Btukfyl

I have a script which runs through lines of input to find the occurrence of an ID string while keeping track of the linenumber.
Then it runs backwards up the input to trace parentID/childID relationships. The script accepts either a logfile using a '-f' flag as an argument or the contents of stdin from a pipe.
The logfile as input portion works just fine, but reading from stdin seems not to work.

For the sake of reasonable clarity I've included the portion of the script that this concerns, but don't expect to be able to run it. It's just to show you sorta whats going on (anyone who works in financial services around FIX protocol would recognize a few things):

import os

import sys

import linecache



from types import *

from ____ import FixMessage   # custom message class that is used throughout



# Feel free to ignore all the getArgs and validation crap

def getArgs():

    import argparse

    parser = argparse.ArgumentParser(

               description='Get amendment history.')

    parser.add_argument('-f', '--file',

               help="input logfile.'")

    args = parser.parse_args()



    return validateArgs(args)





def validateArgs(args):

    try:

        if sys.stdin.isatty():

            if args.file:

                assert os.path.isfile(args.file.strip('n')), 

                    'File "{0}" does not exist'.format(args.file)

                args.file = open(args.file, 'r')

        else:

            args.file = sys.stdin

        assert args.file, 

            "Please either include a file with '-f' or pipe some text in"

    except AssertionError as err:

        print err

        exit(1)



    return args    





defGetMessageTrail(logfile, orderId):

    # some input validation

    if isinstance(logfile, StringType):

        try: logfile = open(logfile, 'r')

        except IOError as err: exit(1)

    elif not isinstance(logfile, FileType):

        raise TypeError(

              'Expected FileType and got {0}'.format(type(logfile)))



    linenum  = 0



    # This retrieves the message containing the orderID as well as the linenum

    for line in logfile:

        linenum += 1

        if orderId in line:

            # FixMessage is a custom class that is treated here like

            # a dictionary with some metadata

            # Missing dict keys return 'None'

            # .isvalid is bool results of some text validation

            # .direction is either incoming or outgoing

            # thats all you really need to know

            msg = FixMessage(line)

            if msg.isvalid and msg.direction == 'Incoming':

                yield msg

                break



    # If there is a message parentID, it would be in msg['41']

    if msg['41']:

        messages = findParentMessages(logfile, startline=linenum, msg['41'])

        for msg in messages: yield msg







def findParentMessages(logfile, startline, targetId):

    # Some more input validation

    assert isinstance(logfile, FileType)

    assert isinstance(startline, IntType)

    assert isinstance(targetId, StringType)



    # should just make a integer decrementing generator,

    # but this is fine for the example

    for linenum in range(startline)[::-1]:

        # *** This is where the question lies... ***

        # print(logfile.name) # returns "<stdin>"

        line = linecache.getline(logfile.name, linenum)

        if 'Incoming' in line and '11=' + targetId in line:

            msg = FixMessage(line)

            yield msg

            if msg['41']: findParentMessages(logfile, linenum, msg['41'])

            else: break





def main():

    log = getArgs().file

    trail = getMessageTrail(log, 'ORDER123')





if __name__ == '__main__': main()

The question is, how does linecache.getline work when it comes to reading stdin as a file? is it different than how it would work if given a regular filename?

linecache.getline() accepts a file name, not a file object. It is not designed to work that way as filename is passed to calls like open() and os.stat().

For reference: https://github.com/python/cpython/blob/2.6/Lib/linecache.py