Cannot calculate suggested fees for inner transactions

General
9

I have made an gh issue and I think it clarified a few things for me as well.

The fee parameter (fee-per-byte) returned by the API is indeed correct (it is zero until network congestion occurs). The fee-per-byte is NOT a problem.

Is this a bug?

As stated above, suggested_params.min_fee has no effect when passed into a transaction contructor (sp=suggested_params).

The reason for this is because py-algorand-sdk uses a constant (constants.min_txn_fee) instead of the min-fee which is stored in the object (sp.min_fee).

if not sp.flat_fee:
    self.fee = max(
        self.estimate_size() * self.fee, constants.min_txn_fee
    )

I do not know if this is intentional or not, but it does make it so that the user cannot specify the minimum fee - and you need to use a flat fee instead.

How to calculate fees for inner transactions

For the time-being, if:

  • You have inner transactions (with fees paid by the user)
  • You want to make sure that your transactions adapt to increased fee-per-byte (due to network congestion)
  • You want it to work with specific number of inner transactions

Then you need to do something like the following:

# Get txn info

num_bytes = transaction.ApplicationNoOpTxn(
    sender=sender.address,
    sp=client.suggested_params(),
    index=application_id,
    app_args=['return_to_sender']
).estimate_size()
num_txns = 5 # hard coded (1 outer + 4 inner)

# Calculate fee & get suggested params

suggusted_params = client.suggested_params()
suggusted_params.fee = max(
    num_bytes * suggested_params.fee,     # fee-per-byte
    num_txns * suggusted_params.min_fee  # min-fee
)
suggusted_params.flat_fee = True

# Construct transaction

txn = transaction.ApplicationNoOpTxn(
    sender=sender.address,
    sp=suggusted_params,
    index=application_id,
    app_args=['return_to_sender']
)

# ... 
# sign and send

(I have not tested)

Otherwise, for 99% of cases, the simple answers will work fine.

I would be convenient to use suggested_parameters.min_fee though.