python：从.txt读取并判断txt文件中的新字符串

Question

I am trying to read/write txt file. I want to read from txt file, and then write new string to the next line if the string not existed in the original file.

f = open('./video/1.txt', 'a')
f.write('123' + '\n')
f.close()

with open('./video/1.txt') as o:
    lines=o.readlines()

lines=set(lines)

if '123' in lines:
    print("Existed")
else:print("Not Existed")

The question is : i have '123' is the 1.txt file. And the result is "Not Existed" And I tried to use

for line in lines:
    print(line + '\n')

to print out the set variable "lines", it print out 123 at one line. I am confused now. What causes the "NOT EXISTED"? Thank you.

Update: I uploaded the screenshot. Still confused.

Answer 1

Your if statement is checking if an element of the set lines is ' 123 '

example:

lines = ['hello there', 'hello 123 there', '123']
if '123' in lines:
    print("Existed")
else:print("Not Existed")

output:

Existed

example:

lines = ['hello there', 'hello 123 there']
if '123' in lines:
    print("Existed")
else:print("Not Existed")

output:

Not Existed

You could do something like this:

lines = ['hello there', 'hello 123 there']
for line in lines:
    if '123' in line:
        print('Existed')

If you are reading from file, you could be getting a trailing carriage-return character in you line, this is \\n . This could also cause your if '123' in lines to go wrong, even when the line is only ' 123 ', reality is the line is actually ' 123\\n '.

example:

ms = '123\n'
print(ms)
mb = ms.encode()
print(mb)

output:

123

b'123\n'

in this sample output, as well as in your screenshot, you can actually see the extra carriage return in the output. Hence your string is not ' 123 ' your string is ' 123\\n '

Answer 2

Thanks for @Edwin van Mierlo 's answer, i got this now. This is a '\\n' after 123. So if i use '123\\n' instead of 123, it shows existed.

if '123\n' in lines:
    print("Existed")
else:print("Not Existed")

Answer 3

Late answer but anyway

Instead of keep using lines as a variable I will use lines1

In [3]: with open('1.txt') as o:
   ...:     lines = o.readlines()
   ...:

In [4]: lines1 = set(lines)

If you do a print on lines1 , this is the result you have.

In [5]: print(lines1)
{'123\n'}

Where does this take you?

One possible solution is to use a regex to look for 123 in your set, if you wish to continue with your set.

The regex \\d+ will look for digit.

In [6]: import re

In [7]: if re.search(r'\d+', str(lines1)):
   ...:     print('yes')
   ...: else:
   ...:     print('no')
   ...:
yes