How To Change A String To Lower Case In Python

A way to easily sort a list of strings equally is to change all the strings to lower case, but how do you change strings to lower case in Python?

To change a string to lower case in Python use the string method .lower() like this: my_string.lower(). Similarly, to change a string to upper case in Python use the string method .upper() like this: my_string.upper().

An example of how this works is as follows:

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>>> my_string = "I am AlL OveR ThE PlaCe"
>>> my_string.lower()
'i am all over the place'

As can be seen from the example above all characters in my_string that are capital letters are changed to lower case characters.

Where Is .lower() Really Handy?

When handling strings in your Python script there will likely come a time when you need to sort your strings into some alphabetical order. However, when running the sort function on your list you might discover some strange behaviour:

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>>> my_list = ["cat", "ant", "Zebra"]
>>> my_list.sort()
>>> print(my_list)
['Zebra', 'ant', 'cat']

Zebra before ant? What dictionary is this?? Why does Python sort strings like this? Doesn’t a come before Z?

It does, but Python handles characters within a string according to its ordinal number in the Unicode character table. Each character has a designated number and depending on the rank of that number it helps to know whether a > Z.

But if you’re looking for more consistency in how strings are sorted then there’s a handy parameter in the .sort() method and sorted() function to help: the key parameter.

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>>> my_list = ["cat", "ant", "Zebra"]
>>> my_list.sort(key=str.lower)
>>> print(my_list)
['ant', 'cat', 'Zebra']

As you can see from the same list Zebra is now at the back of the list, and we didn’t have to change it to get it to work. The power of using the key parameter and setting this using the str.lower() method is that each string is handled as lower case and sorted accordingly.

What is a lower case character?

In the Unicode character set there are some characters that have a lower case counterpart. With the English language characters A through to Z are the capital letter equivalents to a through to z.

To find out which characters are considered to have lower case letter counterparts in the Unicode series a small snippet of code can help provide this insight.

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>>> for i in range(1114112):
...     a_chr = chr(i)
...     lower_chr = a_chr.lower()
...     if len(lower_chr) == 1:
...         ord_lower = ord(lower_chr)
...         if i != ord_lower:
...             print(f"{i}. upper = {a_chr} lower = {lower_chr}")

Before exploring the output let’s look at the above code and dissect it a little to understand what is going on.

First, the for-loop uses the range() function to loop through all the characters in the Unicode series. Currently, in Python there are 1,114,112 characters in this series starting at 0 and going to 1,114,111.

Next, we convert the ordinal number to its corresponding character using the chr() function then apply the lower() string method. The first if statement is a check that the length of the lower case character created is a single character so that when this is inserted into the ord() function it will not cause any errors.

After getting the ordinal number of the lower case character the second if statement checks if the original characters ordinal number is the same as the lower case ordinal number. If it isn’t then the output is put to the console.

Here’s the output from the above for-loop: