通过特定的定界符对字符串进行操作并写入文本文件

Question

I am writing a function that takes a file updates.txt as an input. The file looks like this :

---------------------------------------------------
MRT Header
    Timestamp: 1453939200(2016-01-28 01:00:00)
    Type: 16(BGP4MP)
    Subtype: 4(BGP4MP_MESSAGE_AS4)
    Length: 39
BGP4MP_MESSAGE_AS4
    Peer AS Number: 37989
    Local AS Number: 12654
    Interface Index: 0
    Address Family: 1(IPv4)
    Peer IP Address: 203.123.48.6
    Local IP Address: 193.0.4.28
BGP Message
    Marker: -- ignored --
    Length: 19
    Type: 4(KEEPALIVE)
---------------------------------------------------
MRT Header
    Timestamp: 1453939200(2016-01-28 01:00:00)
    Type: 16(BGP4MP)
    Subtype: 4(BGP4MP_MESSAGE_AS4)
    Length: 118
BGP4MP_MESSAGE_AS4
    Peer AS Number: 1836
    Local AS Number: 12654
    Interface Index: 0
    Address Family: 1(IPv4)
    Peer IP Address: 146.228.1.3
    Local IP Address: 193.0.4.28
BGP Message
    Marker: -- ignored --
    Length: 98
    Type: 2(UPDATE)
    Withdrawn Routes Length: 0
    Total Path Attribute Length: 71
    Path Attribute Flags/Type/Length: 0x40/1/1
        ORIGIN: 0(IGP)
    Path Attribute Flags/Type/Length: 0x40/2/42
        AS_PATH
            Path Segment Type: 2(AS_SEQUENCE)
            Path Segment Length: 10
            Path Segment Value: 1836 174 6453 37282 37088 37629 37629 37629 37629 37629
    Path Attribute Flags/Type/Length: 0x40/3/4
        NEXT_HOP: 146.228.1.3
    Path Attribute Flags/Type/Length: 0xc0/8/12
        COMMUNITY: 1836:110 1836:6000 1836:6031
    NLRI: 154.65.7.0/24
---------------------------------------------------

The file is a sequence of 'blocks'. Each block is enclosed between the line of dashes

---------------------------------------------------
# Block (n)
---------------------------------------------------
# Block (n+1)
---------------------------------------------------
# Block (n+2) , etc

I would like to read the whole file, block-by-block, and return a text file only containing the lines of the fields : Timestamp, Peer AS number, Local AS number, Peer IP Address, Local IP Address.

The resulting .txt file should look something like this :

---------------------------------------------------
MRT Header
    Timestamp: 1453939200(2016-01-28 01:00:00)
BGP4MP_MESSAGE_AS4
    Peer AS Number: 37989
    Local AS Number: 12654
    Peer IP Address: 203.123.48.6
    Local IP Address: 193.0.4.28
---------------------------------------------------
MRT Header
    Timestamp: 1453939200(2016-01-28 01:00:00)
BGP4MP_MESSAGE_AS4
    Peer AS Number: 1836
    Local AS Number: 12654
    Peer IP Address: 203.123.48.6
    Local IP Address: 193.0.4.28
---------------------------------------------------

Ideally, I want to overwrite the updates.txt with the new text file not to waste space, and save it in a new directory "Parsed Updates".

I know it is minimal as I am stuck with the line of dashes delimiter, but my code looks like this :

import sys
import os

def parser(filename):
    info = open(filename, 'r+')
    info.read()

    #Here comes the string manipulation code
    #info.split( '---------------------------------------------------')

    info.close()
    print 'The file has been parsed successfully !!'

def main():
    parser('updates.txt')


if __name__=='__main__':
    main()

Answer 1

>>> with open('results.txt', 'wb') as r:
...     with open('updates.txt', 'rb') as u:
...         for line in u.readlines():
...             if '-'*51 in line:
...                 r.write(line)
...             else:
...                 if any(field in line for field in ['Timestamp', 'Peer AS Number', 'Local AS Number', 'Peer IP Address', 'Local IP Address','MRTHeader']):
...                     r.write(line)

your results file will look this:

$ cat results.txt
---------------------------------------------------
MRT Header
    Timestamp: 1453939200(2016-01-28 01:00:00)
    Peer AS Number: 37989
    Local AS Number: 12654
    Peer IP Address: 203.123.48.6
    Local IP Address: 193.0.4.28
---------------------------------------------------
MRT Header
    Timestamp: 1453939200(2016-01-28 01:00:00)
    Peer AS Number: 1836
    Local AS Number: 12654
    Peer IP Address: 146.228.1.3
    Local IP Address: 193.0.4.28
---------------------------------------------------

Answer 2

In this specific case, you don't even need to break the blocks into separate parts before parsing. You could just go line by line and check for matches against the types of info you want.

out_lines = []
regexes = [
    r'^-+$',
    r'^MRT HEADER\s*$',
    r'^\s*Timestamp:.*$',
    r'^BGP4MP_MESSAGE_AS4\s*$',
    r'^\s*Peer AS Number:.*$',
    r'^\s*Local AS Number:.*$',
    r'^\s*Peer IP Address:.*$',
    r'^\s*Local IP Address:.*$',
]
with open('file.txt', 'r') as f:
    for line in f:
        for regex in regexes:
            if re.match(regex, line):
                out_lines.append(line)
                break

with open('file.txt', 'w') as f:
     f.write('\n'.join(out_lines))