Python Revision Tour — Notes

A program is made of tokens: keywords (if, while, True), identifiers (names you choose: letters, digits and _ but not starting with a digit; case-sensitive), literals (25, 3.75, "Agra"), operators and punctuators.

Types: int, float, complex, bool, str, list, tuple, dict, None. Use type(x) to check.

Mutable (can change in place): list, dict. Immutable: numbers, bool, str, tuple.

t = (4, 6, 8)
t[0] = 5

raises TypeError: 'tuple' object does not support item assignment.

Arithmetic: / always gives a float; // floors towards minus infinity; % takes the sign of the divisor; ** is right-associative.

print(-17 // 5, -17 % 5)
print(2 ** 3 ** 2)
print(10 / 4, 10 // 4)

Output:

-4 3
512
2.5 2

Logical operators return one of their operands: 0 or 7 gives 7; not > and > or in priority. Identity (is) compares objects; equality (==) compares values. Conversion: int() truncates, str() makes text, float() makes a decimal.

Selection: if … elif … else — exactly one block runs. Iteration: for goes through a sequence; range(start, stop, step) stops before stop. while repeats while a condition is True; the body must update the variable.

Jumps: break leaves the innermost loop; continue skips to the next pass; a loop else runs only when there was no break; pass does nothing.

for n in [9, 15, 21]:
    if n % 7 == 0:
        print("found", n)
        break
else:
    print("none")

Output: found 21

A string is an immutable sequence of characters. Indexing: s[0] first, s[-1] last. Slicing: s[a:b:c] (a included, b excluded, step c); s[::-1] reverses.

Methods (all return new values): upper(), lower(), title(), capitalize(), count(), find() (−1 if absent), replace(), split(), join(), strip(), isalpha(), isdigit(), isalnum().

s = "chai"
s.upper()
print(s, s.upper())

Output: chai CHAI — the first call's result was thrown away.

Lists are mutable: append(), insert(), extend(), remove(), pop(), sort(), reverse(), index(), count(); slice assignment L[1:4] = [0] replaces a part.

Aliasing: M = L gives a second name for the same list; L[:] or L.copy() makes a new list.

L = [1, 2, 3]
M = L
N = L[:]
L.append(4)
print(M, N)

Output: [1, 2, 3, 4] [1, 2, 3]

Tuples are immutable: indexing, slicing, count(), index(), + and * make new tuples; one item needs a comma, (5,); unpacking: a, b = (1, 2).

A dictionary holds key: value pairs; keys are unique and must be immutable (numbers, strings, tuples). d[k] = v adds or updates; d[k] on a missing key raises KeyError; d.get(k, default) does not. Methods: keys(), values(), items(), update(), pop(), len(); del d[k] removes a pair. A for loop over a dictionary goes through its keys in insertion order.

stock = {"rice": 40, "dal": 15}
stock.update({"oil": 8, "dal": 20})
print(stock)

Output: {'rice': 40, 'dal': 20, 'oil': 8}