Numbers, Strings, and Booleans

Python’s three everyday data types: numbers for math, strings for text, booleans for decisions. Use type(x) to check any value’s type.

Numbers

• int: whole numbers (-3, 42); never overflow.
• float: numbers with a decimal point (3.5, 2.0); binary approximations.
• 2 is an int; 2.0 is a float, even though same quantity.
• A variable names a value; = is assignment (not math equals).

Arithmetic operators

• + - * standard; / true division, always a float (6 / 2 is 3.0).
• // floor division: drops the fraction, rounds down toward negative infinity (-7 // 3 is -3).
• % remainder/modulo; n % 2 == 0 tests even.
• ** power; 9 ** 0.5 is the square root (3.0).
• Int-only operands keep int result (except /); any float makes the result float.

Float pitfalls

• Floats are approximate: 0.1 + 0.2 is 0.30000000000000004, so 0.1 + 0.2 == 0.3 is False.
• Never compare floats with ==; check closeness: abs(a - 0.3) < 1e-9.
• round(value, places) for display; round(2.5) is 2 — banker’s rounding (ties go to nearest even).

Strings

• Text in quotes (single or double); "" is empty.
• len(s) = character count.
• Indexing s[i] starts at 0; -1 is last. Out-of-range raises IndexError.
• Slicing s[start:stop] includes start, excludes stop; omit a side for begin/end.
• f-strings: f"{name} is {age}" drops values into text.
• Methods: .upper() .lower() .strip() .replace(a,b) .split(sep); .split() returns a list.
• "ell" in "hello" tests substring, returns a boolean.
• Immutable: can’t change a character in place; operations build a new string.
• Building a string by repeated + in a loop is O(n^2); collect pieces and "".join(parts) is O(n).

Booleans

• Exactly True or False (capitalized).
• Comparisons return booleans: == != < > <= >=. = assigns, == compares.
• String comparison is alphabetical by character code; case matters.
• Logic: and (both true), or (at least one), not (flip).
• Truthiness: falsy values are 0/0.0, "", [], None; almost everything else is truthy. So if items: means “non-empty.”

Index map for “hello”

  h    e    l    l    o
  0    1    2    3    4     <- positive
 -5   -4   -3   -2   -1     <- negative