how to skip certain line in text file and keep reading the next line in python?

Question

I have been searching for this answer but did not quite get it.

I have a text file that looks like this

who are you????
who are you man?
who are you!!!!
who are you? man or woman?

I want to skip the line with man in it and print

who are you????
who are you!!!!

My code so far

f = open("test.txt", "r")
word = "man"
for line in f:
    if word in line:
        f.next()
    else:
        print line

This prints the first line only

who are you????

How should I troubleshoot this problem?

Thank you for your help.

Why call `f.next()`. Simply `if word not in line: print line`. You don't have to do anything if `word` is in the `line`. — Willem Van Onsem, Mar 27 '17 at 07:34
Printing the first line is not the only problem. Your code also raises an exception. — Eric Duminil, Mar 27 '17 at 08:16

Giordano · Accepted Answer · 2017-03-27T07:59:44.680

5

It's not necessary to add an if else statement in for loop, so you can modify your code in this way:

f = open("test.txt", "r")
word = "man"
for line in f:
    if not word in line:
        print line

Furthermore, the issue in your code is that you are using f.next() directly in a for loop used to scan the file. This is the reason because when the line contains "man" word, your code skips two lines.

If you want preserve if else statement because this is only an example of a more complex problem, you can use the following code:

f = open("test.txt", "r")
word = "man"
for line in f:
    if word in line:
        continue
    else:
        print line

Using continue, you skip one loop's iteration, and so you can reach your goal.

As Alex Fung suggests, would be better use with, so your code would become like this:

with open("test.txt", "r") as test_file:
    for line in test_file:
        if "man" not in line:
            print line

edited Mar 27 '17 at 07:59

answered Mar 27 '17 at 07:35

Giordano

5,422
3
33
49

2

An explanation of *why* the `next()` is the problem would be useful. – SiHa Mar 27 '17 at 07:36
1

I suggest using `with` to make sure the file object is closed at the end, – Alex Fung Mar 27 '17 at 07:37
1

Interesting to see how our answers almost converged after multiple edits. You get my vote! – Eric Duminil Mar 27 '17 at 08:08
Thank you for the help. You are right, this is just an example for more complex problem. Indeed `continue` is what I've been searching for. – Fang Mar 27 '17 at 08:16
@Fang: Don't forget to use `print line,` You'll end up with too many newlines otherwise – Eric Duminil Mar 27 '17 at 08:16
Glad to help you! Anyway, please consider to use with instead open. – Giordano Mar 27 '17 at 08:18

score 5 · Answer 2 · edited May 23 '17 at 12:02

Problem

With your current code, when the current line contains "man" :

you don't print anything. That's correct.
you also skip the next line. That's your problem!
f.next() is already called implicitely by for line in f: at each iteration. So you actually call f.next() twice when "man" is found.
If the last line of your file contains a "man", Python will throw an exception because there's no next line.

You might have been looking for continue, which would also achieve the desired result but would be complex and unneeded. Note that it's called next in Perl and Ruby, which might be confusing.

Example

who are you????            # <- This line gets printed, because there's no "man" in it
who are you man?           # word in line is True. Don't print anything. And skip next line
who are you!!!!            # Line is skipped because of f.next()
who are you? man or woman? # word in line is True. Don't print anything. 
                           #   Try to skip next line, but there's no next line anymore.
                           #   The script raises an exception : StopIteration

Correct code

Don't forget to close the file. You can do this automatically with with :

word = "man"
with open("test.txt") as f:
    for line in f:
        if not word in line:
            print line, # <- Note the comma to avoid double newlines

Thank you for your explanation. I've been searching for `continue` and the answer from you and the next answer help me to understand it. — Fang, Mar 27 '17 at 08:18

score 1 · Answer 3 · answered Mar 27 '17 at 07:36

1

How about

f = open("test.txt", "r")
word = "man"
for line in f:
    if not word in line:
        print line

answered Mar 27 '17 at 07:36

Tore

51
5

how to skip certain line in text file and keep reading the next line in python?

3 Answers3

Problem

Example

Correct code