# Source code for forml.evaluation._metric

```
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"""
Metric implementations.
"""
import statistics
import typing
from forml import flow
from forml.pipeline import payload
from . import _api
if typing.TYPE_CHECKING:
from forml import evaluation
[docs]class Function(_api.Metric):
"""Function(metric: Callable[[typing.Any, typing.Any], float], reducer: Callable[..., float] = mean)
Basic metric implementation wrapping a plain scoring function.
Caution:
As with any ForML task, the implementer is responsible for engaging a function that is
compatible with the particular :ref:`payload <io-payload>`.
Args:
metric: Actual metric function implementation.
reducer: Callable to reduce individual metric *partitions* into a single final value.
It must accept as many positional arguments as many outcome partitions there are.
The default reducer is the :func:`python:statistics.mean`.
Examples:
>>> LOG_LOSS = evaluation.Function(sklearn.metrics.log_loss)
>>> ACCURACY = evaluation.Function(
... lambda t, p: sklearn.metrics.accuracy_score(t, numpy.round(p))
... )
"""
def __init__(
self,
metric: typing.Callable[[typing.Any, typing.Any], float],
reducer: typing.Callable[..., float] = lambda *m: statistics.mean(m), # noqa: B008
):
self._metric: flow.Builder = payload.Apply.builder(function=metric)
self._reducer: flow.Builder = payload.Apply.builder(function=reducer)
def score(self, *outcomes: 'evaluation.Outcome') -> flow.Worker:
def apply(partition: 'evaluation.Outcome') -> flow.Worker:
"""Score the given outcome partition.
Args:
partition: Outcome to be scored.
Returns:
Worker node implementing the scoring for this partition.
"""
worker = flow.Worker(self._metric, 2, 1)
worker[0].subscribe(partition.true)
worker[1].subscribe(partition.pred)
return worker
def merge(reducer: flow.Worker, partition: flow.Worker, index: int) -> flow.Worker:
"""Merge the given partition using the provided reducer under the given partition index.
Args:
reducer: Reducer worker flow.
partition: Partition worker flow.
index: Partition index.
Returns:
Reducer worker flow.
"""
reducer[index].subscribe(partition[0])
return reducer
assert outcomes, 'Expecting outcomes.'
result = apply(outcomes[0])
if (partition_count := len(outcomes)) > 1:
result = merge(flow.Worker(self._reducer, partition_count, 1), result, 0)
for idx, out in enumerate(outcomes[1:], start=1):
merge(result, apply(out), idx)
return result
```