Context Manager Class

Implement a context manager that measures execution time and sets duration based on exceptions.

context-manager classes timing error-handling

Write a class `TimedContext` that can be used with the `with` statement. It must:

- Accept a single argument `name` (a string) in its `__init__` method.
- When entering the context, record the start time using `time.perf_counter()`.
- When exiting, compute elapsed time as `time.perf_counter() - start_time`.
- If no exception occurred during the block, store the elapsed time in an attribute `duration` (a float).
- If an exception occurred, set `duration` to `None`.
- In all cases, the `__exit__` method should return `False` (so exceptions propagate).

Complete the class definition. Do not modify the `__init__` signature.

The class will be used as:
```python
with TimedContext('block') as tc:
    ...
print(tc.duration)
```

Note: The duration attribute must be set after the block completes.

Constraints

- `name` will be a non-empty string.
- The class must work in a `with` statement.
- No additional attributes are required.
- `__exit__` must accept the standard exception arguments `(exc_type, exc_val, exc_tb)`.

Example

```python
# Example 1: Normal execution
tc = TimedContext('fast')
with tc:
    sum(range(1000))
print(round(tc.duration, 6))  # prints a small non-negative float

# Example 2: Exception case
tc = TimedContext('boom')
try:
    with tc:
        raise ValueError('oops')
except ValueError:
    pass
print(tc.duration)  # prints None
```

8 points ~12 min

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Hints

Inside __enter__, assign `self._start = time.perf_counter()` and return `self`.

In __exit__, compute `elapsed = time.perf_counter() - self._start`.

Check whether `exc_type is not None` to decide if an exception occurred.

Set `self.duration` to `elapsed` or `None` accordingly, then return `False`.

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