S. Lott: Handling Irregular File Formats

This is a common issue. We have a file which was printed for human consumption. Consequently, it has many different kinds of lines.

These are the two kinds of lines of interest:

9002962684/9/16Mobility, Data Mining and PrivacyExpired

900295204 4/1/16 Pro .NET Best Practices
Expired

The first is a single physical line.  It has four data elements. The second is two physical lines. The first has three data elements.

There are a number of other noise lines in the file which must be filtered out.

The first "solution" pitched to me could be summarized with this:

Move "Expired" on a line by itself to the previous line

That was part of the email subject line. The body of the email was some whining about regular expressions. Which I mostly ignored. Multiline regular expressions are their own kind of challenge.

We (should) all know this: https://blog.codinghorror.com/regular-expressions-now-you-have-two-problems/

Let's do this without regular expressions. There are two things we need to know. One is buffering, and the other is the best way to split each line. It turns out that there are spaces as well as tabs, and can can, by splitting on tabs, make a lot of progress.

Instead of the good approach, I'll pick the other approach that doesn't involve splitting on tabs.

Here's the simulated file, with data lightly redacted.

sample_text = '''
"Your eBooks"

Show 200



Page: 1



Order #DateTitleFormatsStatusDownload
-------
xxx3156059/30/16R for Cloud ComputingAvailable



xxx3047906/21/16Java XML and JSONAvailable
xxx3047906/21/16Accelerated DOM Scripting with Ajax, APIs, and LibrariesAvailable

xxx291633 2/28/16 Practical Google Analytics and Google Tag Manager for Developers
Expired
'''

It's not perfectly obvious (because of line wrapping) but there are three examples of the "all-complete-in-one-line" records. There's one example of the "two-lines" record.

Rather than mess with the file, we'll build a file-like object with our sample data.

import io
file_like_object = io.StringIO(sample_text)

I like this because it lets me write proper unit test cases.

The file has four kinds of lines:

• Complete Records
• Record Headers (without Available/Expired)
• Record Trailers (only Available/Expired)
• Noise

We'll create some decision rules for the two obvious kinds of file lines: complete records and trailers. We can deduce the headers based on a simple adjacency rule: they precede a trailer. The fourth kind of lines are those which are possible headers but are not immediately prior to a trailer.

def complete(words):
    return len(words) > 3 and words[-1] in ('Available', 'Expired')

def trailer(words):
    return len(words) == 1 and words[0] in ('Available', 'Expired')    

We can spot these two kinds of lines easily. The other kinds require a Buffered Generator.

def emit_clean(source):
    header = None
    for line in (line.strip() for line in source):
        words = [w.strip() for w in line.split()]
        if len(words) == 0: continue
        if complete(words):
            yield(line)
            header = None
        elif trailer(words) and header:
            yield(header + '\t\t' + line)
            header = None
        else:
            # Possible header
            # print('??', line)
            header = line

The Buffered Generator is a way to implement a "look ahead one item" (LA1) algorithm. We do this by buffering rows. When we get to the next row we can use the buffered row and the current row to implement the look-ahead logic.

The actual implementation uses a look-behind buffer, header.

The (line.strip() for line in source) generator expression strips away leading and trailing spaces. This gets rid of the newline characters at the end of each input line.

The default behavior of split() is to split on whitespace. In this case, it will create a number of words for complete records or header records, and a single word for a trailer record. If we had split on tab characters, some of this logic would be simplified.

That's left as an exercise for the reader.

If the len(words) is zero, the line is blank.

If the line matches the complete() function, we can yield it as one of the iterable results of the generator function. We also clear out the look-behind buffer, header.

If the line is a trailer and we have a buffered look-behind line, this is the two-physical-line case. We can assemble a complete record and emit it.

Otherwise, we don't know what the line is. It's a possible header line, so we'll save it for later examination.

This algorithm involves no regular expressions.

With Regular Expressions

An alternative would use three regular expressions to match the three kinds of lines.

import re
all_one_pat = re.compile("(.*)\t(.*)\t(.*)\t\t((?:Available)|(?:Expired))")
header_pat = re.compile("(.*)\t(.*)\t(.*)")
trailer_pat = re.compile("((?:Available)|(?:Expired))")
The essential processing involves seeing which of the regular expressions match the line at hand. If it's all-in-one, this is good. We can yield the groups of meaningful data. If it's a header, we can save the groups. If it's a trailer, we can combine header and trailer groups and yield the composite.