Stripping Whitespace in Python

Stripping Whitespace in Python

Extra whitespace can be confusing in your programs. To programmers ‘python’ and ‘python ‘ look pretty much the same. But to a program, they are two different strings. Python detects the extra space in ‘python ‘ and considers it significant unless you tell it otherwise.

It’s important to think about whitespace, because often you’ll want to compare two strings to determine whether they are the same. For example, one important instance might involve checking people’s usernames when they log in to a website. Extra whitespace can be confusing in much simpler situations as well. Fortunately, Python makes it easy to eliminate extraneous whitespace from data that people enter.

Python can look for extra whitespace on the right and left sides of a string. To ensure that no whitespace exists at the right end of a string, use the rstrip() method.

>>> favorite_language = 'python '
>>> favorite_language
'python '
>>> favorite_language.rstrip()
'python'
>>> favorite_language
'python '

The value stored in favorite_language at u contains extra whitespace at the end of the string. When you ask Python for this value in a terminal session, you can see the space at the end of the value v. When the rstrip() method acts on the variable favorite_language at w, this extra space is removed. However, it is only removed temporarily. If you ask for the value
of favorite_language again, you can see that the string looks the same as when it was entered, including the extra whitespace x.

To remove the whitespace from the string permanently, you have to store the stripped value back into the variable:

>>> favorite_language = 'python '
>>> favorite_language = favorite_language.rstrip()
>>> favorite_language
'python'

To remove the whitespace from the string, you strip the whitespace from the right side of the string and then store that value back in the original variable, as shown at u. Changing a variable’s value and then storing the new value back in the original variable is done often in programming. This is how a variable’s value can change as a program is executed or in response to user input.

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You can also strip whitespace from the left side of a string using the lstrip() method or strip whitespace from both sides at once using strip():

>>> favorite_language = ' python '
>>> favorite_language.rstrip()
' python'
>>> favorite_language.lstrip()
'python '
>>> favorite_language.strip()
'python'

In this example, we start with a value that has whitespace at the beginning and the end u. We then remove the extra space from the right side at v, from the left side at w, and from both sides at x. Experimenting with these stripping functions can help you become familiar with manipulating strings. In the real world, these stripping functions are used most often to clean up user input before it’s stored in a program.

In this next article, we will examine how you can avoid syntax errors with strings.

This article is an excerpt from A Hands-On, Project-Based Introduction to Programming by Eric Matthes

Reproduced with permission from No Starch Press

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