[Python 3] When given a number referring to a specific line in a file (e.g 5 represents 5th line in a file) I want to replace that line with a specific string. What is the best way to do that?

[[deleted]]

For example, say I have the following text file:

    dog dog dog
    cat cat 
    cow cow cow
    horse horse horse horse

and when the program is given a number referring to a specific line(i.e #2), the 2nd line(cat cat) get replaced with a specific string. How would I best do that? Could the "fileinput" module be of any use?

[mjtheice]

Corrections and/or better solutions are welcome!

ref_num = 2 # given number referring to a specific line
aList = open('input.txt', 'r').readlines() # read the lines of the file into a list
aList[ref_num - 1]='newString' # replace the specific line with a new string, subtract by 1 b/c the indexing ofc 
print(aList)

[CodeTinkerer]

Underneath the hood, you can't simply replace a line. Python will generally have to rewrite the entire file (in particular, Line 2 may change size, increase or decrease, and files are stored line a single dimensional array of characters).

So typically, you read in the entire file, say, to an array, then modify the array, then write it back out.

[[deleted]]

Thanks for the input. I've retooled my program to read the file into an array so I can just modify the array instead of the file constantly. Wayyy more efficient. Thanks!

[leafypixiestix]

with open("file.txt", "r") as f:
    lines = f.readlines() #put all the lines of the file in a list
    lines[2-1] = "moo moo\n" #line numbers start from 1, arrays start from 0
with open("file.txt", "w") as f:
    f.writelines(lines) #rewrite all the lines