Difference between evaluation metrics and evaluation function in lightgbm

Refer to lightgbm.cv, there are two parameters confusing me: metrics and feval. Based on my little knowledge on gbdt, evaluation metrics and evaluation function are both to compute the loss, such as auc, from a vector of predictions and a vector of true labels.

However, metrics and function sound like they are handling different tasks .

Are they doing the same thing?
To be more specific, if I execute lightgbm.cv(params,metrics='auc', feval='ks'), will feval='ks' override metrics='auc'?

Solution

To start with, the general notions of metrics and function are not at all different: from a mathematical viewpoint, a metric is a function (Wikipedia entry). And although here the notion of metric is more broad, the argument still holds.

More specifically on your question; from the documentation page you have linked:

metrics (string, list of strings or None, optional (default=None)) – Evaluation metrics to be monitored while CV. If not None, the metric in params will be overridden.

feval (callable or None, optional (default=None)) – Custom evaluation function.

Notice 1) the plural metrics, which can be a list of strings 2) the term custom in feval.

To make a long story short:

You can indeed use more than one available metrics in the metrics argument; your example should be:
```
lightgbm.cv(params,metrics=['auc','ks'])
```
feval should only be used if, additionally to whatever metrics you may use from the readily available ones, you also want a custom metric you have defined yourself; see an example here, where metric='auc' and feval = my_err_rate are used simultaneously and after my_err_rate has been defined.