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# _FitnessEstimator.py

## High-level description

The `FitnessEstimator` class in `cassiopeia/tools/fitness_estimator/_FitnessEstimator.py` defines an abstract base class for all fitness estimation algorithms in the Cassiopeia library. It mandates that all derived classes implement the `estimate_fitness` method, which calculates and assigns a 'fitness' attribute to each node in a given `CassiopeiaTree`.

## Code Structure

The code defines an abstract base class `FitnessEstimator` and a custom exception class `FitnessEstimatorError`. The `FitnessEstimator` class has a single abstract method, `estimate_fitness`, which needs to be implemented by concrete fitness estimation algorithm classes.

## References

This code references the `CassiopeiaTree` class from `cassiopeia.data`.

## Symbols

### `FitnessEstimatorError`

#### Description

This class represents an exception specific to the `FitnessEstimator` class and its subclasses. It is raised when errors occur during fitness estimation.

#### Inputs

*This class does not have any inputs.*

#### Outputs

*This class does not have any outputs.*

### `FitnessEstimator`

#### Description

This abstract base class defines the interface for all fitness estimation algorithms. It requires derived classes to implement the `estimate_fitness` method.

#### Inputs

*This class does not have any inputs.*

#### Outputs

*This class does not have any outputs.*

#### Internal Logic

*This class does not have any internal logic.*

### `FitnessEstimator.estimate_fitness`

#### Description

This abstract method defines the interface for estimating the fitness of nodes in a `CassiopeiaTree`. Concrete fitness estimation algorithms must implement this method.

#### Inputs

| Name | Type           | Description                                                                    |
| :--- | :------------- | :----------------------------------------------------------------------------- |
| tree | CassiopeiaTree | A `CassiopeiaTree` object representing the tree for which to estimate fitness. |

#### Outputs

*This method does not return any values. It modifies the input `CassiopeiaTree` object directly.*

#### Internal Logic

*This method is abstract and therefore has no implementation in the base class.*

## Side Effects

This code modifies the input `CassiopeiaTree` object in the `estimate_fitness` method by adding a 'fitness' attribute to each node.

## TODOs

*This code does not contain any TODOs.*