9.7 Common String Methods

Every method below returns a new string (or list/tuple) — none modify the original, consistent with 9.1 Introduction to Strings.

Case Conversion

Method	Example	Result
`upper()`	`"Hello".upper()`	`'HELLO'`
`lower()`	`"Hello".lower()`	`'hello'`
`capitalize()`	`"hello".capitalize()`	`'Hello'`
`title()`	`"hello world".title()`	`'Hello World'`
`swapcase()`	`"Hello".swapcase()`	`'hELLO'`
`casefold()`	`"STRASSE".casefold()`	`'strasse'`

Searching Methods

Method	Example	Result
`find()`	`"Hello World".find("World")`	`6`
`rfind()`	`"Hello World".rfind("o")`	`7`
`index()`	`"Hello World".index("World")`	`6`
`count()`	`"banana".count("a")`	`3`
`startswith()`	`"file.txt".startswith("file")`	`True`
`endswith()`	`"file.txt".endswith(".txt")`	`True`

find() returns -1 if not found; index() raises ValueError. Use find() when “not found” is a normal case, index() when the substring’s absence signals a bug that should surface loudly.

Validation Methods

The is* family answers yes/no questions about a string’s content — used for quick input validation before parsing or type conversion (see 8.1 Introduction to Type Conversion).

Method	Example	Result
`isalpha()`	`"abc123".isalpha()`	`False`
`isdigit()`	`"123".isdigit()`	`True`
`isalnum()`	`"abc123".isalnum()`	`True`
`islower()` / `isupper()`	`"hello".islower()`	`True`
`isspace()`	`" ".isspace()`	`True`
`isidentifier()`	`"var_1".isidentifier()`	`True`

Modification Methods

Method	Example	Result
`replace()`	`"Hello World".replace("World","Python")`	`'Hello Python'`
`strip()`	`" hi ".strip()`	`'hi'`
`lstrip()` / `rstrip()`	`" hi ".lstrip()`	`'hi '`
`removeprefix()`	`"unit_test.py".removeprefix("unit_")`	`'test.py'`
`removesuffix()`	`"image.png".removesuffix(".png")`	`'image'`

Splitting and Joining Methods

Method	Example	Result
`split()`	`"a,b,,c".split(",")`	`['a','b','','c']`
`rsplit()`	`"a,b,,c".rsplit(",",1)`	`['a,b,','c']`
`splitlines()`	`"line1\nline2".splitlines()`	`['line1','line2']`
`partition()`	`"k=v=x".partition("=")`	`('k','=','v=x')`
`join()`	`",".join(["a","b","c"])`	`'a,b,c'`

join() does the reverse of split() — it merges a list of strings into one, inserting the given separator between each piece. It’s the efficient way to build a string from parts (see 9.11 Memory and Performance).

Alignment Methods

Method	Example	Result
`center()`	`"hi".center(6,"*")`	`'hi'`
`ljust()`	`"hi".ljust(6,"*")`	`'hi****'`
`rjust()`	`"hi".rjust(6,"*")`	`'****hi'`
`zfill()`	`"7".zfill(3)`	`'007'`

Encoding and Translation Methods

encode() converts a Unicode str into raw bytes using a given character encoding, usually UTF-8 — required whenever text needs to leave Python as a byte stream, such as writing to a file or sending over a network (see 5.7 Binary Data Types).

>>> "café".encode("utf-8")
b'caf\xc3\xa9'

maketrans() builds a character-mapping table; translate() applies it in one fast pass — useful for bulk character substitution without chaining many replace() calls.

>>> table = str.maketrans("aeiou", "12345")
>>> "hello world".translate(table)
'h2ll4 w4rld'

Quick Interview Answer

“The str method set breaks down into a handful of purpose-based groups: case conversion (upper/lower/title), searching (find/index/count/startswith), validation (the is* family), modification (replace/strip), splitting/joining (split/join — mirror images of each other), alignment (center/ljust/zfill), and encoding (encode/translate). The one recurring theme is that every single one returns a new string rather than mutating the original, since str is immutable.”

Common Mistakes

Using index() when “not found” is a normal, expected outcome — it raises ValueError instead of returning a sentinel, so find() is usually the safer default for optional matches.
Calling strip() expecting it to remove whitespace from the middle of a string — it only trims from the two ends; use replace(" ", "") or a regex for interior whitespace.
Forgetting split() with no arguments splits on any run of whitespace and drops empty strings, while split(",") with an explicit separator does not — "a,,b".split(",") keeps the empty string between the commas.