add an OP_RETURN output to a PSBT

op-return-builder.py

#!/usr/bin/python3

'''
this script takes in an PSBT and adds an 
OP_RETURN output (OP_RETURN OP_PUSHDATA1 <len(message)> <message>)
with the chosen message payload

usage:
  interactive:
    ./op-return-builder.py
  static:
    ./op-return-builder.py <base64 encoded PSBT> <message (80 char max)>
'''

import sys
from embit import compact
from embit.psbt import PSBT, OutputScope
from embit.script import Script

class OPCODES:
    OP_RETURN = 106
    OP_PUSHDATA1 = 76

if len(sys.argv) == 3:
  psbt_string = sys.argv[1]
  opreturn_string = sys.argv[2]
else:
  psbt_string = input("Paste your base64 encoded PSBT below:\n")
  opreturn_string = input("\nWrite your message: ")

psbt = PSBT.from_base64(psbt_string)

raw_payload_data = opreturn_string.encode()

data = (compact.to_bytes(OPCODES.OP_RETURN) + 
    compact.to_bytes(OPCODES.OP_PUSHDATA1) + 
    compact.to_bytes(len(raw_payload_data)) +
    raw_payload_data)
    
script = Script(data)
output = OutputScope()
output.script_pubkey = script
output.value = 0
psbt.outputs.append(output)

print("\nModified PSBT:")
print(psbt.to_string())