How do I robustly parse malformed CSV?

Question

I\'m processing data from government sources (FEC, state voter databases, etc). It\'s inconsistently malformed, which breaks my CSV parser in all sorts of delightful ways.

Answer 1

It is possible to subclass Ruby's File to process each line of the the CSV file before it is passed to the Ruby's CSV parser. For example, here's how I used this trick to replace non-standard backslash-escaped quotes \" with standard double-quotes ""

class MyFile < File
  def gets(*args)
    line = super
    if line != nil
      line.gsub!('\\"','""')  # fix the \" that would otherwise cause a parse error
    end
    line
  end
end

infile = MyFile.open(filename)
incsv = CSV.new(infile)

while row = incsv.shift
  # process each row here
end

You could in principle do all sorts of additional processing, e.g. UTF-8 cleanups. The nice thing about this approach is you handle the file on a line by line basis, so you don't need to load it all into memory or create an intermediate file.