PyTorch: What's the difference between state_dict and parameters()?

The parameters() only gives the module parameters i.e. weights and biases.

Returns an iterator over module parameters.

You can check the list of the parameters as follows:

for name, param in model.named_parameters():
    if param.requires_grad:
        print(name)

On the other hand, state_dict returns a dictionary containing a whole state of the module. Check its source code that contains not just the call to parameters but also buffers, etc.

Both parameters and persistent buffers (e.g. running averages) are included. Keys are the corresponding parameter and buffer names.

Check all keys that state_dict contains using:

model.state_dict().keys()

For example, in state_dict, you'll find entries like bn1.running_mean and running_var, which are not present in .parameters().

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If you only want to access parameters, you can simply use .parameters(), while for purposes like saving and loading model as in transfer learning, you'll need to save state_dict not just parameters.