ENH: Option for reading files with a variable number of comment lines at start

[wesm]

http://stackoverflow.com/questions/14276661/python-pandas-read-file-skipping-commented

[montefra]

@wesm: I'm the author of the stackoverflow question.

Implementing this well would require dipping into the C tokenizer code. It's not as bad as it might sound.

If you can point me to where this is implemented, so I can give a look, see what I understand and if I can contribute?

[wesm]

pandas/src/parser/tokenizer.c

[montefra]

I have spent a number of hours in the last days trying to figure out which function calls which(, and is not an easy task).
The error that I get when reading a file (in my case with read_table, but my understanding is that it would be the same with read_csv) with commented line I get an error:

pandas/io/parsers.py", line 1475, in _rows_to_cols
   raise ValueError(msg) 
ValueError: Expected 0 fields in line 7, saw 7content

The error comes from within the PythonParser class. Before calling _rows_to_cols the input file is stored in a list of lists, content and fully commented lines appears as an empty list.
So a possibility would be to add an extra keyword skipcommentedlines (or similar) and if True count how many empty lines from the beginning of the lines there are and take them out from content. If self.orif_names is also empty, count the number of fields, and fill it with range(n_fields).

If this makes sense, I'll implement it asap.

It looks to me that the functions in pandas/src/parser/tokenizer.c are used only in class CParserWrapper when the 'c' engine is required. From a comment I understand that the 'c' engine is not supported yet. For this I think that I have found a couple of places where the skipping of commented lines can be implemented, although I have to understand if it makes sense and how to implement it.

At last. When reading a file of the kind

 # line1 (space before the comment)
# line2
# [...]
# line 6 
1 2 3 4 5 6 7
[...]

with pd.read_table(fname, header=None, comment='#', sep='\s') I have noticed that if I have 2 commented lines, content has two empty lists in the first two positions (as I would expecting), for 1 or >=3 commented lines I get n-1 empty lists in content. It does look quite strange to me. Any idea of what's going on?

Thanks for the time reading (and hopefully replying) to this long post

[holocronweaver]

I would very much like to see this feature implemented as I have to deal with workarounds for it often.

Is there a reason that the above implementation would not work? It seems simple enough. I have never contributed to pandas before and would like some feedback before proceeding to implement the feature.

[jreback]

the c-parser is the primary parser; this change would have to be made there (actually in src/parser.pyx). its not that hard, but a bit non-trivial

[montefra]

I'll give a look at src/parser.pyx asap. I hope to have more luck than when I looked into pandas/src/parser/tokenizer.c

[jreback]

this is an already existing feature to skip comments at the end of the line

http://pandas.pydata.org/pandas-docs/dev/io.html#comments

IIUC you want to skip a line if there is a comment at the beginning?

e.g.

# comment
data.....
# comment
data....
   # comment

if you specified comment='\s*#' (I am not sure it takes re ATM though)

[jreback]

the last one would be tough FYI, tokenzier only takes a single character ATM

[holocronweaver]

@montefra Whichever of us gets to it first then. The race is on. ;-)

@jreback That is correct, I want to skip lines with a comment character at the beginning. I would imagine the last case could not be handled in the event the file is space delimited. Basically, comments probably need to either be declared at the very beginning or end of the line.

[holocronweaver]

I have implemented comment skipping and also fixed a related problem with CSV format sniffing. I did so by modifying io/parsers.py and have not yet begun work on the c-parser which apparently my install of pandas is not using by default. Once I have finished making changes in parser.pyx I will push my changes.

A similar problem is whether pandas should ignore empty lines by default. It would be very easy to implement as a slight extension of ignoring comments. I will open up an issue related on this to get some feedback.

[jreback]

gre8!

you can put your changes up as a PR, be sure to enable travis, lmk if you need help

[holocronweaver]

Only one build failed in Travis CI, but it does not appear to have anything to do with my changes. Is this expected, or did I break something related to pytables somehow?

[jreback]

@holocronweaver rebase on master again I just pushed some code to 'fix' that failure (although net net the changes didn't actually do antything)...but seemed to fix it

to force travis to rebuild

git commit -amend -C HEAD
git push origin yourbranch -f

this resets the last commit to a new hash forcing a rebuild