Finding Blank Columns in CSV

[Peter Otten]

Jaydip Chakrabarty wrote:

> On Tue, 06 Oct 2015 01:34:17 +1100, Chris Angelico wrote:
> 
>> On Tue, Oct 6, 2015 at 1:06 AM, Tim Chase
>> <python.list at tim.thechases.com> wrote:
>>> That way, if you determine by line 3 that your million-row CSV file has
>>> no blank columns, you can get away with not processing all million
>>> rows.
>> 
>> Sure, although that effectively means the entire job is moot. I kinda
>> assume that the OP knows that there are some blank columns (maybe lots
>> of them). The extra check is unnecessary unless it's actually plausible
>> that there'll be no blanks whatsoever.
>> 
>> Incidentally, you have an ordered_headers list which is the blank
>> columns in order; I think the OP was looking for a list of the
>> _non_blank columns. But that's a trivial difference, easy to tweak.
>> 
>> ChrisA
> 
> Thanks to you all. I got it this far. But while writing back to another
> csv file, I got this error - "ValueError: dict contains fields not in
> fieldnames: None". Here is my code.
> 
> rdr = csv.DictReader(fin, delimiter=',')
> header_set = set(rdr.fieldnames)
> for r in rdr:
>     header_set = set(h for h in header_set if not r[h])
>     if not header_set:
>         break
> 
> for r in rdr:
>     data = list(r[i] for i in header_set)
> 
> dw = csv.DictWriter(fout, header_set)
> dw.writeheader()
> dw.writerows(data)

Sorry, this is not the code you ran. I could guess what the missing parts 
might be, but it is easier for both sides if you provide a small script that 
actually can be executed and a small dataset that shows the behaviour you 
describe. Then post the session and especially the traceback. Example:

$ cat my_data.csv
0
$ cat my_code.py 
print 1/int(open("my_data.csv").read())
$ python my_code.py 
Traceback (most recent call last):
  File "my_code.py", line 1, in <module>
    print 1/int(open("my_data.csv").read())
ZeroDivisionError: integer division or modulo by zero

Don't retype, use cut and paste. Thank you.