Better assertion message for invariant predicates of 2 variables

[tzaffi]

Problem

As you can see in the full printout below, when we call invariant.validates() on an invariant that was defined by a 2-variable predicate, we get an inscrutable expected description.

This invariant was defined via:

predicate = (lambda args, actual: "PASSE" == actual if args[0] else True)
name = 'DryRunProperty.status' 
invariant = Invariant(predicate, name=name)

and spat out the following msg:

<<<<<<<<<<<Invariant for 'DryRunProperty.status' failed for for args (2,): actual is [PASS] BUT expected [<function test_exercises.<locals>.<lambda> at 0x1082557e0>]>>>>>>>>>>>

Full error:

E           AssertionError: ===============
E               <<<<<<<<<<<Invariant for 'DryRunProperty.status' failed for for args (2,): actual is [PASS] BUT expected [<function test_exercises.<locals>.<lambda> at 0x1082557e0>]>>>>>>>>>>>
E               ===============
E               App Trace:
E                  step |   PC# |   L# | Teal              | Scratch   | Stack
E           --------+-------+------+-------------------+-----------+----------------------
E                 1 |     1 |    1 | #pragma version 6 |           | []
E                 2 |     2 |    2 | arg_0             |           | [0x0000000000000002]
E                 3 |     3 |    3 | btoi              |           | [2]
E                 4 |     7 |    6 | label1:           |           | [2]
E                 5 |     9 |    7 | store 0           | 0->2      | []
E                 6 |    11 |    8 | load 0            |           | [2]
E                 7 |    13 |    9 | pushint 2         |           | [2, 2]
E                 8 |    14 |   10 | exp               |           | [4]
E                 9 |     6 |    4 | callsub label1    |           | [4]
E                10 |    15 |   11 | retsub            |           | [4]
E               ===============
E               MODE: ExecutionMode.Signature
E               TOTAL COST: None
E               ===============
E               FINAL MESSAGE: PASS
E               ===============
E               Messages: ['PASS']
E               Logs: []
E               ===============
E               -----BlackBoxResult(steps_executed=10)-----
E               TOTAL STEPS: 10
E               FINAL STACK: [4]
E               FINAL STACK TOP: 4
E               MAX STACK HEIGHT: 2
E               FINAL SCRATCH: {0: 2}
E               SLOTS USED: [0]
E               FINAL AS ROW: {'steps': 10, ' top_of_stack': 4, 'max_stack_height': 2, 's@000': 2}
E               ===============
E               Global Delta:
E               []
E               ===============
E               Local Delta:
E               []
E               ===============
E               TXN AS ROW: {' Run': 1, ' cost': None, ' last_log': '`None', ' final_message': 'PASS', ' Status': 'PASS', 'steps': 10, ' top_of_stack': 4, 'max_stack_height': 2, 's@000': 2, 'Arg_00': 2}
E               ===============
E               <<<<<<<<<<<Invariant for 'DryRunProperty.status' failed for for args (2,): actual is [PASS] BUT expected [<function test_exercises.<locals>.<lambda> at 0x1082557e0>]>>>>>>>>>>>
E               ===============

[tzaffi]

Being addressed in #21

[tzaffi]

Resolved in #21 and available in 🐕