Table of Contents

## Problem Formulation

- Given a Python string
`s`

with length`k<=n`

. - How to fill the string with
`n-k`

empty spaces on the left so that the new string has length`n`

?

Here are some examples:

INPUT: 'a', n = 2OUTPUT: 'a 'INPUT: 'hi', n = 4OUTPUT: 'hi 'INPUT: 'hi', n = 2OUTPUT: 'hi'INPUT: 'finxter', n = 10OUTPUT: 'finxter '

## Method 1: str.ljust()

The built-in Python `str.ljust(length, fillchar)`

method returns a left-justified string by appending fill characters up to a certain length. Per default, `fillchar`

is set to the empty space, so `str.ljust(n)`

creates a new string by appending empty spaces up to length `n`

.

Here’s the method applied to our four examples:

>>> 'a'.ljust(2) 'a ' >>> 'hi'.ljust(4) 'hi ' >>> 'hi'.ljust(2) 'hi' >>> 'finxter'.ljust(10) 'finxter '

You can learn more about the string method `ljust()`

here:

## Method 2: Literal String Interpolation Left Justify

Literal string interpolation is a relatively new Python feature that introduces f-strings that are string literals prefixed by the letter `'f'`

or `'F'`

and with special functionality. For example, you can left-justify a string using `'{variable:<n}'`

where `n`

is the desired length.

Here’s how this works out for our examples: ??

# Example 1 >>> s = 'a' >>> f'{s:<2}' 'a ' # Example 2 >>> s = 'hi' >>> f'{s:<4}' 'hi ' # Example 3 >>> f'{s:<2}' 'hi' # Example 4 >>> s = 'finxter' >>> f'{s:<10}' 'finxter '

## Method 3: Literal String Interpolation Space Padding

F-strings also provide you a simple means to use padding on a string with the expression `'{variable:n}'`

where `n`

is the desired length. Python then fills up the string with empty spaces.

Here’s how this works out for our examples: ??

# Example 1 >>> s = 'a' >>> f'{s:2}' 'a ' # Example 2 >>> s = 'hi' >>> f'{s:4}' 'hi ' # Example 3 >>> f'{s:2}' 'hi' # Example 4 >>> s = 'finxter' >>> f'{s:10}' 'finxter '

I would say this is even prettier than ** Method 2** due to its conciseness and clarity. 🙂

If you need some refreshing on F-Strings, feel free to check out our detailed guide on the Finxter blog.

## Method 4: String Concatenation

A simple way without f-strings to fill a given string with empty spaces to obtain a left-justified string is to use string concatenation arithmetic via the overloaded `+`

and `*`

operators. For example, the expression `s + ' ' * n-k`

appends `n-k`

empty spaces to the string `s`

.

Here’s the code on our examples:

# Example 1 >>> s = 'a' >>> n = 2 >>> s + ' ' * (n - len(s)) 'a ' # Example 2 >>> s = 'hi' >>> n = 4 >>> s + ' ' * (n - len(s)) 'hi ' # Example 3 >>> n = 2 >>> s + ' ' * (n - len(s)) 'hi' # Example 4 >>> s = 'finxter' >>> n = 10 >>> s + ' ' * (n - len(s)) 'finxter '

## Summary

There are four ways to fill a string with `n-k`

empty spaces on the left so that the new string has length `n=10`

.

- Left justification –
`s.ljust(10)`

- F-strings with left justification –
`f'{s:<10}'`

- F-strings with padding –
`f'{s:10}'`

- String arithmetic –
`s + ' ' * (n - len(s))`

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