show error message

[corochann]

When we construct QFunction, and model parameter is not initialized (it happens when some Link is instantiated with in_size=None), copy_param will fail because the target_link's param is None.

This PR is to show user-friendly error message to let the user know what is the cause of this error.

I'd like to get comment that what kind of message is better.

I'm not sure the performance degrade by checking this type error.

[toslunar]

Thanks. I agree with you about showing error message.

I'd like to see message that starts with "target_link has parameter {}", since target_params is a local name.

[corochann]

And should we add some sentence to how to fix this error like:

'target_link parameter {} is None. Maybe the model params are not initialized.\n Please forward dummy input before hand to determine parameter shape of the model.'.format(param_name)

or something?

[toslunar]

It's a possible fix to forward dummy input. However, I'd rather not recommend it.
Instead default agents in ChainerRL compute the first action before doing the first training (in act_and_train).

[corochann]

Updated text.

default agents in ChainerRL compute the first action before doing the first training (in act_and_train).

Yeah, if DQN agent uses parameter-initialized target_link it would be nice.

[toslunar]

Sorry. You are right. DQN agent expects parameters of q_function to be initialized. Let's recommend forwarding dummy input on such uninitialized-parameter errors.

[corochann]

So you prefer below comment rather than current PR?

'target_link parameter {} is None. Maybe the model params are not initialized.\n Please forward dummy input before hand to determine parameter shape of the model.'.format(param_name)

[toslunar]

Yes, I prefer the longer comment.

Some minor fixes:

• \n Please -> \nPlease
• Please -> Please try to (?)
• before hand -> beforehand

[toslunar]

Could you add testcases (in tests/misc_tests/test_copy_param.py) if you could spare the time?

[toslunar]

LGTM