' 0.031250\n'], ['08:55:57', ' 0.031250\n']]

What I want:

[['08:54:55', 0.031250], ['08:55:57', 0.031250]]

A multiple list comprehension should do the trick:

Something like:

[ [j[0], j[1]] for j in i.split(',') for i in b] # the cleaning/str/float would be something like: str(j[0]), float(j[1])

BUT, this is what I get.

>>> [ [j[0], j[1]] for j in i.split(',') for i in b]
[['0', '8'], ['0', '8'], [' ', '0'], [' ', '0']]
>>> 
>>> [ [j[1]] for j in i.split(',') for i in b]
[['8'], ['8'], ['0'], ['0']]

Any attempt to work on j[0] or j[1] does not pan out.

To figure out what was going on, I did this:

>>> [ [ pprint(type(j)) for j in i.split(',')] for i in b]
<type 'str'>
<type 'str'>
<type 'str'>
<type 'str'>
[[None, None], [None, None]]

If I don't mess with j[0] or j[1],

>>> [ [ j for j in i.split(',')] for i in b]
[['08:54:55', ' 0.031250\n'], ['08:55:57', ' 0.031250\n']]

The example with pprint(type(j)) means even if I were to write an external function, it would not do the trick. I would simply be getting an empty list back from the list comprehension.

What am I doing wrong? A lambda might do the trick?

Upvotes: 0

Views: 4393

Answers (2)

AChampion
AChampion

Reputation: 30288

You can create a second for comprehension to extra the split():

>>> [[t, sanitize(v)] for i in b for t, v in [i.split(',')]]
[['08:54:55', 0.031250], ['08:55:57', 0.031250]]

Upvotes: 4

Ry-
Ry-

Reputation: 225273

If you write out a proper, working loop:

out = []

for i in b:
    j = i.split(',')
    out.append((j[0], sanitize(j[1])))

you’ll notice that you need somewhere to store the result of i.split(',') to avoid repeating it. This isn’t possible inline in one list comprehension. You can write a function:

def convert_thing(thing):
    time, value = thing.split(',')
    return time, sanitize(value)

out = [convert_thing(x) for x in b]

or generate intermediate values (don’t do this):

out = [(time, sanitize(value)) for time, value in (x.split(',') for x in b)]
# this bit is a generator expression              ^^^^^^^^^^^^^^^^^^^^^^^^^

Upvotes: 1

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