parse blocks of text from text file using Python

Question

I am trying to parse some text files and need to extract blocks of text. Specifically, the lines that start with "1:" and 19 lines after the text. The "1:" does not start on the same row in each file and there is only one instance of "1:". I would prefer to save the block of text and export it to a separate file. In addition, I need to preserve the formatting of the text in the original file.

Needless to say I am new to Python. I generally work with R but these files are not really compatible with R and I have about 100 to process. Any information would be appreciated.

The code that I have so far is:

tmp = open(files[0],"r") 
lines = tmp.readlines()
tmp.close()

num = 0
a=0

for line in lines:
    num += 1    
    if "1:" in line:
      a = num 
      break

a = num is the line number for the block of text I want. I then want to save to another file the next 19 lines of code, but can't figure how how to do this. Any help would be appreciated.

Answer 1

Could be done in a one-liner...

open(files[0]).read().split('1:', 1)[1].split('\n')[:19]

or more readable

txt = open(files[0]).read()           # read the file into a big string
before, after = txt.split('1:', 1)    # split the file on the first "1:"
after_lines = after.split('\n')       # create lines from the after text
lines_to_save = after_lines[:19]      # grab the first 19 lines after "1:"

then join the lines with a newline (and add a newline to the end) before writing it to a new file:

out_text = "1:"                       # add back "1:"
out_text += "\n".join(lines_to_save)  # add all 19 lines with newlines between them
out_text += "\n"                      # add a newline at the end

open("outputfile.txt", "w").write(out_text)

to comply with best practice for reading and writing files you should also be using the with statement to ensure that the file handles are closed as soon as possible. You can create convenience functions for it:

def read_file(fname):
    "Returns contents of file with name `fname`."
    with open(fname) as fp:
         return fp.read()

def write_file(fname, txt):
    "Writes `txt` to a file named `fname`."
    with open(fname, 'w') as fp:
         fp.write(txt)

then you can replace the first line above with:

txt = read_file(files[0])

and the last line with:

write_file("outputfile.txt", out_text)