Apply combiner to an input_filepath_list of length 1

[justinsalamon]

Trying to combine a single file (which basically doesn't alter it unless the combiner applies transformations too):

cbn.build([filename], outfile, 'concatenate')

Raises an error: ValueError: input_filepath_list must have at least 2 files.

But there are scenarios where this is useful, for example in my case I need to concatenate a file to itself if the file is shorter than a certain value, but otherwise leave it unchanged. The number of concatenattions N is determined at runtime, so ideally I'd like to call build() like this:

cbn.build([filename] * N, outfile, 'concatenate')

So that if N=1 it basically leaves the file unchanged. This can of course be achieved using an if statement to determine whether I need to use the combiner or not, but it's much clunkier.

@rabitt Is there a particular reason why the combiner can't be called with an input_filepath_list list of length 1?

[rabitt]

@justinsalamon The fundamental problem with having an input_filepath_list of length less than 2, is that SoX's "combine" mode expects at least 2 files. For example:
sox --combine mix input.wav output.wav
fails, while
sox --combine mix input.wav input.wav output.wav
works fine.

I agree it would be useful to allow having a file list of length one but unfortunately it would require significantly more than an if statement -- to do this, the Combiner object would have to "revert" itself to a Transformer.

The cleanest way I can think of to solve this is to create a third class that wraps the Transformer and Combiner object - if there is 1 input file it behaves like a Transformer and if there are more it behaves like a Combiner. If you want to go this route, I accept pull requests ;)

[justinsalamon]

The cleanest way I can think of to solve this is to create a third class that wraps the Transformer and Combiner object - if there is 1 input file it behaves like a Transformer and if there are more it behaves like a Combiner.

Yes, I mean the combiner already does everything the transformer does, just with the added ability to mix/concatenate several source files before applying a transformation. A single class that can do everything (and hides the logic of choosing a transformer/combiner internally) would be nice to have. No cycles just now for PR, but will add it to the stack.