to change a text file containing multiline strings

Question

I have a text file consisting of multiline (hundreds of lines actually) strings. Each of the strings starts with '&' sign. I want to change my text file in a way that only the first 300 characters of each string remain in the new file. How I can do this by using python?

Answer 1

You can read a file and loop over the lines to do what you want. Strings are easily slicable in python to get the first 300 to write to another file.

file = open(path,"r")
lines = file.readlines()
newFile = open(newPath,"w")

for index, line in enumerate(lines):
    newLine = line[0:301]
    newFile.writelines([newLine])

Hope this is what you meant

Answer 2

You could do something like this:

# Open output file in append mode
with open('output.txt', 'a') as out_file:
    # Open input file in read mode
    with open("input.txt", "r") as in_file:
        for line in in_file:
            # Take first 300 characters from line
            # I believe this works even when line is < 300 characters
            new_line = line[0:300]

            # Write new line to output
            # (You might need to add '\n' for new lines)
            out_file.write(new_line)

            print(new_line)

Answer 3

You can use the string method split to split your lines, then you can use slices to keep only the 300 first characters of each split.

with open("oldFile.txt", "rt") as old_file, open("newFile.txt", "wt") as new_file:
    for line in old_file.read().split("&"):
        new_file.write("&{}\n".format(line[:300]))

This version preserves ends of line \n within your strings.

If you want to remove ends of line in each individual string, you can use replace:

with open("oldFile.txt", "rt") as old_file, open("newFile.txt", "wt") as new_file:
    for line in old_file.read().split("&"):
        new_file.write("&{}\n".format(line.replace("\n", "")[:300]))

Note that your new file will end with an empty line.

Another note is, depending on the size of your file, you may rather use a generator function version, instead of split which results in the whole file content being loaded in memory as a list of strings.