How do I split a huge text file in python

Question

I have a huge text file (~1GB) and sadly the text editor I use won't read such a large file. However, if I can just split it into two or three parts I'll be fine, so, as an exercise I wanted to write a program in python to do it.

What I think I want the program to do is to find the size of a file, divide that number into parts, and for each part, read up to that point in chunks, writing to a filename.nnn output file, then read up-to the next line-break and write that, then close the output file, etc. Obviously the last output file just copies to the end of the input file.

Can you help me with the key filesystem related parts: filesize, reading and writing in chunks and reading to a line-break?

I'll be writing this code test-first, so there's no need to give me a complete answer, unless its a one-liner ;-)

Answer 1

While Ryan Ginstrom's answer is correct, it does take longer than it should (as he has already noted). Here's a way to circumvent the multiple calls to itertools.islice by successively iterating over the open file descriptor:

def splitfile(infilepath, chunksize):
    fname, ext = infilepath.rsplit('.',1)
    i = 0
    written = False
    with open(infilepath) as infile:
        while True:
            outfilepath = "{}{}.{}".format(fname, i, ext)
            with open(outfilepath, 'w') as outfile:
                for line in (infile.readline() for _ in range(chunksize)):
                    outfile.write(line)
                written = bool(line)
            if not written:
                break
            i += 1

Answer 2

I've written the program and it seems to work fine. So thanks to Kamil Kisiel for getting me started.
(Note that FileSizeParts() is a function not shown here)
Later I may get round to doing a version that does a binary read to see if its any quicker.

def Split(inputFile,numParts,outputName):
    fileSize=os.stat(inputFile).st_size
    parts=FileSizeParts(fileSize,numParts)
    openInputFile = open(inputFile, 'r')
    outPart=1
    for part in parts:
        if openInputFile.tell()<fileSize:
            fullOutputName=outputName+os.extsep+str(outPart)
            outPart+=1
            openOutputFile=open(fullOutputName,'w')
            openOutputFile.writelines(openInputFile.readlines(part))
            openOutputFile.close()
    openInputFile.close()
    return outPart-1

Answer 3

This generator method is a (slow) way to get a slice of lines without blowing up your memory.

import itertools

def slicefile(filename, start, end):
    lines = open(filename)
    return itertools.islice(lines, start, end)

out = open("/blah.txt", "w")
for line in slicefile("/python27/readme.txt", 10, 15):
    out.write(line)

Answer 4

linux has a split command

split -l 100000 file.txt

would split into files of equal 100,000 line size

Answer 5

don't forget seek() and mmap() for random access to files.

def getSomeChunk(filename, start, len):
    fobj = open(filename, 'r+b')
    m = mmap.mmap(fobj.fileno(), 0)
    return m[start:start+len]

Answer 6

usage - split.py filename splitsizeinkb

import os
import sys

def getfilesize(filename):
   with open(filename,"rb") as fr:
       fr.seek(0,2) # move to end of the file
       size=fr.tell()
       print("getfilesize: size: %s" % size)
       return fr.tell()

def splitfile(filename, splitsize):
   # Open original file in read only mode
   if not os.path.isfile(filename):
       print("No such file as: \"%s\"" % filename)
       return

   filesize=getfilesize(filename)
   with open(filename,"rb") as fr:
    counter=1
    orginalfilename = filename.split(".")
    readlimit = 5000 #read 5kb at a time
    n_splits = filesize//splitsize
    print("splitfile: No of splits required: %s" % str(n_splits))
    for i in range(n_splits+1):
        chunks_count = int(splitsize)//int(readlimit)
        data_5kb = fr.read(readlimit) # read
        # Create split files
        print("chunks_count: %d" % chunks_count)
        with open(orginalfilename[0]+"_{id}.".format(id=str(counter))+orginalfilename[1],"ab") as fw:
            fw.seek(0) 
            fw.truncate()# truncate original if present
            while data_5kb:                
                fw.write(data_5kb)
                if chunks_count:
                    chunks_count-=1
                    data_5kb = fr.read(readlimit)
                else: break            
        counter+=1 

if __name__ == "__main__":
   if len(sys.argv) < 3: print("Filename or splitsize not provided: Usage:     filesplit.py filename splitsizeinkb ")
   else:
       filesize = int(sys.argv[2]) * 1000 #make into kb
       filename = sys.argv[1]
       splitfile(filename, filesize)