Here is the standard form of a callback handler that loops through its callbacks:

```
class CallbackHandler():
def __init__(self,cbs=None):
self.cbs = cbs if cbs else []
def begin_fit(self, learn):
self.learn,self.in_train = learn,True
learn.stop = False
res = True
for cb in self.cbs: res = res and cb.begin_fit(learn)
return res
def after_fit(self):
res = not self.in_train
for cb in self.cbs: res = res and cb.after_fit()
return res
def begin_epoch(self, epoch):
learn.model.train()
self.in_train=True
res = True
for cb in self.cbs: res = res and cb.begin_epoch(epoch)
return res
def begin_validate(self):
self.learn.model.eval()
self.in_train=False
res = True
for cb in self.cbs: res = res and cb.begin_validate()
return res
```

We see that at each point, it is able to only return ‘res’, a boolean. But if a callback returns a object it will throw an error as it expects a boolean for the AND operation with res. However in cases as

where we do

`xb,xy=cb_handler.on_batch_begin(xb,yb)`

and

`out = cb_handler.on_loss_begin(out)`

and

`loss, skip_backward = callbacks.on_loss_begin(loss)`

we are returning objects given by the callbacks, two in the last case. How are we able to do this?