如何将ljust（）转换应用于字符串列表的每个元素？

Question

I am learning python and i'm trying to add 2 ** in front of each line of some example text. However when i call ljust(2,'*') on every element it doesn't change the original string. I want to replace the new element with this old string but how?

This is what i' ve tried. First i tried it with a regular for loop but that didn't work. Then i came across a question where list comprehensions are explained Perform a string operation for every element in a Python list so i tried that.

This is what i have now

example_string = '''hello there how are you doing!
i am doig well thank you
lets get to work!!! '''


def modify_example_string():
    global example_string
    new_string_list = [element.ljust(2,'*') for element in example_string.split('\n')]
    example_string = '\n'.join(new_string_list)
    print(new_string_list)

modify_example_string()

This should return a new list with all the ljust transformations but it doesn't so i would like to know the proper way of solving this.

Answer 1

It seems like you misunderstood what ljust(2, '*') is doing. It does not add two * to the beginning of the string but will pad the string with * to a total length of 2. All your lines are longer, so it does nothing.

Instead, just use "**" + line to add the stars to the lines.

def modify_example_string():
    global example_string
    example_string = "\n".join("**" + line for line in example_string.splitlines())

Also, instead of using global I'd recommend using parameters and return values:

def prepend_stars(s):
    return "\n".join("**" + line for line in s.splitlines())

example_string = prepend_stars(example_string)

Answer 2

The ljust method doesn't do what you expect. The documentation says:

Return the string left justified in a string of length width. Padding is done using the specified fillchar (default is an ASCII space). The original string is returned if width is less than or equal to len(s) .

Your list comprehension is correct.

One solution might be to use format . A good tutorial here .

Example code with format :

example_string = '''hello there how are you doing!
i am doig well thank you
lets get to work!!! '''


def modify_example_string(example_string, ch, n):
    new_string_list = ["{} {}".format(ch * n, element)
                       for element in example_string.split('\n')]
    example_string = '\n'.join(new_string_list)
    return example_string

print(modify_example_string(example_string, "*", 2))
# ** hello there how are you doing!
# ** i am doig well thank you
# ** lets get to work!!!

Answer 3

I would split your string using the splitlines() method and use a for loop to iterate through the lines and build a new, concatenated string for output.

Something like this:

example_string = '''hello there how are you doing!
i am doig well thank you
lets get to work!!! '''


def modify_example_string(input_string):
    new_string_list = ''
    for line in input_string.splitlines():
        new_string_list += f'**{line}\n'
    return new_string_list

print(modify_example_string(example_string))