Can't pickle local object - exporting model

I am trying to export a model but am getting a pickle error. I know that lambda functions cannot be pickled and I have followed advice on Fastai v2 Recipes (Tips and Tricks) - Wiki - #2 by farid for using a named function instead. However, I am still getting:

AttributeError: Can't pickle local object 'get_dls.<locals>.get_y'

Here’s the trace

AttributeError                   Traceback (most recent call last)
---->  learn.export(filesavename+'_export.pkl')
/usr/local/lib/python3.7/dist-packages/fastai/ in export(self, fname, pickle_module, pickle_protocol)
    373         #To avoid the warning that come from PyTorch about model not being checked
    374         warnings.simplefilter("ignore")
--> 375, self.path/fname, pickle_module=pickle_module, pickle_protocol=pickle_protocol)
    376     self.create_opt()
    377     if state is not None: self.opt.load_state_dict(state)

/usr/local/lib/python3.7/dist-packages/torch/ in save(obj, f, pickle_module, pickle_protocol, _use_new_zipfile_serialization)
    377         if _use_new_zipfile_serialization:
    378             with _open_zipfile_writer(opened_file) as opened_zipfile:
--> 379                 _save(obj, opened_zipfile, pickle_module, pickle_protocol)
    380                 return
    381         _legacy_save(obj, opened_file, pickle_module, pickle_protocol)

/usr/local/lib/python3.7/dist-packages/torch/ in _save(obj, zip_file, pickle_module, pickle_protocol)
    482     pickler = pickle_module.Pickler(data_buf, protocol=pickle_protocol)
    483     pickler.persistent_id = persistent_id
--> 484     pickler.dump(obj)
    485     data_value = data_buf.getvalue()
    486     zip_file.write_record('data.pkl', data_value, len(data_value))

My data loader is defined as follows:

def get_dls(bs:int, size:int):
  path_src = path/'src'
  path_tar = path/'tar'

  def get_y(o):
     return path_tar/

  dblock = DataBlock(blocks=(ImageBlock, ImageBlock),
                   get_y = get_y, #lambda x: path_tar/,
  dls = dblock.dataloaders(path_src, bs=bs, path=path)
  dls.c = 3 
  return dls

And definition of learner:

data = get_dls(224,224)
learn = unet_learner(data, resnet34, loss_func=F.mse_loss)

Everything works fine for training. Can someone please point out my mistake?

You have a function in a function, pickle doesn’t like that.

Separate out get_y outside your get_dls function and it’ll work fine.

Also make sure that your get_y function is available when calling load_learner, as it needs the reference to load in the pickle file

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