Lists & Tuples

▶Today's videos

Watch each video, then work the matching sections below. Watching alone will not do it.

1The structure you have been missing

Mini-Project 1 wanted this and could not have it.

On Day 5 you kept numbers in a comma-separated string, because you had nowhere else to put them. Yesterday's refactor made that workaround impossible to ignore — four functions all splitting the same text. Here is what you actually wanted:

Notice what did not change: the function still takes the numbers and a threshold, and still returns a count. The parsing, the empty-piece guard and the float() conversion all disappeared — they were never part of the idea, only of the workaround.

The operations

scores = [23, 45, 12, 67]

print(scores[0])          # first
print(scores[-1])         # last
print(len(scores))

scores.append(89)         # add one to the end
scores.insert(0, 5)       # add at a position
removed = scores.pop()    # take the last one off

print(scores)
print(removed)
print(67 in scores)

2Slicing, and the fence-post model

Indices sit between items, not on them.

Off-by-one errors come from imagining that index 2 is the third item. A better model: the index marks the gap before an item. A slice takes everything between two gaps.

Slicing never raises IndexError, even when the bounds are nonsense — scores[10:99] on a four-item list is just []. Plain indexing does raise. That asymmetry catches people, because a bug in the bounds fails silently.

3Mutability, and where the box metaphor breaks

The most important idea in Week 2.

Until today you could think of a variable as a box holding a value. That model has been quietly wrong all along, and lists are where it breaks visibly.

A name does not contain an object. A name points at one. Two names can point at the same object — and if that object is mutable, changing it through one name changes what you see through the other.

Three ways to copy

original = [1, 2, 3]

copy_a = original[:]
copy_b = list(original)
copy_c = original.copy()

copy_a.append(4)
print(original, copy_a)

The mutable default argument

Yesterday I flagged this forward. Here it is.

def add_item(item, basket=None):
    if basket is None:
        basket = []
    basket.append(item)
    return basket

Tuples: the immutable sibling

A tuple is not "a list you cannot change" — it is a different message. (x, y) is one point. [p1, p2, p3] is a collection of points. Choosing the right one documents your intent for free.

4Comprehensions

The loop you write most often, in one line.

Three quarters of the loops you have written so far do the same thing: start with an empty list, walk something, append a transformed or filtered item. Python has a shorthand for exactly that shape.

    [ what to keep   for each item   in what   if condition ]
    [ word.upper()   for word        in words  if len(word) > 3 ]

5Packing and unpacking

Several values at once, without indices.

point = (3, 4)
x, y = point                 # unpack
print(x, y)

a, b = 1, 2                  # pack then unpack
a, b = b, a                  # swap, no temp variable
print(a, b)

first, *rest = [10, 20, 30, 40]
print(first, rest)

The swap on line 6 is worth pausing on. The right-hand side is fully evaluated first, into a temporary tuple, and only then unpacked into the names. That is why no temporary variable is needed.

Unpacking in a for loop

people = [("Amin", 34), ("Ada", 36)]

for name, age in people:              # unpack each tuple
    print(f"{name} is {age}")

for i, name in enumerate(["a", "b"]):  # index AND value
    print(i, name)

Returning several values

def stats_of(numbers):
    """Return count, total and mean."""
    total = 0
    for n in numbers:
        total += n
    count = len(numbers)
    return count, total, total / count


count, total, mean = stats_of([12, 7, 25, 3])
print(count, total, mean)

Python has no special "multiple return" feature. return a, b, c builds a tuple and hands it back; the call site unpacks it. Once you see that, the feature stops being magic — and you know you can hold the whole tuple instead if you would rather.

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