`smt.exists(item)` doesn't work if default node value is set

[fubuloubu]

https://github.com/ethereum/py-trie/blob/e5bf55c66c19bb21b626b8d61925ebc4336cba0c/trie/smt.py#L340-L341

should be this instead:

 return self.get(key) == self._default

[antazoey]

haha I just got this problem, messaged you, searched, found you created this issue already

[antazoey]

Wouldn't it get fixed by changing get() to check self._default before raising KeyError instead of BLANK_NODE? That way, you still get the KeyError when expected. Same with branch. My attached PR does that, and thus it fixes this issue as well.

[antazoey]

Looking at the doc of the class:

If this default is anything other than a blank
node, then all keys "exist" in the database, which mimics the behavior of
Ethereum on-chain datastores.

So we do want exists return True in this case?

[fubuloubu]

Looking at the doc of the class:

If this default is anything other than a blank
node, then all keys "exist" in the database, which mimics the behavior of
Ethereum on-chain datastores.

So we do want exists return True in this case?

I think per the comment that it would augment .exists(key) to a non-default check (which would be useful independently of a KeyError from .get(key)), and leave .get(key) and .branch(key) to allows provide you with a value, even if the key is set default (but non-empty) because I might want to "prove" that my proof leads to an empty value