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# tf.keras.losses.Poisson

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

Inherits From: `Loss`

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

#### Standalone usage:

````y_true = [[0., 1.], [0., 0.]]`
`y_pred = [[1., 1.], [0., 0.]]`
`# Using 'auto'/'sum_over_batch_size' reduction type.`
`p = tf.keras.losses.Poisson()`
`p(y_true, y_pred).numpy()`
`0.5`
```
````# Calling with 'sample_weight'.`
`p(y_true, y_pred, sample_weight=[0.8, 0.2]).numpy()`
`0.4`
```
````# Using 'sum' reduction type.`
`p = tf.keras.losses.Poisson(`
`    reduction=tf.keras.losses.Reduction.SUM)`
`p(y_true, y_pred).numpy()`
`0.999`
```
````# Using 'none' reduction type.`
`p = tf.keras.losses.Poisson(`
`    reduction=tf.keras.losses.Reduction.NONE)`
`p(y_true, y_pred).numpy()`
`array([0.999, 0.], dtype=float32)`
```

Usage with the `compile()` API:

``````model.compile(optimizer='sgd', loss=tf.keras.losses.Poisson())
``````

`reduction` Type of `tf.keras.losses.Reduction` to apply to loss. Default value is `AUTO`. `AUTO` indicates that the reduction option will be determined by the usage context. For almost all cases this defaults to `SUM_OVER_BATCH_SIZE`. When used with `tf.distribute.Strategy`, outside of built-in training loops such as `tf.keras` `compile` and `fit`, using `AUTO` or `SUM_OVER_BATCH_SIZE` will raise an error. Please see this custom training tutorial for more details.
`name` Optional name for the instance. Defaults to 'poisson'.

## Methods

### `from_config`

View source

Instantiates a `Loss` from its config (output of `get_config()`).

Args
`config` Output of `get_config()`.

Returns
A `Loss` instance.

### `get_config`

View source

Returns the config dictionary for a `Loss` instance.

### `__call__`

View source

Invokes the `Loss` instance.

Args
`y_true` Ground truth values. shape = `[batch_size, d0, .. dN]`, except sparse loss functions such as sparse categorical crossentropy where shape = `[batch_size, d0, .. dN-1]`
`y_pred` The predicted values. shape = `[batch_size, d0, .. dN]`
`sample_weight` Optional `sample_weight` acts as a coefficient for the loss. If a scalar is provided, then the loss is simply scaled by the given value. If `sample_weight` is a tensor of size `[batch_size]`, then the total loss 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 loss element of `y_pred` is scaled by the corresponding value of `sample_weight`. (Note on`dN-1`: all loss functions reduce by 1 dimension, usually axis=-1.)

Returns
Weighted loss float `Tensor`. If `reduction` is `NONE`, this has shape `[batch_size, d0, .. dN-1]`; otherwise, it is scalar. (Note `dN-1` because all loss functions reduce by 1 dimension, usually axis=-1.)

Raises
`ValueError` If the shape of `sample_weight` is invalid.