Usage of dictionaries / the value map from the script context

[nielstron]

The script context provides a dictionary/map from policy id -> tokenname -> value being contained in the inputs and outputs of the transaction being validated.

This thread is used to keep track of questions popping up regarding its use.

[nielstron]

Q: does value.get require an extra parameter here ->

def entry_policy_present(
    txins: List[TxInInfo]
) -> Bool:
    res = False
    for txi in txins:
        if txi.resolved.value.get(token.policy_id) == ENTRY-POLICY:
            res = True
    return res

A: Yes! As opposed to the way it is done in python, you always need to supply a default value when accessing elements in a dictionary in eopsin. The reason is that None is not always of the correct type, so always supply a default value of the correct type. In this case the type is a dictionary from bytestring to integers, you can for example supply {b"":0}.

[while0x1]

so in that instance to check for the existance of ENTRY-POLICY the correct should be txo.value.get(token.policy_id, {b"": 0}) ?

[nielstron]

You need to check that there is any token being spent with the ENTRY-POLICY id. txo.value gives you a map from policy id to a map from tokenname to how much of it is being spent. Make sure that your python code is valid python code first and eopsin compile your program to get additional hints from the compiler!

[nielstron]

Q: for getting the ada being spent by the contract ->

own_utxo = own_spent_utxo(ctx.tx_info.inputs, purpose)
spendingvalue = own_utxo.value.get...

A: In this case, we need to access the policy id and token name of ADA. It is defined in the plutus docs that ADA/Lovelace always has the empty bytestring as policy id and token name. So we use own_utxo.value.get(b"", {b"":0}).get(b"",0). Note how the default value has to match the expected return type!