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How To Capitalize First Letter Of Every Word In Python (One-Liner)

How To Capitalize First Letter Of Every Word In Python (One-Liner)

How do you capitalize the first letter of each word in a string using Python?

To capitalize the first letter of every word in Python using one line of code enter the following: " ".join([x.capitalize() for x in my_string.split()]).

Here is an example demonstrating how the code works in the Python REPL:

>>> my_string = "How long is a piece of string?"
>>> " ".join([x.capitalize() for x in my_string.split()])
'How Long Is A Piece Of String?'

As you can see from the above example the output produces a string with each character capitalized.

Capitalizing the first character for each word is popular when your string is needed as a heading. However, using the str.capitalize() method may not achieve what you want especially if your headings contain acronyms. I present an alternative solution if this happens to be your case later below.

Let’s explore how this one-liner works. To explore this code in a little more detail I’ll start from the right-hand side and work my way left, starting with the expression my_string.split():

How To Split A String Into Words

How can you split a string into words? You can easily do this by using the built-in string method .split(). The split(sep, maxsplit) function takes two parameters sep which is the string delimiter to break up your string and maxsplit as the maximum number of splits to perform.

If sep is blank by default Python will separate the string based on consecutive whitespace characters while also trimming and leading or trailing whitespace.

Here’s an example of what this means:

>>> my_string = "  Spaced    out    man.  "
>>> my_string.split()
['Spaced', 'out', 'man.']

Notice how the split() function stripped out the leading and trailing spaces and it also merged those consecutive whitespace characters into one and just output the words (with punctuation).

If you don’t want the split() function to do this, but still want to break your string by a single space character then insert the space character as the parameter, like so:

>>> my_string = "  Spaced    out    man.  "
>>> my_string.split(" ")
['', '', '', 'Spaced', '', '', 'out', '', 'man.', '', '']

As you can see from this example the splits are now performed on each space character.

Finally, the maxsplit parameter is blank or -1 it will perform the split on the entire string, otherwise it will only split according to the number of times specified for this parameter.

So the first element with the one-liner code is to split each word into a list. By using the split() string method you can easily break your string into a list of words.

Here’s how the one-liner works by looking at the split() expression first:

>>> my_split = "How long is a piece of string?"
>>> my_split.split()
['How', 'long', 'is', 'a', 'piece', 'of', 'string?']

As you can see from the above display the sentence is now a list broken up into words.

If you wanted to capitalize the first letter at the start of each sentence in your string you could instead insert "." as your parameter to the split method.

>>> my_string = "How long. Is a piece of string."
>>> my_string.split(".")
['How long', ' Is a piece of string', '']

How To Use A List Comprehension

A list comprehension is a single line that allows users to loop through the contents of a list to create a new list.

The second part of the one-liner code is the for loop that is inside a list comprehension. The basic aspect of this code looks like this [x for x in my_string.split()].

The for x in my_string.split() part of this code just loops through each element in the list which has each word of my_string for its elements and assigns each element to the variable x.

The operation before the for loop part instructs Python on what to do with each element. At the moment nothing is happening with each element.

If I wanted to apply the upper() string method to each element then I could write the following code:

>>> my_string = "How long is a piece of string?"
>>> [x.upper() for x in my_string.split()]
['HOW', 'LONG', 'IS', 'A', 'PIECE', 'OF', 'STRING?']

As you can see from the above code now I have a new list of uppercase characters from my original string.

Capitalize Each Word

As the split method has broken the original string into a list of words and the list comprehension has enabled the ability to perform an operation on each word the next step is to use the built in string method str.capitalize() which, as the name suggests, capitalizes each string passed in as the parameter.

Here’s how this method works:

>>> str.capitalize("HOW LONG IS A PIECE OF STRING?")
'How long is a piece of string?'
>>> str.capitalize("123abc")
'123abc'

As you can see this method capitalizes the first character and makes every other character in the string lower case. It does not search for the first alphabetic character.

How Does str.capitalize() Work?

The str.capitalize() method takes a string as its sole parameter and breaks the string into two parts: the first character and every other character.

The first part containing the first character of the string is passed into the .upper() string method. The second part containing every other character of the original string is passed into the .lower() string method.

To break the string into two parts Python has a handy slice operator which easily enables extracting characters from a string.

To extract the first character from a string using the Python slice operator, simply use my_string[0]. To extract every other character after the first character use my_string[1:].

Here’s what this would look like when used on a simple string:

>>> my_string = "fIrSt"
>>> my_string[0]
'f'
>>> my_string[1:]
'IrSt'
>>> my_string[0].upper() + my_string[1:].lower()
'First'

As you can see from the above examples you can extract the first character by using the index reference, as this is permitted in Python with the string data type, you then can use the slice operator to get every other character in your string thereafter, and finally, you can apply the .upper() string method on the index reference and stitch this together with the remaining part of the string to get the capitalized string you want.

Make First Character Capitalized But Keep Other Characters As Is

The reason why I wanted to go to this level of detail is that as shown above the str.capitalize(s) method forces the first character to be capitalized and every other character to be lower case.

What if you wanted to keep every other character as they originally were?

For example what if your string contained an acronym like USA or UK? Using str.capitalize() will not be the best use case for this scenario.

>>> my_string = "Cost of living in USA versus UK"
>>> " ".join([x.capitalize() for x in my_string.split()])
'Cost Of Living In Usa Versus Uk'

As you can see from the above application of my code it doesn’t quite meet my expectations as some words need to maintain their original case.

By forcing the second part of every other character to be lower case using the .lower() string method, you may opt to instead leave the remaining characters as is. In other words, want if you just wanted the first character capitalized, but wanted to leave the other characters as they were?

You could use the slice operation as performed above, but instead of forcing the second part containing all other characters of the string to be lower case using the other built-in function lower() you would just remove the .lower() method.

Here’s an example where you can leave the case of the other strings as is:

>>> my_string = "hELLo"
>>> my_string[0].upper() + my_string[1:]
'HELLo'

Whatever your final choice on this operation you insert it before the for loop inside the list comprehension and this will then perform the required capitalization on each element in the split list.

Here’s how the output appears up to this point:

>>> my_string = "How long is a piece of string?"
>>> [x.capitalize() for x in my_string.split()]
['How', 'Long', 'Is', 'A', 'Piece', 'Of', 'String?']

As you can see you now have a list of strings with the first character capitalized.

How To Join A List Of Strings Together

The final step is to join the list of capitalized strings together into one string and this is done with another built-in string function .join().

Using the join() method can seem a little strange at first, as some other languages would have two parameters where the user enters the list and then the string to use to join each element in the list by, but in Python the format of the join() string method is to operate the join() on the string and to have the list inside its parameters.

It therefore looks like this:

"string".join(my_list)

Therefore, with our current working example, if I was to capture the list comprehension into a variable it would look something like this:

>>> my_string = "How long is a piece of string?"
>>> my_caps_list = [x.capitalize() for x in my_string.split()]
>>> " ".join(my_caps_list)
'How Long Is A Piece Of String?'

As you can see the join() method is quite simple to use provided you know which elements go where: string outside, list inside.

Summary

To capitalize each word in a string using Python’s powerful features on one line of code use the following: " ".join([x.capitalize() for x in my_string.split()]) where my_string refers to the variable you’re initiating with the original sentence string.

As this approach capitalizes the first character of each word and forces every other character to be lower case there is another approach that capitalizes the first character but keeps every other character in each word as it originally is, and the code for this is:

" ".join([x[0].upper() + x[1:] for x in my_string.split()])