# tf.keras.metrics.Poisson

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| [![](https://www.tensorflow.org/images/GitHub-Mark-32px.png) View source on GitHub](https://github.com/keras-team/keras/tree/v2.7.0/keras/metrics.py#L2925-L2958) |

Computes the Poisson metric between `y_true` and `y_pred`.

Inherits From: [`MeanMetricWrapper`](https://www.tensorflow.org/versions/r2.7/api_docs/python/tf/keras/metrics/MeanMetricWrapper), [`Mean`](https://www.tensorflow.org/versions/r2.7/api_docs/python/tf/keras/metrics/Mean), [`Metric`](https://www.tensorflow.org/versions/r2.7/api_docs/python/tf/keras/metrics/Metric), [`Layer`](https://www.tensorflow.org/versions/r2.7/api_docs/python/tf/keras/layers/Layer), [`Module`](https://www.tensorflow.org/versions/r2.7/api_docs/python/tf/Module)

#### View aliases


**Main aliases**

[`tf.metrics.Poisson`](https://www.tensorflow.org/api_docs/python/tf/keras/metrics/Poisson)
**Compat aliases for migration**

See
[Migration guide](https://www.tensorflow.org/guide/migrate) for
more details.

[`tf.compat.v1.keras.metrics.Poisson`](https://www.tensorflow.org/api_docs/python/tf/keras/metrics/Poisson)

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    tf.keras.metrics.Poisson(
        name='poisson', dtype=None
    )

`metric = y_pred - y_true * log(y_pred)`

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| ## Args ||
|---|---|
| `name` | (Optional) string name of the metric instance. |
| `dtype` | (Optional) data type of the metric result. |

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#### Standalone usage:

    m = tf.keras.metrics.Poisson()
    m.update_state([[0, 1], [0, 0]], [[1, 1], [0, 0]])
    m.result().numpy()
    0.49999997

    m.reset_state()
    m.update_state([[0, 1], [0, 0]], [[1, 1], [0, 0]],
                   sample_weight=[1, 0])
    m.result().numpy()
    0.99999994

Usage with `compile()` API:

    model.compile(optimizer='sgd',
                  loss='mse',
                  metrics=[tf.keras.metrics.Poisson()])

## Methods

### `merge_state`

[View source](https://github.com/keras-team/keras/tree/v2.7.0/keras/metrics.py#L288-L322)

    merge_state(
        metrics
    )

Merges the state from one or more metrics.

This method can be used by distributed systems to merge the state computed
by different metric instances. Typically the state will be stored in the
form of the metric's weights. For example, a tf.keras.metrics.Mean metric
contains a list of two weight values: a total and a count. If there were two
instances of a tf.keras.metrics.Accuracy that each independently aggregated
partial state for an overall accuracy calculation, these two metric's states
could be combined as follows:

    m1 = tf.keras.metrics.Accuracy()
    _ = m1.update_state([[1], [2]], [[0], [2]])

    m2 = tf.keras.metrics.Accuracy()
    _ = m2.update_state([[3], [4]], [[3], [4]])

    m2.merge_state([m1])
    m2.result().numpy()
    0.75

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| Args ||
|---|---|
| `metrics` | an iterable of metrics. The metrics must have compatible state. |

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| Raises ||
|---|---|
| `ValueError` | If the provided iterable does not contain metrics matching the metric's required specifications. |

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### `reset_state`

[View source](https://github.com/keras-team/keras/tree/v2.7.0/keras/metrics.py#L251-L266)

    reset_state()

Resets all of the metric state variables.

This function is called between epochs/steps,
when a metric is evaluated during training.

### `result`

[View source](https://github.com/keras-team/keras/tree/v2.7.0/keras/metrics.py#L489-L500)

    result()

Computes and returns the metric value tensor.

Result computation is an idempotent operation that simply calculates the
metric value using the state variables.

### `update_state`

[View source](https://github.com/keras-team/keras/tree/v2.7.0/keras/metrics.py#L695-L727)

    update_state(
        y_true, y_pred, sample_weight=None
    )

Accumulates metric statistics.

For sparse categorical metrics, the shapes of `y_true` and `y_pred` are
different.

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| Args ||
|---|---|
| `y_true` | Ground truth label values. shape = `[batch_size, d0, .. dN-1]` or shape = `[batch_size, d0, .. dN-1, 1]`. |
| `y_pred` | The predicted probability values. shape = `[batch_size, d0, .. dN]`. |
| `sample_weight` | Optional `sample_weight` acts as a coefficient for the metric. If a scalar is provided, then the metric is simply scaled by the given value. If `sample_weight` is a tensor of size `[batch_size]`, then the metric for each sample of the batch is rescaled by the corresponding element in the `sample_weight` vector. If the shape of `sample_weight` is `[batch_size, d0, .. dN-1]` (or can be broadcasted to this shape), then each metric element of `y_pred` is scaled by the corresponding value of `sample_weight`. (Note on `dN-1`: all metric functions reduce by 1 dimension, usually the last axis (-1)). |

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| Returns ||
|---|---|
| Update op. ||

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