str has ~47 methods. Knowing the right one — partition instead of a regex, casefold instead of lower, removeprefix instead of strip — makes text code shorter and correct.

Case conversion

Method	Does	Example → result
`lower()` / `upper()`	All lower / upper case	`'PyThon'.lower()` → `'python'`
`casefold()`	Aggressive lower-casing for caseless comparison	`'Straße'.casefold()` → `'strasse'`
`title()`	First letter of every word upper	`'hello world'.title()` → `'Hello World'`
`capitalize()`	First character upper, rest lower	`'hELLO'.capitalize()` → `'Hello'`
`swapcase()`	Invert each letter's case	`'Ab'.swapcase()` → `'aB'`

Python

print("Straße".lower(), "Straße".casefold())          # straße strasse
print("STRASSE".casefold() == "Straße".casefold())    # True
print("STRASSE".lower() == "Straße".lower())          # False
print("they're bill's".title())                      # They'Re Bill'S
print(" ".join(w.capitalize() for w in "they're bill's".split()))   # They're Bill's

Searching

Method	Returns	If not found
`s.find(sub[, start[, end]])`	Lowest index	`-1`
`s.rfind(sub)`	Highest index	`-1`
`s.index(sub)` / `s.rindex(sub)`	Like `find` / `rfind`	raises `ValueError`
`s.count(sub)`	Number of non-overlapping occurrences	`0`
`s.startswith(p)` / `s.endswith(p)`	`bool` — `p` may be a tuple of options	`False`
`sub in s`	`bool` — the most readable test	`False`

Python

path = "reports/2026/q1.final.csv"
print(path.find("/"), path.rfind("/"), path.find("zip"))   # 7 12 -1
print(path[path.rfind(".") + 1:])                         # csv
print(path.endswith((".csv", ".tsv")))                    # True
print("aaaa".count("aa"), "2026" in path)                 # 2 True
print("banana".find("a", 2), "banana".find("a", 2, 3))    # 3 -1

Splitting family

Method	Behaviour
`s.split()`	Split on runs of any whitespace, dropping empty strings and leading/trailing space
`s.split(sep)`	Split on an exact separator — keeps empty strings between adjacent separators
`s.split(sep, maxsplit)`	At most `maxsplit` splits, from the left
`s.rsplit(sep, maxsplit)`	At most `maxsplit` splits, from the right
`s.splitlines()`	Split on `\n`, `\r\n`, `\r`… without a trailing empty item
`s.partition(sep)`	Always a 3-tuple `(before, sep, after)` — at the first `sep`
`s.rpartition(sep)`	Same, at the last `sep`

Python

print("  a   b  c ".split())               # ['a', 'b', 'c']
print("a,,b,".split(","))                  # ['a', '', 'b', '']
print("k=v=w".split("=", 1))               # ['k', 'v=w']
print("a.b.c.txt".rsplit(".", 1))          # ['a.b.c', 'txt']
print("one\ntwo\r\nthree\n".splitlines())  # ['one', 'two', 'three']
print("one\ntwo\n".split("\n"))            # ['one', 'two', '']
print("user@example.com".partition("@"))   # ('user', '@', 'example.com')
print("no-at-sign".partition("@"))         # ('no-at-sign', '', '')
key, _, value = "timeout=30".partition("=")
print(key, value)                          # timeout 30

Trimming & prefixes

Python

print("  hi  ".strip() + "|")               # hi|
print("xxhixyx".strip("xy"))                # hi   ← a SET of characters, not a substring
print("www.python.org".lstrip("w."))        # python.org
print("test_login.py".removesuffix(".py"))  # test_login
print("mississippi".rstrip("ip"))           # mississ
print("mississippi".removesuffix("ippi"))   # mississ
print("reader.txt".rstrip(".txt"))          # reader   ← looks right…
print("rat.txt".rstrip(".txt"))             # ra       ← …but eats the 't' of 'rat'
print("rat.txt".removesuffix(".txt"))       # rat

Padding & alignment

Python

print("[" + "hi".center(8, "*") + "]")      # [***hi***]
print("[" + "hi".ljust(6) + "]")            # [hi    ]
print("[" + "hi".rjust(6, ".") + "]")       # [....hi]
print("42".zfill(5), "-42".zfill(5))        # 00042 -0042
print("a\tbc\td".expandtabs(4))             # a   bc  d
print("-" * 10)                             # ----------

Replacing & translating

Python

s = "a-b-c-d"
print(s.replace("-", "+"))                  # a+b+c+d
print(s.replace("-", "", 2))                # abc-d   ← only the first 2
table = str.maketrans({"a": "4", "e": "3", "o": "0"})
print("hello world".translate(table))       # h3ll0 w0rld
strip_vowels = str.maketrans("", "", "aeiou")   # 3rd arg: characters to delete
print("programming".translate(strip_vowels))    # prgrmmng
print("abc".translate(str.maketrans("abc", "xyz")))   # xyz

translate does many single-character replacements (and deletions) in one pass — much faster than chaining replace calls.

Character-class predicates

Method	True when every character is…	`'²'`	`'½'`	`'٣'` (Arabic 3)
`isdecimal()`	a decimal digit usable by `int()`	False	False	True
`isdigit()`	a digit, including superscripts	True	False	True
`isnumeric()`	any numeric character, incl. fractions	True	True	True

Python

for s in ["123", "²", "½", "٣", "-5", "3.5", ""]:
    print(repr(s), s.isdecimal(), s.isdigit(), s.isnumeric())

Output

'123' True True True
'²' False True True
'½' False False True
'٣' True True True
'-5' False False False
'3.5' False False False
'' False False False

Python

print("abc123".isalnum(), "abc".isalpha(), "ABC".isupper())    # True True True
print(" \t\n".isspace(), "".isspace())                          # True False
print("my_var".isidentifier(), "2fast".isidentifier())          # True False
print(int("٣") + 1)                                             # 4

Comparison, repetition & membership

Python

print("apple" < "banana", "Zebra" < "apple")     # True True   ← 'Z' (90) < 'a' (97)
print(sorted(["b", "C", "a"]))                   # ['C', 'a', 'b']
print(sorted(["b", "C", "a"], key=str.casefold)) # ['a', 'b', 'C']
print("10" < "9", int("10") < int("9"))          # True False
print("ab" * 3, "-" * 0 == "")                   # ababab True
print("ell" in "hello", "" in "hello")           # True True

Building strings efficiently

Python

nums = [3, 1, 4]
print(", ".join(map(str, nums)))      # 3, 1, 4
print("".join(reversed("stressed")))  # desserts
try:
    ", ".join(nums)
except TypeError as e:
    print(type(e).__name__)           # TypeError

Interview questions

What's the difference between find and index?

Why does 'rat.txt'.rstrip('.txt') return 'ra'?

lower() vs casefold()?

How do you reverse the words of a sentence?