Check if value exists in file

Question

I am trying to read the following file line by line and check if a value exists in the file. What I am trying currently is not working. What am I doing wrong?

If the value exists I do nothing. If it does not then I write it to the file.

file.txt:

123
345
234
556
654
654

Code:

file = open("file.txt", "a+")
lines = file.readlines()
value = '345'
if value in lines:
    print('val ready exists in file')
else:
    # write to file
    file.write(value)

Answer 1

There are two problems here:

• .readlines() returns lines with \n not trimmed, so your check will not work properly.
• a+ mode opens a file with position set to the end of the file. So your readlines() currently returns an empty list!

Here is a direct fixed version of your code, also adding context manager to auto-close the file

value = '345'
with open("file.txt", "a+") as file:
    file.seek(0) # set position to start of file
    lines = file.read().splitlines() # now we won't have those newlines
    if value in lines:
        print('val ready exists in file')
    else:
        # write to file
        file.write(value + "\n") # in append mode writes will always go to the end, so no need to seek() here

However, I agree with @RoadRunner that better is to just use r+ mode; then you don't need the seek(0). But the cleanest is just to split out your read and write phases completely, so you don't run into file position problems.

Answer 2

I would consider several changes.

1: Use with to automatically close the file.
2: Use strip() to remove leading or trailing stuff, like \n
3: Use a break for the loop.
4: Add \n in the write part.

value = "345"
with open("file.txt", "a+") as file:
    file.seek(0)
    for line in file.readlines():
        if line.strip("\n") == value:
            print('val ready exists in file')
            break
    else:
        # write to file
        file.write(f"\n{value}")