process_function in Engine has strong signature?

I checked all docs, but I can’t understand if we have any restriction for process_function for Engine.

For example, I want to resolve the following questions:

• Must my process_function has engine as a first argument? If it is true, then it would be great to notice about it in docs.
• Can I use another signature instead of process_function(engine, labelled_batch)? For instance, process_function(engine, labelled_batch, device)? According to this great code, it is used additional function _prepare_batch, because of strong signature of process_function (otherwise, I would pass device to process_function as third argument).

[vfdev-5]

@ZhiliangWu having engine as an argument of process_function can help with fetching easily the current state: iteration, epoch, custom variables etc. For example, when during the training you have total loss function composed of two part and an alpha parameter that is also updated during the training:

def train_step(engine, batch):
     # ...
     loss_1 = ...
     loss_2 = ...
     if engine.state.epoch > num_epochs / 2:
         total_loss = loss_1 + engine.state.alpha * loss_2
     else:
         total_loss = loss_1
     # ...
 

trainer = Engine(train_step)
trainer.state.alpha = 0.1

@trainer.on(Events.ITERATION_COMPLETED)
def update_alpha(engine):
    engine.state.alpha += 0.001

This is just an example, but you can imagine other use-cases where usage of trainer’s state can be interesting.

[ZhiliangWu]

Hi @vfdev-5, I am trying to use this great package to accelerate my development cycle. However, I cannot quite understand why the function process_function() must take engine as its input argument. The only usage of it is in the _run_once_on_dataset() method to compute the self.state.output. However, the argument engine (or self when called in the class) doesn’t do anything or save anything with it. Are there any deeper design thoughts on it? Or is it only used to show deliberately that the process is associated with a specific engine? Thanks for your comments in advance.