Extracting line number of a txt file in Python

Question

I want to know if there is a way to find the line number of a line in a text file, using Python.

So, for example, if we have the file textfile.txt:

abc
deff
ghi
jkl
EOF

Then I'd like a function like func(FILE,TEXT), that would work in the following way:

Input:  func(textfile.txt, "ghi")
Output: 3

Why do you want the line number? What are you ultimately trying to do? — zelusp, Mar 23 '18 at 16:36
I have an output file with information. I want to find the line in which is written "Something and something else" and to copy that line and the following "n" lines to another file. — Mario Ibáñez, Mar 23 '18 at 16:39
`with open('my_file.txt') as f: for line_num, line in enumerate(f): ` that's a start — Chris_Rands, Mar 23 '18 at 16:40
It's not a definition, is the output I want to get after executing the function... — Mario Ibáñez, Mar 23 '18 at 16:43
You can't use python keywords as variable names. `def` is reserved for function definitions. — dROOOze, Mar 23 '18 at 16:44
It was just tje second line of the text file. I changed it to the third line so the examole is clearer. — Mario Ibáñez, Mar 23 '18 at 16:48

Joe Iddon · Answer 1 · 2018-03-23T16:59:04.367

1

You can do this in one line with next() called on a generator that uses the enumerate() function to get the index of the line.

So, as a function:

def func(f, t):
    return next(i for i, l in enumerate(open(f).read().split('\n'), 1) if l == t)

which would be called through something like:

func('textfile.txt', 'ghi') # --> 3

note: the enumerate(..., 1) trick was from @KeyurPotdar, so creds

edited Mar 23 '18 at 16:59

answered Mar 23 '18 at 16:49

Joe Iddon

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Instead of `+1` at the end, you could also do `enumerate(..., 1)`. – Keyur Potdar Mar 23 '18 at 16:55
@KeyurPotdar Nice one :) – Joe Iddon Mar 23 '18 at 16:58

score 0 · Answer 2 · answered Mar 23 '18 at 16:49

0

file_name="test.txt"
phrase="line3"
with open(file_name) as f:
    for i, line in enumerate(f, 1):
        if phrase in line:
           print(i)

answered Mar 23 '18 at 16:49

Joao

37
9

score 0 · Answer 3 · answered Mar 23 '18 at 16:53

Maybe that's waht you're looking for:

def fn(fname, substr):
    with open(fname) as text:
        for n, line in enumerate(text, start=1):
            if line.startswith(substr):
                return n
    return -1

Don't be afraid to study the python docs on string functions, file I/O, context managers etc. You'll be shocked on how easy things like this are to accomplish.

score 0 · Answer 4 · answered Mar 23 '18 at 17:25

0

This might help.

def LineNumber(phrase,textObjext):
    for i, line in enumerate(textObjext, 1):
        if phrase in line:
           return i

file_name="test.txt"
phrase="deff"
text = open(file_name,'r')
line = LineNumber(phrase,text)
print(line)

answered Mar 23 '18 at 17:25

Hayat

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Extracting line number of a txt file in Python

4 Answers4