# skfolio.utils.tools.apply_window_size ### skfolio.utils.tools.apply_window_size(X, window_size) Return the last `window_size` observations from the array X. * **Parameters:** **X** *ndarray of shape (n_observations,) or (n_observations, n_assets)* : Input array from which to extract the last observations. Can be 1D or 2D. **window_size** *int or None* : Number of observations to keep from the end of X. If None, returns X unchanged. * **Returns:** **X_windowed** *ndarray* : The last `window_size` rows of X. If `window_size` is None, returns the original array unchanged. * **Raises:** ValueError : If `window_size` is not a positive integer or cannot be converted to int. ValueError : If `window_size` exceeds the number of observations in X. ### Examples ```pycon >>> import numpy as np >>> X = np.array([[1, 2], [3, 4], [5, 6], [7, 8], [9, 10]]) >>> apply_window_size(X, window_size=3) array([[ 5, 6], [ 7, 8], [ 9, 10]]) ```