Reading text file - for loop gives unexpected output

Question

I am learning how to read txt files and find something in them. The example below outputs the entire txt file. I am trying to get it to print out "found it" when it finds the word "thanks" in the txt file. Where am I wrong?

This is the txt file I am reading:

this is a
demo file
for exercises
thanks
bye

This is the code I have written:

f = open("demo.txt", "r")
print(f.readline())
print(f.readline())

for word in f:
    print(word)
    if word == "thanks":
        print("found it")

This is the output:

this is a

demo file

for exercises

thanks

bye



Process finished with exit code 0

Your line is thanks\n meaning it contains newline. Change == into in. — zipa
– zipa, Commented Sep 19, 2019 at 8:44
On everyline there is a special character at the end of the line for new line (\n). So you can use ` if "thanks" in word: ` or strip the word using word.strip() to remove trailing space and new line character. — Syed Jafer
– Syed Jafer, Commented Sep 19, 2019 at 8:53
Thank you @syedjafer, this is the only solution that worked! — nephele
– nephele, Commented Sep 19, 2019 at 9:36

Kostas Charitidis · Accepted Answer · 2019-09-19 08:57:51Z

1

with open("demo.txt", "r") as f:
    for word in f:
        print(word)
        if "thanks" in word:
            print("found it")
            break

edited Sep 19, 2019 at 8:57

answered Sep 19, 2019 at 8:50

Kostas Charitidis

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3 Comments

bruno desthuilliers Over a year ago

file.readlines() loads the whole file in memory, which is a potential issue if file is huge, and is not necessary anyway, files are iterable so the preferred solution is, plain simply, for line in f: (nb also notice that those are lines, not "words")

Kostas Charitidis Over a year ago

@brunodesthuilliers you are right about huge files but I think he is testing on small files considering his example. However I changed it to what you said. Also I used word because I worked on his example so I used same variable name.

bruno desthuilliers Over a year ago

the fact the OP is testing on a small file doesn't mean he shouldn't learn to do things the proper way. 20 years ago no one in his own mind would ever consider reading a whole file in memorry at once, and while we now have huge ram space, available meory can still be an issue and break a production server (think about what happens when you have hundreds of processes running parallel, each acting as if it was alone on the server).

bruno desthuilliers · Accepted Answer · 2019-09-19 09:03:59Z

0

Files are iterable, so if you want to read a text file line by line, all you have to do is iterate over it. Also, you must ensure the file is closed after use - which is easily done using the with statement. And, finally, lines ends with the (system-dependant) newline marker, which you may want to strip for comparisons.

IOW, your code should look something like:

# nb: "r" (read) is the default
with open("path/to/your/file") as f:
    for line in f:
        # removes the ending newline marker 
        line = line.rstrip("\n") 
        print(line)
        # given your spec 'when it finds the word "thanks"'
        # I assume that it doesn't matter if there's
        # something else in the line, so we test for
        # containment.
        if "thanks" in line:
            print("found it")

answered Sep 19, 2019 at 9:03

bruno desthuilliers

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2 Comments

nephele Over a year ago

Thank you everyone for your help!

bruno desthuilliers Over a year ago

@nephele you're welcome - but the way to thank someone for a useful answer is to upvote the answer (and eventually accept the one you deem as best) ;-)

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