解析字符串模式 (Python)

Question

I have a file with following data:

<<row>>12|xyz|abc|2.34<</row>>
<<eof>>

The file may have several rows like this. I am trying to design a parser which will parse each row present in this file and return an array with all rows. What would be the best way of doing it? The code has to be written in python. Code should not take rows that do not start with <<row>> or should raise error.

=======> UPDATE <========

I just found that a particular <<row>> can span multiple lines. So my code and the code present below aren't working anymore. Can someone please suggest an efficient solution?

The data files can contain hundreds to several thousands of rows.

Answer 1

def parseFile(fileName):
  with open(fileName) as f:

    def parseLine(line):
      m = re.match(r'<<row>>(\d+)\|(\w+)\|(\w+)\|([\d\.]+)<</row>>$', line)
      if m:
        return m.groups()

    return [ values for values in (
      parseLine(line)
        for line in f
        if line.startswith('<<row>>')) if values ]

And? Am I different? ;-)

Answer 2

A simple way without regular expressions:

output = []
with open('input.txt', 'r') as f:
    for line in f:
        if line == '<<eof>>':
            break
        elif not line.startswith('<<row>>'):
            continue
        else:
            output.append(line.strip()[7:-8].split('|'))

This uses every line starting with <<row>> until a line contains only <<eof>>

Answer 3

A good practice is to test for unwanted cases and ignore them. Once you are sure that you have a compliant line, you process it. Note that the actual processing is not in an if statement. Without rows split across several lines, you need only two tests:

rows = list()
with open('newfile.txt') as file:
    for line in file.readlines():
        line = line.strip()
        if not line.startswith('<<row>>'):
            continue
        if not line[-8:] == '<</row>>':
            continue
        row = line[7:-8]
        rows.append(row)

With rows split across several lines, you need to save the previous line in some situations:

rows = list()
prev = None
with open('newfile.txt') as file:
    for line in file.readlines():
        line = line.strip()
        if not line.startswith('<<row>>') and prev is not None:
            line = prev + line
        if not line.startswith('<<row>>'):
            continue
        if not line[-8:] == '<</row>>':
            prev = line
            continue
        row = line[7:-8]
        rows.append(row)
        prev = None

If needed, you can split columns with: cols = row.split('|')