Problem in the lesson #5

[sergei-bondarenko]

In the "Collaborative filtering" section got the error when trying to learn.fit:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-6-8cf2e7ab9f6b> in <module>()
----> 1 learn.fit(1e-2, 2, wds=wd, cycle_len=1, cycle_mult=2, use_wd_sched=True)

~/courses/fastai/courses/dl1/fastai/learner.py in fit(self, lrs, n_cycle, wds, **kwargs)
    207         self.sched = None
    208         layer_opt = self.get_layer_opt(lrs, wds)
--> 209         return self.fit_gen(self.model, self.data, layer_opt, n_cycle, **kwargs)
    210 
    211     def warm_up(self, lr, wds=None):

~/courses/fastai/courses/dl1/fastai/learner.py in fit_gen(self, model, data, layer_opt, n_cycle, cycle_len, cycle_mult, cycle_save_name, use_clr, metrics, callbacks, use_wd_sched, norm_wds, wds_sched_mult, **kwargs)
    154         n_epoch = sum_geom(cycle_len if cycle_len else 1, cycle_mult, n_cycle)
    155         return fit(model, data, n_epoch, layer_opt.opt, self.crit,
--> 156             metrics=metrics, callbacks=callbacks, reg_fn=self.reg_fn, clip=self.clip, **kwargs)
    157 
    158     def get_layer_groups(self): return self.models.get_layer_groups()

~/courses/fastai/courses/dl1/fastai/model.py in fit(model, data, epochs, opt, crit, metrics, callbacks, **kwargs)
     78     avg_mom=0.98
     79     batch_num,avg_loss=0,0.
---> 80     for cb in callbacks: cb.on_train_begin()
     81     names = ["epoch", "trn_loss", "val_loss"] + [f.__name__ for f in metrics]
     82     layout = "{!s:10} " * len(names)

AttributeError: 'NoneType' object has no attribute 'on_train_begin'

[interogativ]

The reason for this is that the model fit function requires a callback class reference and for the attributes of that reference to have been initialized. While this is normally done for the user in the calling subclass (learner) it is not done in lesson 5 because Jeremy has you building this yourself. So you have two choices:

1. define you own callback class with the appropriate attributes.
2. Make the changes to the model fit routine to check to see if the callbacks class and attributes are appropriately defined before using them as suggested here

[interogativ]

http://forums.fast.ai/t/lesson-5-in-class-discussion/8408/205

[sergei-bondarenko]

It's fixed now.