When do two names point to the same changing data?

1 · Observe

One flat is a row of numbers

A flat has bedrooms, area and distance to the metro. Python holds such groups in collections: lists for ordered items you can change, tuples for ordered items you cannot, and dictionaries for items you look up by name. Choosing the right one decides what bugs are possible.

2 · Express

Indexing, slicing and mutation

3 · Visualize

What happens

Assignment gives the same list two names, so editing through the alias changes the original. A copy is independent, but a shallow copy of a list of lists still shares the inner lists: changing one inner value changed both.

4 · Discover

Dictionaries

A dictionary maps keys to values: house['price_lakh'] = 65.0. A missing key raises KeyError, while house.get('age_years') returns None, or a default you provide. Use get when absence is normal.

5 · Apply

Two worked examples

Example 1, a hidden edit. A function receives features, sorts them in place, and the caller's data is reordered. Pass a copy when the function mutates.

Example 2, a lookup. house.get('age_years', 'unknown') records absence instead of crashing.

6 · Where it goes wrong

Three ways it goes wrong

7 · In Python

Collections in code

Here is how to implement it, step by step. The code below does exactly these steps, and every line is editable.

1. Build a list in a fixed order, such as features = [3, 80, 2.0].
2. Index with [0] and slice with [0:2].
3. Copy with list(features) when you need an independent list.
4. Use a tuple for values that must not change.
5. Use a dict when you look up by name: house['price_lakh'], and .get() when absence is normal.

features = [3, 80, 2.0]                  # bedrooms, size in sq m, km to metro
print("first:", features[0], "| last:", features[-1], "| slice:", features[0:2], "| length:", len(features))

alias = features
alias[1] = 90                           # assignment does not copy a list
print("after editing alias: original is", features)
safe = list(features)
safe[1] = 100
print("after editing a copy: original", features, "| copy", safe)

point = (3, 80, 2.0)                    # a tuple cannot change
try:
    point[1] = 90
except TypeError as e:
    print("tuple edit ->", type(e).__name__)

house = {"city": "Pune", "bedrooms": 3, "price_lakh": 62.5}
house["price_lakh"] = 65.0
print(house, "| keys:", list(house))
print("missing key with get:", house.get("age_years"), "| with default:", house.get("age_years", "unknown"))

rows = [[3, 80], [2, 60]]
shallow = rows.copy()
shallow[0][1] = 999
print("a shallow copy shares the inner lists:", rows)

first: 3 | last: 2.0 | slice: [3, 80] | length: 3
after editing alias: original is [3, 90, 2.0]
after editing a copy: original [3, 90, 2.0] | copy [3, 100, 2.0]
tuple edit -> TypeError
{'city': 'Pune', 'bedrooms': 3, 'price_lakh': 65.0} | keys: ['city', 'bedrooms', 'price_lakh']
missing key with get: None | with default: unknown
a shallow copy shares the inner lists: [[3, 999], [2, 60]]