Remove specific lines from a large text file in python

Question

I have several large text text files that all have the same structure and I want to delete the first 3 lines and then remove illegal characters from the 4th line. I don't want to have to read the entire dataset and then modify as each file is over 100MB with over 4 million records.

Range   150.0dB -64.9dBm
Mobile unit 1   Base    -17.19968    145.40369  999.8
Fixed unit  2   Mobile  -17.20180    145.29514  533.0
Latitude    Longitude   Rx(dB)  Best unit
-17.06694    145.23158  -050.5  2
-17.06695    145.23297  -044.1  2

So lines 1,2 and 3 should be deleted and in line 4, "Rx(db)" should be just "Rx" and "Best Unit" be changed to "Best_Unit". Then I can use my other scripts to geocode the data.

I can't use commandline programs like grep ( as in this question ) as the first 3 lines are not all the same -the numbers (such as 150.0dB, -64*) will change in each file so you have to just delete the whole of lines 1-3 and then grep or similar can do the search-replace on line 4.

Thanks guys,

=== EDIT new pythonic way to handle larger files from @heltonbiker. Error.

import os, re
##infile = arcpy.GetParameter(0)
##chunk_size = arcpy.GetParameter(1) # number of records in each dataset

infile='trc_emerald.txt'
fc= open(infile)
Name = infile[:infile.rfind('.')]
outfile = Name+'_db.txt'

line4 = fc.readlines(100)[3]
line4 = re.sub('\([^\)].*?\)', '', line4)
line4 = re.sub('Best(\s.*?)', 'Best_', line4)
newfilestring = ''.join(line4 + [line for line in fc.readlines[4:]])
fc.close()
newfile = open(outfile, 'w')
newfile.write(newfilestring)
newfile.close()

del lines
del outfile
del Name
#return chunk_size, fl
#arcpy.SetParameterAsText(2, fl)
print "Completed"

Traceback (most recent call last): File "P:\\2012\\Job_044_DM_Radio_Propogation\\Working\\FinalPropogation\\TRC_Emerald\\working\\clean_file_1c.py", line 13, in newfilestring = ''.join(line4 + [line for line in fc.readlines[4:]]) TypeError: 'builtin_function_or_method' object is unsubscriptable

Answer 1

As wim said in the comments, sed is the right tool for this. The following command should do what you want:

sed -i -e '4 s/(dB)//' -e '4 s/Best Unit/Best_Unit/' -e '1,3 d' yourfile.whatever

To explain the command a little:

-i executes the command in place, that is it writes the output back into the input file

-e execute a command

'4 s/(dB)//' on line 4 , subsitute '' for '(dB)'

'4 s/Best Unit/Best_Unit/' same as above, except different find and replace strings

'1,3 d' from line 1 to line 3 (inclusive) delete the entire line

sed is a really powerful tool, which can do much more than just this, well worth learning.

Answer 2

Just try it for each file. 100 MB per file is not that big, and as you can see, the code to just make an attempt is not time-consuming to write.

with open('file.txt') as f:
  lines = f.readlines()
lines[:] = lines[3:]
lines[0] = lines[0].replace('Rx(db)', 'Rx')
lines[0] = lines[0].replace('Best Unit', 'Best_Unit')
with open('output.txt', 'w') as f:
  f.write('\n'.join(lines))

Answer 3

You can use file.readlines() with an aditional argument in order to read just a few first lines from the file. From the docs:

f.readlines() returns a list containing all the lines of data in the file. If given an optional parameter sizehint, it reads that many bytes from the file and enough more to complete a line, and returns the lines from that. This is often used to allow efficient reading of a large file by lines, but without having to load the entire file in memory. Only complete lines will be returned.

Then the most robust way to manipulate generic strings are Regular Expressions. In Python, this means the re module with, for example, the re.sub() function.

My suggestion, which should be adapted to suit your needs:

import re

f = open('somefile.txt')
line4 = f.readlines(100)[3]
line4 = re.sub('\([^\)].*?\)', '', line4)
line4 = re.sub('Best(\s.*?)', 'Best_', line4)
newfilestring = ''.join(line4 + [line for line in f.readlines[4:]])
f.close()
newfile = open('someotherfile.txt', 'w')
newfile.write(newfilestring)
newfile.close()