How do I lowercase a string in Python?

Sailing the .lower() sea

The backbone of string lowering

The fundamental method in Python to convert a string to lowercase is the method str.lower(). Here is what you need to remember:

• Immutability: Strings in Python are immutable which means the .lower() method doesn't modify the original string but returns a new one.
• Reusable: Assign the result of the .lower() call to a variable if you need to use the lowercase string afterwards.

Version-specific nuances

The handling of strings differs between Python 2 and Python 3:

• Python 2: It differentiates between byte strings and Unicode strings. Strings need to be prefixed with u to indicate they are Unicode.
• Unicode Management: Unicode strings need to be decoded before converting to lowercase: unicode_string.decode('utf-8').lower().

Note: Python 3 handles all strings as Unicode, making these steps unnecessary.

Case-insensitive comparison

For more robust case-insensitive comparisons, Python 3 provides:

• .casefold(): A more aggressive version of .lower(), used for case-insensitive comparisons.

german_string = "Groß"
print(german_string.lower())       # Output: "groß"
print(german_string.casefold())    # Output: "gross" (groß? More like no-ß, am I right?)


Use .casefold() to handle special characters like the German ß correctly.

Clean coding with lower()

Robust, maintainable code

When working with .lower(), aim for understandable and maintainable code. Be explicit:

original_string = "HELLO, WORLD!"
lowercase_string = original_string.lower()
print(lowercase_string)  # Outputs: "hello, world!" (can you hear the echo?)


Veer away from string module

Avoid string.ascii_lowercase and string.ascii_uppercase for changing cases manually. They are merely constants and do not provide any method for case conversion!

Python 2 Unicode literals

For Unicode strings, Python 2 requires the u prefix:

unicode_string = u"Hello, World!"
print(unicode_string.lower())  # Outputs: "hello, world!" (deja vu, anyone?)


Deep Dive: Unicode

Consistent text with Unicode

Python's built-in support for Unicode ensures the consistency of text representation. Understanding this can help you:

• Python 2 Unicode Literals: For 16-bit codes, use the \u escape sequence.
• PEP 263: Defines how to handle Unicode in Python source files.

Encoding nuances

Encoding and decoding are crucial aspects when working with byte strings, especially in Python 2:

• To manipulate byte strings, convert them to Unicode first.
• After manipulation, convert Unicode strings back to bytes if necessary with .encode('utf-8').