update `__repr__` to work even when the object hasn't fully been initialized

[tswast]

When running in a debugger (e.g. https://github.com/microsoft/debugpy in VS Code), setting a breakpoint in __init__ will crash the debugger.

[tswast]

Might want to fix #733 (comment) too at the same time, which also seems to mostly be a VS Code peculiarity, but still possible in other environments.

Edit: that part should be fixed by #741