# COMPUTE_NOT_PURE

compute() changed the strategy's own attributes or the data it was given.

## What happened

`compute()` changed one of the strategy's own attributes. Forge compares the strategy's attributes before and after `compute()` runs, and stops when one was added, removed or replaced. The message names the attribute.

### Common causes

- Setting an attribute on `self` inside `compute()`, such as `self.count = 0`.
- Replacing or deleting an attribute the strategy already had.

## What to do

Keep what `compute()` works out in local variables. `compute()` gets the data and the settings only, and keeps no state between runs.

### Related

- [Strategy](https://oathra.dev//reference/forge/strategy/Strategy)

## Example

```text
strategy.py: compute() changed self.count. compute() gets the data and the settings only, and keeps no state between runs. [COMPUTE_NOT_PURE]
```

The wording of the message depends on your file, and the code does not.