Problem Formulation
- Given a Python string
s
with lengthk<=n
. - How to fill the string with
n-k
empty spaces on the left so that the new string has lengthn
?
Here are some examples:
INPUT: 'a', n = 2 OUTPUT: 'a ' INPUT: 'hi', n = 4 OUTPUT: 'hi ' INPUT: 'hi', n = 2 OUTPUT: 'hi' INPUT: 'finxter', n = 10 OUTPUT: '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: ?β?
# 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: ?β?
# 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. π
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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