Reading specific lines only

Question

I\'m using a for loop to read a file, but I only want to read specific lines, say line #26 and #30. Is there any built-in feature to achieve this?

Thanks

Answer 1

If you don't mind importing then fileinput does exactly what you need (this is you can read the line number of the current line)

Answer 2

How about this:

>>> with open('a', 'r') as fin: lines = fin.readlines()
>>> for i, line in enumerate(lines):
      if i > 30: break
      if i == 26: dox()
      if i == 30: doy()

Answer 3

Fairly quick and to the point.

To print certain lines in a text file. Create a "lines2print" list and then just print when the enumeration is "in" the lines2print list. To get rid of extra '\n' use line.strip() or line.strip('\n'). I just like "list comprehension" and try to use when I can. I like the "with" method to read text files in order to prevent leaving a file open for any reason.

lines2print = [26,30] # can be a big list and order doesn't matter.

with open("filepath", 'r') as fp:
    [print(x.strip()) for ei,x in enumerate(fp) if ei in lines2print]

or if list is small just type in list as a list into the comprehension.

with open("filepath", 'r') as fp:
    [print(x.strip()) for ei,x in enumerate(fp) if ei in [26,30]]

Answer 4

To print line# 3,

line_number = 3

with open(filename,"r") as file:
current_line = 1
for line in file:
    if current_line == line_number:
        print(file.readline())
        break
    current_line += 1

Original author: Frank Hofmann

Answer 5

If your large text file file is strictly well-structured (meaning every line has the same length l), you could use for n-th line

with open(file) as f:
    f.seek(n*l)
    line = f.readline() 
    last_pos = f.tell()

Disclaimer This does only work for files with the same length!

Answer 6

f = open(filename, 'r')
totalLines = len(f.readlines())
f.close()
f = open(filename, 'r')

lineno = 1
while lineno < totalLines:
    line = f.readline()

    if lineno == 26:
        doLine26Commmand(line)

    elif lineno == 30:
        doLine30Commmand(line)

    lineno += 1
f.close()