Strings

Strings are sequences of characters. They’re immutable — every “modification” returns a new string.

Indexing and slicing

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The slice s[a:b:c] is start (inclusive), stop (exclusive), step. Negative indices count from the right. This works on every Python sequence — lists, tuples, bytes — so internalize it once.

The string methods you’ll use daily

"  hi  ".strip()             # 'hi'  — strip whitespace
"FOO".lower()                 # 'foo'
"foo bar".split()             # ['foo', 'bar']
"foo,bar".split(",")          # ['foo', 'bar']
",".join(["a", "b", "c"])     # 'a,b,c'
"hello".replace("l", "L")     # 'heLLo'
"hello".startswith("he")      # True
"hello".endswith("o")         # True
"Hi {}".format("there")       # 'Hi there'   (old style)
f"Hi {name}"                  # f-string     (preferred)

split() and join() are inverses you’ll combine constantly when cleaning text.

f-strings — beyond {name}

f-strings are far more capable than just f"{name}". The format spec after : lets you control width, precision, alignment, and more.

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f"{x=}" is a lifesaver during debugging — it auto-includes the variable name and its value.

Raw strings — for paths and regex

path = r"C:\Users\Aarav\data.csv"   # 'r' = treat backslashes literally
pattern = r"\d{4}-\d{2}-\d{2}"      # regex for YYYY-MM-DD

Without the r, \n, \t, etc. would be interpreted as escape characters. Always use raw strings for regex patterns.

Strings are immutable

s = "hello"
s[0] = "H"        # TypeError — strings can't be modified in-place
s = "H" + s[1:]   # ✓ creates a new string

This matters for performance. If you’re concatenating many strings in a loop, every iteration creates a new object — slow. Prefer "".join(parts):

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Quick check

Next

Strings live in lists, lists live in dicts. The next lesson covers lists — sorting, slicing, copying, and the methods that bite people most often.