# Shift¶

Shifts a data stream by returning past values.

This can be used to compute statistics over past data. For instance, if you're computing daily averages, then shifting by 7 will be equivalent to computing averages from a week ago.

Shifting values is useful when you're calculating an average over a target value. Indeed, in this case it's important to shift the values in order not to introduce leakage. The recommended way to do this is to feature_extraction.TargetAgg, which already takes care of shifting the target values once.

## Parameters¶

• amount – defaults to 1

Shift amount. The get method will return the t - amount value, where t is the current moment.

• fill_value – defaults to None

This value will be returned by the get method if not enough values have been observed.

• name

## Examples¶

It is rare to have to use Shift by itself. A more common usage is to compose it with other statistics. This can be done via the | operator.

>>> from river import stats

>>> stat = stats.Shift(1) | stats.Mean()

>>> for i in range(5):
...     stat = stat.update(i)
...     print(stat.get())
0.0
0.0
0.5
1.0
1.5


A common usecase for using Shift is when computing statistics on shifted data. For instance, say you have a dataset which records the amount of sales for a set of shops. You might then have a shop field and a sales field. Let's say you want to look at the average amount of sales per shop. You can do this by using a feature_extraction.Agg. When you call transform_one, you're expecting it to return the average amount of sales, without including today's sales. You can do this by prepending an instance of stats.Mean with an instance of stats.Shift.

>>> from river import feature_extraction

>>> agg = feature_extraction.Agg(
...     on='sales',
...     how=stats.Shift(1) | stats.Mean(),
...     by='shop'
... )


Let's define a little example dataset.

>>> X = iter([
...     {'shop': 'Ikea', 'sales': 10},
...     {'shop': 'Ikea', 'sales': 15},
...     {'shop': 'Ikea', 'sales': 20}
... ])


Now let's call the learn_one method to update our feature extractor.

>>> x = next(X)
>>> agg = agg.learn_one(x)


At this point, the average defaults to the initial value of stats.Mean, which is 0.

>>> agg.transform_one(x)
{'sales_mean_of_shift_1_by_shop': 0.0}


We can now update our feature extractor with the next data point and check the output.

>>> agg = agg.learn_one(next(X))
>>> agg.transform_one(x)
{'sales_mean_of_shift_1_by_shop': 10.0}

>>> agg = agg.learn_one(next(X))
>>> agg.transform_one(x)
{'sales_mean_of_shift_1_by_shop': 12.5}


## Methods¶

get

Return the current value of the statistic.

update

Update and return the called instance.

Parameters

• x (numbers.Number)