もうハマらない! list.copyとcopy.deepcopyの正しい使い分け

# シャローコピーの基本例
nums = [[1, 2], [3, 4]]
shallow = nums.copy()  # または nums[:], list(nums)

shallow[0].append(99)  # 子リストを変更

print("元:", nums)
print("複製:", shallow)
print("子要素は同一参照か:", nums[0] is shallow[0])

import copy

nums = [[1, 2], [3, 4]]
deep = copy.deepcopy(nums)

deep[0].append(88)

print("元:", nums)
print("複製:", deep)
print("子要素は同一参照か:", nums[0] is deep[0])

data = [[0], [1]]
sh = data.copy()

print("外側は別物か:", sh is data)
print("内側0番は同一参照か:", sh[0] is data[0])
print("内側1番は同一参照か:", sh[1] is data[1])

import copy

lst = [[1], [2]]

a = lst.copy()   # メソッド
b = lst[:]       # スライス
c = list(lst)    # コンストラクタ
d = copy.copy(lst)  # 標準ライブラリの浅いコピー

print("a is lst:", a is lst)
print("b is lst:", b is lst)
print("c is lst:", c is lst)
print("d is lst:", d is lst)
print("a[0] is lst[0]:", a[0] is lst[0])
print("b[0] is lst[0]:", b[0] is lst[0])
print("c[0] is lst[0]:", c[0] is lst[0])
print("d[0] is lst[0]:", d[0] is lst[0])

a = [[1, 2], [3, 4]]
b = a.copy()

b[1].remove(4)     # 子要素の就地変更

print("a:", a)
print("b:", b)

grid = [[0] * 3] * 3
grid[0][0] = 1  # 1箇所だけ変えたつもりが…

print("grid:", grid)
print("行0と行1は同一オブジェクトか:", grid[0] is grid[1])

# 正しい初期化
grid = [[0 for _ in range(3)] for _ in range(3)]
grid[0][0] = 1

print("grid:", grid)
print("行0と行1は同一オブジェクトか:", grid[0] is grid[1])

names = ["Alice", "Bob", "Carol"]
names2 = names.copy()

names2[0] = "ALICE"   # 要素の差し替えはトップレベルの変更
print("元:", names)
print("複製:", names2)

import copy

state = {"users": [{"id": 1, "tags": {"vip"}}, {"id": 2, "tags": set()}]}
cloned = copy.deepcopy(state)

cloned["users"][0]["tags"].add("new")
print("元:", state)
print("複製:", cloned)
print("タグ集合は同一参照か:", state["users"][0]["tags"] is cloned["users"][0]["tags"])

# 注意: 実行環境により計測結果は大きく異なります
import copy, time

def bench(n_rows: int, row_len: int = 50):
    base = [[0] * row_len for _ in range(n_rows)]
    t0 = time.perf_counter()
    shallow = base.copy()
    t1 = time.perf_counter()
    deep = copy.deepcopy(base)
    t2 = time.perf_counter()
    return (t1 - t0), (t2 - t1)

for n in (1_000, 5_000):
    s, d = bench(n)
    print(f"行数={n}, シャロー={s:.6f}s, ディープ={d:.6f}s, 比={d/max(s, 1e-9):.1f}倍")

# スコア表を3行作るつもりで…(実は同じ行を3回参照している)
scores = [[0] * 3] * 3
scores[0][1] = 10

print("scores:", scores)

# 1) 正しい初期化
scores = [[0 for _ in range(3)] for _ in range(3)]
scores[0][1] = 10
print("独立行:", scores)

# 2) 既存の雛形を増やす場合はdeepcopy
import copy
row = [0, 0, 0]
scores2 = [copy.deepcopy(row) for _ in range(3)]
scores2[0][1] = 10
print("deepcopy版:", scores2)