python - Cancel last line iteration on a file -

i need iterate on file, stop iteration on condition , continue parse file @ same line function (that may change can't add content in previous function).

an example file (file.txt) :

1 2 3 4 5 6 7 8 9 

function try :

def parse1(file, stop): # 1st parsing function (main function doing)     line in file:             if line.strip() == stop:             # stop parsing on condition                 break             else:             # parse line (just print example)                 print(line)  def parse2(file): # 2nd parsing function (will own functions or external functions)     line in file:         # parse line (just print example)         print(line) 

result in terminal:

>>> file = open("file.txt")  >>> parse1(file, "4") 1 2 3  >>> parse2(file) 5 6 7 8 9 

my problem "4" line skipped 1st function when condition.

how can avoid : found solution cancel last iteration or go line.

the file.tell() function don't work for on file.

i tried while + file.readline() very slower for loop on file (and want parse files millions of lines).

is there elegant solution keeping use of for loop ?

in python3, 'for line in file' construct represented iterator internally. definition, value produced iterator cannot 'put back' later use (http://www.diveintopython3.net/iterators.html).

to desired behaviour, need function chains 2 iterators, such chain function provided itertools module. in stop condition of parse1, return last line file iterator:

import itertools  def parse1(file,stop): # 1st parsing function     line in file:        # stop parsing on condition         if line.strip() == stop:             return itertools.chain([line],file) # important line         else:         # parse line (just print example)             print('parse1: '+line) 

the chain statement connects 2 iterators. first iterator contains 1 element: line want process again. second iterator remaining part of file. first iterator runs out of values, second iterator accessed.

you don't need change parse2. clarity, modified print statement:

def parse2(file): # 2nd parsing function line in file:     # parse line (just print example)     print('parse2: '+line) 

then, can call parse1 , parse2 in functional manner:

with open('testfile','r') infile:    parse2(parse1(infile,'4')) 

the output of above line is:

parse1: 1 parse1: 2 parse1: 3 parse2: 4 parse2: 5 parse2: 6 parse2: 7 parse2: 8 parse2: 9 

note, how value '4' produced parse2 function.