Is there an audio processing library/API that contains all standard features of a digital audio workstation?
For python, I'd try out: 


pydub for basic audio manipulation and arrangement (slicing, concatenation, fading, repeating, etc). 
For effects processing, pyo is a good python signal processing module with a musical focus. Here's a comprehensive review, with inline clips. 


Together you should be able to get good coverage of most DAW functions. There are also fantastic python audio libraries like librosa, in case you do decide to do any feature extraction or audio analysis. Good luck!