unit8co/darts

Improve error message when future covariates are missing during backtest() call

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#2,846 opened on Jul 18, 2025

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 (4 comments) (0 reactions) (0 assignees)Python (6,832 stars) (762 forks)batch import
good first issueimprovement

Description

Is your feature request related to a current problem? Please describe. Yes.When using backtest() with a model trained with train and val future covariates, forgetting to pass the full future covariates again at backtest time results in an error (IndexError: index 1 is out of bounds for axis 0 with size 1.).

Describe proposed solution A better explanation or error message should be thrown as it is the case for "regular" historical forecasts.

Additional context Error currently thrown :

IndexError: index 1 is out of bounds for axis 0 with size 1.

Contributor guide

Tech stack
python
Domain
data
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Knowledge of PythonFamiliarity with darts backtest function
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75
Research direction
The error occurs in the backtest() function when future covariates are missing. Developers should locate the backtest() method in the darts library's source code and add a validation check for future covariates before the current error is raised. A clear error message should be provided, similar to the one for historical forecasts. Check the existing comments on the issue for any additional context or maintainer guidance.