It would be great if someone were to contribute a pure-Python implementation, but it's unlikely the sandstorm.io team will work on this since it has no real use

It would be great if someone were to contribute a pure-Python implementation, but it's unlikely the sandstorm.io team will work on this since it has no real use to us. I actually think it's likely that a pure-Python version of Cap'n Proto would be significantly faster than the pure-Python protobuf implementation. Parsing Protobufs in Python is really horrible performance-wise since you have to inspect and branch on almost every byte. The way to make Python fast is to delegate as much work as possible to the built-in libraries that are written in C. But, there's just nothing that can be delegated in the case of Protobufs. In contrast, a Cap'n Proto parser could pretty easily leverage the existing `struct` module. That said, if you enable Cap'n Proto's "packed" mode, then this advantage is lost, since that's another byte-by-byte algorithm that will perform poorly in pure Python.