# Activity Timeouts - Python SDK

> Optimize Workflow Execution with Temporal Python SDK - Set Activity Timeouts and Retry Policies efficiently.

## Set Activity timeouts 

Each Activity timeout controls the maximum duration of a different aspect of an Activity Execution.

The following timeouts are available in the Activity Options.

- **[Schedule-To-Close Timeout](/encyclopedia/detecting-activity-failures#schedule-to-close-timeout):** is the maximum amount of time allowed for the overall [Activity Execution](/activity-execution).
- **[Start-To-Close Timeout](/encyclopedia/detecting-activity-failures#start-to-close-timeout):** is the maximum time allowed for a single [Activity Task Execution](/tasks#activity-task-execution).
- **[Schedule-To-Start Timeout](/encyclopedia/detecting-activity-failures#schedule-to-start-timeout):** is the maximum amount of time that is allowed from when an [Activity Task](/tasks#activity-task) is scheduled to when a [Worker](/workers#worker) starts that Activity Task.

An Activity Execution must have either the Start-To-Close or the Schedule-To-Close Timeout set.

Activity options are set as keyword arguments after the Activity arguments.

Available timeouts are:

- schedule_to_close_timeout
- schedule_to_start_timeout
- start_to_close_timeout

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  <a href="https://github.com/temporalio/documentation/blob/main/sample-apps/python/activity_timeouts_retires/your_workflows_dacx.py">
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  in the context of the rest of the application code.
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```python
# ...
        activity_timeout_result = await workflow.execute_activity(
            your_activity,
            YourParams(greeting, "Activity Timeout option"),
            # Activity Execution Timeout
            start_to_close_timeout=timedelta(seconds=10),
            # schedule_to_start_timeout=timedelta(seconds=10),
            # schedule_to_close_timeout=timedelta(seconds=10),
        )
```

### Set an Activity Retry Policy 

**How to set an Activity Retry Policy using the Temporal Python SDK**

A Retry Policy works in cooperation with the timeouts to provide fine controls to optimize the execution experience.

Activity Executions are automatically associated with a default [Retry Policy](/encyclopedia/retry-policies) if a custom one is not provided.

To create an Activity Retry Policy in Python, set the [RetryPolicy](https://python.temporal.io/temporalio.common.RetryPolicy.html) class within the [`start_activity()`](https://python.temporal.io/temporalio.workflow.html#start_activity) or [`execute_activity()`](https://python.temporal.io/temporalio.workflow.html#execute_activity) function.

<div className="copycode-notice-container">
  <a href="https://github.com/temporalio/documentation/blob/main/sample-apps/python/activity_timeouts_retires/your_workflows_dacx.py">
    View the source code
  </a>{' '}
  in the context of the rest of the application code.
</div>

```python
from temporalio.common import RetryPolicy
# ...
        activity_result = await workflow.execute_activity(
            your_activity,
            YourParams(greeting, "Retry Policy options"),
            start_to_close_timeout=timedelta(seconds=10),
            # Retry Policy
            retry_policy=RetryPolicy(
                backoff_coefficient=2.0,
                maximum_attempts=5,
                initial_interval=timedelta(seconds=1),
                maximum_interval=timedelta(seconds=2),
                # non_retryable_error_types=["ValueError"],
            ),
        )
```

### Override the retry interval with `next_retry_delay` 

To override the next retry interval set by the current policy, pass `next_retry_delay` when raising an [ApplicationError](/references/failures#application-failure) in an Activity.
This value replaces and overrides whatever the retry interval would normally be on the retry policy.

For example, you can set the delay interval based on an Activity's attempt count.
In the following example, the retry delay starts at 3 seconds after the first attempt.
It increases to 6 seconds for the second attempt, 9 seconds for the third attempt, and so forth.
This creates a steadily increasing backoff, versus the exponential approach used by [backoff coefficients](/encyclopedia/retry-policies#backoff-coefficient):

```python
from temporalio.exceptions import ApplicationError
from datetime import timedelta

@activity.defn
async def my_activity(input: MyActivityInput):
    try:
        # Your activity logic goes here
    except Exception as e:
        attempt = activity.info().attempt
        raise ApplicationError(
            f"Error encountered on attempt {attempt}",
            next_retry_delay=timedelta(seconds=3 * attempt),
        ) from e
```

## Heartbeat an Activity 

**How to Heartbeat an Activity using the Temporal Python SDK**

An [Activity Heartbeat](/encyclopedia/detecting-activity-failures#activity-heartbeat) is a ping from the [Worker Process](/workers#worker-process) that is executing the Activity to the [Temporal Service](/temporal-service).
Each Heartbeat informs the Temporal Service that the [Activity Execution](/activity-execution) is making progress and the Worker has not crashed.
If the Temporal Service does not receive a Heartbeat within a [Heartbeat Timeout](/encyclopedia/detecting-activity-failures#heartbeat-timeout) time period, the Activity will be considered failed and another [Activity Task Execution](/tasks#activity-task-execution) may be scheduled according to the Retry Policy.

Heartbeats may not always be sent to the Temporal Service—they may be [throttled](/encyclopedia/detecting-activity-failures#throttling) by the Worker.

Activity Cancellations are delivered to Activities from the Temporal Service when they Heartbeat. Activities that don't Heartbeat can't receive a Cancellation.
Heartbeat throttling may lead to Cancellation getting delivered later than expected.

Heartbeats can contain a `details` field describing the Activity's current progress.
If an Activity gets retried, the Activity can access the `details` from the last Heartbeat that was sent to the Temporal Service.

To Heartbeat an Activity Execution in Python, use the [`heartbeat()`](https://python.temporal.io/temporalio.activity.html#heartbeat) API.

```python
@activity.defn
async def your_activity_definition() -> str:
    activity.heartbeat("heartbeat details!")
```

In addition to obtaining cancellation information, Heartbeats also support detail data that persists on the server for retrieval during Activity retry.
If an Activity calls `heartbeat(123, 456)` and then fails and is retried, `heartbeat_details` returns an iterable containing `123` and `456` on the next Run.

#### Set a Heartbeat Timeout 

**How to set a Heartbeat Timeout using the Temporal Python SDK**

A [Heartbeat Timeout](/encyclopedia/detecting-activity-failures#heartbeat-timeout) works in conjunction with [Activity Heartbeats](/encyclopedia/detecting-activity-failures#activity-heartbeat).

[`heartbeat_timeout`](https://python.temporal.io/temporalio.worker.StartActivityInput.html#heartbeat_timeout) is a class variable for the [`start_activity()`](https://python.temporal.io/temporalio.workflow.html#start_activity) function used to set the maximum time between Activity Heartbeats.

```python
workflow.start_activity(
    activity="your-activity",
    schedule_to_close_timeout=timedelta(seconds=5),
    heartbeat_timeout=timedelta(seconds=1),
)
```

`execute_activity()` is a shortcut for [`start_activity()`](https://python.temporal.io/temporalio.workflow.html#start_activity) that waits on its result.

To get just the handle to wait and cancel separately, use `start_activity()`. `execute_activity()` should be used in most cases unless advanced task capabilities are needed.

```python
workflow.execute_activity(
    activity="your-activity",
    name,
    schedule_to_close_timeout=timedelta(seconds=5),
    heartbeat_timeout=timedelta(seconds=1),
)
```