Function literal 

fn literal(default: &str, dotted: &str) -> Result<String, Error>

Accepts a default only if it is a plain Python literal.

pyo3 renders the tokens of #[pyo3(signature = …)] verbatim, so a default that is an expression — or a Default::default() rendered as <object at 0x…> — would put an address in a committed file and break determinism. Such a default is an error, not something to paper over with ....