What causes "スタックが空です" in krahets/hello-algo?

Check `if not stack.is_empty(): stack.pop()`.

krahets/hello-algo · error · IndexError

スタックが空です

Error message

スタックが空です

What it means

This IndexError (Japanese: 'スタックが空です' = 'stack is empty') is raised by ArrayStack.pop() (array_stack.py:30) when size() == 0. ArrayStack wraps a Python list; the guard prevents calling list.pop() on an empty backing list (which would itself raise IndexError with a different message). push() is unbounded (list.append grows dynamically), so 'full' is never an issue here.

Source

Thrown at ja/codes/python/chapter_stack_and_queue/array_stack.py:30

        """コンストラクタ"""
        self._stack: list[int] = []

    def size(self) -> int:
        """スタックの長さを取得"""
        return len(self._stack)

    def is_empty(self) -> bool:
        """スタックが空かどうかを判定"""
        return self.size() == 0

    def push(self, item: int):
        """プッシュ"""
        self._stack.append(item)

    def pop(self) -> int:
        """ポップ"""
        if self.is_empty():
            raise IndexError("スタックが空です")
        return self._stack.pop()

    def peek(self) -> int:
        """スタックトップの要素にアクセス"""
        if self.is_empty():
            raise IndexError("スタックが空です")
        return self._stack[-1]

    def to_list(self) -> list[int]:
        """表示用のリストを返す"""
        return self._stack


"""Driver Code"""
if __name__ == "__main__":
    # スタックを初期化
    stack = ArrayStack()

View on GitHub (pinned to 69932aed18)

Solutions

Check `if not stack.is_empty(): stack.pop()`.
Loop with `while not stack.is_empty():` for full drain.
Track expected depth separately and never pop below it.
Use try/except IndexError if pop-on-empty is a benign case in your algorithm.

Example fix

// before
top = stack.pop()  # IndexError when empty

// after
top = stack.pop() if not stack.is_empty() else None

Defensive patterns

Strategy: validation

Validate before calling

if not stack.is_empty():
    top = stack.pop()

Type guard

def stack_has_top(stack: ArrayStack) -> bool:
    return not stack.is_empty()

Try / catch

try:
    top = stack.pop()
except IndexError:
    top = None

Prevention

Guard every pop() with is_empty().
Drain with `while not stack.is_empty():`.
Track push/pop balance in expression evaluators and backtracking.
push() is unbounded (list.append), so only underflow, never overflow, can occur.

When it happens

Trigger: Calling pop() on a freshly constructed ArrayStack (empty list). Calling pop() more times than push(). Popping in a loop without an is_empty() check.

Common situations: Unbalanced push/pop in expression evaluation or backtracking. Draining the stack to empty then popping once more. Assuming a prior push succeeded when it actually was skipped by upstream logic.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/0c190a945dd30351. Report an issue: GitHub.