Enforce uniqueness on custom path converters

[Adam Johnson]

register_converter() allows ​custom path converters to silently replace existing converters. This could lead to surprising behaviour, both when two places accidentally use the same custom name or when replacing a default converter. This could be particularly hard to debug if a third-party package uses a custom converter.

For example, say two modules register path converters named “year”. The first allows 1-4 digits:

class YearConverter:
    regex = r"[0-9]{1,4}"
    ...

register_converter(YearConverter, "year")

Whilst the second requires exactly four digits:

class YearConverter:
    regex = r"[0-9]{4}"
    ...

register_converter(YearConverter, "year")

Whichever module is loaded last will silently overwrite the other’s converter. URLs will then only be interpreted with that converter, leading to fewer URLs being matched than intended. This can be particularly difficult to spot as import order may change accidentally due to other code being rearranged.