py-12-txn-kvstore

# Transactional key-value store

Implement a file **`solution.py`** containing a class `KVStore` that behaves like an in-memory
key-value map with **nested transactions**.

```python
class KVStore:
    def __init__(self): ...
    def get(self, key):          ...   # current value, or None if absent
    def set(self, key, value):   ...
    def delete(self, key):       ...   # remove key (no-op if absent)
    def keys(self):              ...   # sorted list of currently-visible keys
    def __len__(self):           ...   # number of currently-visible keys

    def begin(self):    ...            # open a new (nested) transaction
    def commit(self):   ...            # merge the innermost open transaction into its parent
    def rollback(self): ...            # discard the innermost open transaction
```

## Semantics

- With **no open transaction**, `set`/`delete` mutate the committed store directly.
- `begin()` opens a transaction. Transactions **nest**: a second `begin()` opens a child of the
  first. All `set`/`delete` calls apply to the **innermost** open transaction only.
- `get`, `keys`, and `__len__` always reflect the **currently-visible** state: the committed store
  overlaid by every open transaction in order (innermost wins). A key `delete`d in an open
  transaction is invisible even if it exists in the committed store.
- `commit()` merges the innermost transaction's changes (both sets and deletes) into its parent
  (the enclosing transaction, or the committed store if it was the outermost). The transaction is
  then closed.
- `rollback()` discards the innermost transaction's changes entirely and closes it.
- `commit()` or `rollback()` with **no open transaction** must raise `RuntimeError`.
- `keys()` returns the visible keys in **sorted order**.

## Example

```python
s = KVStore()
s.set("a", 1)
s.begin()
s.set("a", 2)
s.set("b", 3)
assert s.get("a") == 2          # innermost transaction wins
s.begin()
s.delete("a")
assert s.get("a") is None       # deleted in the inner transaction
assert s.get("b") == 3          # still visible from the outer transaction
s.rollback()                    # discard the inner transaction
assert s.get("a") == 2          # back to the outer transaction's value
s.commit()                      # merge outer transaction into the committed store
assert s.get("a") == 2 and s.get("b") == 3
assert s.keys() == ["a", "b"]
```

import pytest

from solution import KVStore


def test_basic_set_get_delete():
    s = KVStore()
    assert s.get("a") is None
    s.set("a", 1)
    s.set("b", 2)
    assert s.get("a") == 1
    assert s.get("b") == 2
    s.delete("a")
    assert s.get("a") is None
    s.delete("missing")  # no-op, must not raise


def test_len_and_keys_sorted():
    s = KVStore()
    s.set("z", 1)
    s.set("a", 1)
    s.set("m", 1)
    assert s.keys() == ["a", "m", "z"]
    assert len(s) == 3
    s.delete("m")
    assert s.keys() == ["a", "z"]
    assert len(s) == 2


def test_transaction_isolation_then_commit():
    s = KVStore()
    s.set("a", 1)
    s.begin()
    s.set("a", 2)
    s.set("b", 3)
    assert s.get("a") == 2
    assert s.get("b") == 3
    s.commit()
    assert s.get("a") == 2
    assert s.get("b") == 3


def test_rollback_discards_changes():
    s = KVStore()
    s.set("a", 1)
    s.begin()
    s.set("a", 99)
    s.set("b", 5)
    s.rollback()
    assert s.get("a") == 1
    assert s.get("b") is None
    assert s.keys() == ["a"]


def test_delete_within_transaction_is_isolated():
    s = KVStore()
    s.set("a", 1)
    s.begin()
    s.delete("a")
    assert s.get("a") is None
    assert s.keys() == []
    assert len(s) == 0
    s.rollback()
    assert s.get("a") == 1  # delete was rolled back


def test_commit_propagates_delete_to_committed_store():
    s = KVStore()
    s.set("a", 1)
    s.begin()
    s.delete("a")
    s.commit()
    assert s.get("a") is None
    assert s.keys() == []


def test_nested_transactions_inner_rollback():
    s = KVStore()
    s.set("a", 1)
    s.begin()
    s.set("a", 2)
    s.set("b", 3)
    s.begin()
    s.delete("a")
    assert s.get("a") is None
    assert s.get("b") == 3
    s.rollback()             # discard inner
    assert s.get("a") == 2   # outer value restored
    s.commit()               # merge outer into committed
    assert s.get("a") == 2
    assert s.get("b") == 3


def test_nested_commit_merges_into_parent_not_base():
    s = KVStore()
    s.begin()
    s.set("a", 1)
    s.begin()
    s.set("a", 2)
    s.set("b", 3)
    s.commit()               # inner -> outer (NOT committed store yet)
    assert s.get("a") == 2
    assert s.get("b") == 3
    s.rollback()             # discard outer -> everything gone
    assert s.get("a") is None
    assert s.get("b") is None
    assert len(s) == 0


def test_commit_without_transaction_raises():
    s = KVStore()
    with pytest.raises(RuntimeError):
        s.commit()


def test_rollback_without_transaction_raises():
    s = KVStore()
    with pytest.raises(RuntimeError):
        s.rollback()


def test_deep_nesting_visibility_and_merge():
    s = KVStore()
    s.set("k", 0)
    for depth in range(1, 6):
        s.begin()
        s.set("k", depth)
        assert s.get("k") == depth
    # commit all five layers down to the base
    for _ in range(5):
        s.commit()
    assert s.get("k") == 5
    assert s.keys() == ["k"]


def test_reset_value_after_delete_in_same_transaction():
    s = KVStore()
    s.set("a", 1)
    s.begin()
    s.delete("a")
    s.set("a", 7)            # re-add after delete within the same transaction
    assert s.get("a") == 7
    s.commit()
    assert s.get("a") == 7

You are an expert programmer. Solve the task exactly as specified. Output your solution as fenced code blocks using the required file name(s) and the exact function/type signatures requested. Prefer correctness; do not include prose outside code unless asked.

```python
class KVStore:
    def __init__(self):
        self.committed = {}
        self.stack = []

    def begin(self):
        self.stack.append({'sets': {}, 'deletes': set()})

    def commit(self):
        if not self.stack:
            raise RuntimeError("No open transaction to commit")
        tx = self.stack.pop()
        if not self.stack:
            # Merge into committed store
            for k, v in tx['sets'].items():
                self.committed[k] = v
            for k in tx['deletes']:
                self.committed.pop(k, None)
        else:
            # Merge into parent transaction
            parent = self.stack[-1]
            for k, v in tx['sets'].items():
                parent['deletes'].discard(k)
                parent['sets'][k] = v
            for k in tx['deletes']:
                parent['sets'].pop(k, None)
                parent['deletes'].add(k)

    def rollback(self):
        if not self.stack:
            raise RuntimeError("No open transaction to rollback")
        self.stack.pop()

    def get(self, key):
        # Check transactions from innermost to outermost
        for tx in reversed(self.stack):
            if key in tx['deletes']:
                return None
            if key in tx['sets']:
                return tx['sets'][key]
        return self.committed.get(key)

    def set(self, key, value):
        if self.stack:
            tx = self.stack[-1]
            tx['deletes'].discard(key)
            tx['sets'][key] = value
        else:
            self.committed[key] = value

    def delete(self, key):
        if self.stack:
            tx = self.stack[-1]
            tx['sets'].pop(key, None)
            tx['deletes'].add(key)
        else:
            self.committed.pop(key, None)

    def keys(self):
        # Start with committed keys
        visible_keys = set(self.committed.keys())
        # Apply transactions from outermost to innermost
        for tx in self.stack:
            visible_keys -= tx['deletes']
            visible_keys.update(tx['sets'].keys())
        return sorted(visible_keys)

    def __len__(self):
        return len(self.keys())
```

The user wants a Python class `KVStore` that implements a key-value store with nested transactions.

Key requirements:
1. In-memory key-value map.
2. Nested transactions.
3. Operations: `get`, `set`, `delete`, `keys`, `__len__`, `begin`, `commit`, `rollback`.
4. Behavior without transaction: direct mutation.
5. Behavior with transaction: changes go to the innermost transaction.
6. Visibility: `get`, `keys`, `__len__` see the committed store + all open transactions (innermost overrides).
7. `commit`: merges innermost transaction into parent (or committed store).
8. `rollback`: discards innermost transaction.
9. Errors: `commit`/`rollback` with no open transaction raises `RuntimeError`.
10. `keys` returns sorted list.

Let's design the data structures.

We need to track:
- The committed state (base store).
- A stack of open transactions.

Each transaction needs to record:
- Sets (key -> value).
- Deletes (set of keys).

When `get(key)` is called:
- Check the innermost transaction for the key.
  - If key is in deletes, return None.
  - If key is in sets, return value.
- If not found in innermost, check the next outer transaction, and so on.
- Finally, check the committed store.

When `set(key, value)` is called:
- If no transaction open, update committed store directly.
- If transaction open, update the innermost transaction's sets.

When `delete(key)` is called:
- If no transaction open, delete from committed store (if exists).
- If transaction open, add key to the innermost transaction's deletes.

When `keys()` is called:
- Start with committed store keys.
- For each transaction from outermost to innermost:
  - Remove keys that were deleted in that transaction.
  - Add keys that were set in that transaction.
- Return sorted list.
Wait, actually, since innermost wins, we can just compute the effective set of keys.
Or simpler:
- Collect all keys from committed store.
- For each transaction in the stack (from bottom to top):
  - Remove keys present in that transaction's deletes.
  - Add keys present in that transaction's sets.
- Return sorted unique keys.

When `__len__()` is called:
- Return `len(keys())`.

When `begin()` is called:
- Create a new transaction object (with empty sets and deletes).
- Push it onto the stack.

When `commit()` is called:
- If stack is empty, raise `RuntimeError`.
- Pop the innermost transaction.
- If stack is now empty (it was the outermost), merge changes into committed store.
  - Apply deletes to committed store.
  - Apply sets to committed store.
- If stack is not empty (there is a parent transaction), merge changes into the parent transaction.
  - Apply deletes to parent's deletes? No, careful.
  - If a key is deleted in the child, it should be deleted in the parent's view.
  - If a key is set in the child, it should be set in the parent's view.
  - So, for each key in child's deletes: add to parent's deletes.
  - For each key in child's sets: set in parent's sets.
  - Wait, what if the parent had set the key, and the child deleted it?
    - Child delete overrides parent set. So parent's delete set should include the key.
    - If parent had deleted the key, and child set it?
    - Child set overrides parent delete. So parent's sets should include the key.
  - Basically, the child's changes become the parent's changes.
  - So:
    - `parent.deletes.update(child.deletes)`
    - `parent.sets.update(child.sets)`
  - Is this correct?
    - Suppose parent set `a=1`. Child deletes `a`.
    - `parent.sets` has `a`. `child.deletes` has `a`.
    - After merge: `parent.deletes` has `a`. `parent.sets` has `a`.
    - When resolving `get(a)` in parent:
      - Check deletes: `a` is there. Return None.
      - This is correct. The delete wins.
    - Suppose parent deleted `a`. Child set `a=2`.
    - `parent.deletes` has `a`. `child.sets` has `a=2`.
    - After merge: `parent.deletes` has `a`. `parent.sets` has `a=2`.
    - When resolving `get(a)` in parent:
      - Check sets: `a` is there. Return 2.
      - This is correct. The set wins.
    - So simply updating the parent's sets and deletes with the child's is sufficient because the resolution logic checks sets/deletes in order (or rather, sets override deletes if both present? No, usually delete means "remove from view").
    - Let's refine the resolution logic for a single transaction layer.
    - A transaction layer has `sets` and `deletes`.
    - If `key` in `deletes`: it's deleted.
    - If `key` in `sets`: it's set.
    - What if both?
      - If I set then delete in same transaction: `sets` has it, `deletes` has it. Delete should win.
      - If I delete then set in same transaction: `deletes` has it, `sets` has it. Set should win.
      - So order matters within a transaction? Or we just need to track the final state of the transaction.
      - Actually, the problem says "set/delete calls apply to the innermost open transaction only".
      - If I do `set(a, 1)` then `delete(a)` in same transaction:
        - `sets` = {a: 1}, `deletes` = {a}.
        - `get(a)` should be None.
      - If I do `delete(a)` then `set(a, 1)` in same transaction:
        - `sets` = {a: 1}, `deletes` = {a}.
        - `get(a)` should be 1.
      - So just storing sets and deletes is ambiguous if both exist.
      - We need to track the net effect.
      - Option 1: Store a dict `changes` where value is `None` for delete, or actual value for set.
      - Option 2: Keep `sets` and `deletes`, but ensure they don't conflict.
        - On `set(k, v)`: remove `k` from `deletes`, add to `sets`.
        - On `delete(k)`: remove `k` from `sets`, add to `deletes`.
      - This seems cleaner.

Let's re-evaluate the merge logic with this "clean" state.
- Child transaction has `sets` and `deletes` (disjoint).
- Parent transaction has `sets` and `deletes` (disjoint).
- Merge child into parent:
  - For each `k` in `child.sets`:
    - Parent should see `k` as set.
    - Remove `k` from `parent.deletes` (if present).
    - Add `k` to `parent.sets`.
  - For each `k` in `child.deletes`:
    - Parent should see `k` as deleted.
    - Remove `k` from `parent.sets` (if present).
    - Add `k` to `pa
…[25018 chars elided]…
 return self.committed.get(key)

    def set(self, key, value):
        if self.stack:
            tx = self.stack[-1]
            tx['deletes'].discard(key)
            tx['sets'][key] = value
        else:
            self.committed[key] = value

    def delete(self, key):
        if self.stack:
            tx = self.stack[-1]
            tx['sets'].pop(key, None)
            tx['deletes'].add(key)
        else:
            self.committed.pop(key, None)

    def keys(self):
        visible_keys = set(self.committed.keys())
        for tx in self.stack:
            visible_keys -= tx['deletes']
            visible_keys.update(tx['sets'].keys())
        return sorted(visible_keys)

    def __len__(self):
        return len(self.keys())
```
I'll wrap this in the final response.

One check on "sorted list of currently-visible keys".
If I have keys "b", "a", "c". `sorted` gives ["a", "b", "c"]. Correct.

I'm confident.

Final check on the `commit` logic when merging into parent.
If parent has `sets={a: 1}` and child has `deletes={a}`.
- Child deletes `a`.
- Parent `sets` pop `a`. Parent `deletes` add `a`.
- Parent becomes `sets={}`, `deletes={a}`.
- `get(a)` in parent: sees delete -> None.
- Correct.

If parent has `deletes={a}` and child has `sets={a: 2}`.
- Child sets `a`.
- Parent `deletes` discard `a`. Parent `sets` add `a: 2`.
- Parent becomes `sets={a: 2}`, `deletes={}`.
- `get(a)` in parent: sees set -> 2.
- Correct.

If parent has `sets={a: 1}` and child has `sets={a: 2}`.
- Child sets `a`.
- Parent `sets` update `a: 2`.
- Parent becomes `sets={a: 2}`.
- `get(a)` in parent: sees set -> 2.
- Correct.

If parent has `deletes={a}` and child has `deletes={a}`.
- Child deletes `a`.
- Parent `deletes` add `a`.
- Parent becomes `deletes={a}`.
- `get(a)` in parent: sees delete -> None.
- Correct.

All cases covered.

One last check: `keys()` with `visible_keys -= tx['deletes']`.
If `tx['deletes']` contains a key not in `visible_keys`, set difference handles it gracefully (no error).
Correct.

The solution is ready.