Skip to main content

Unit Testing with PyTezos

There are multiple frameworks for testing Michelson contracts:

Each language also comes with their own testing features. The one for LIGO is still experimental, in the dev branch, but will be available soon.

Another alternative is to use Tezos's binary octez-client directly. There is also a new mockup mode that no longer requires a Tezos node to run.

This chapter aims to introduce the concept of unit testing and, especially, how to deal with testing Tezos smart contract using the PyTezos python framework.

Unit Testing

Just like any other programming language, writing LIGO code (to be compiled into Michelson) is not the only task of a developer; writing the tests is as important as writing the code.

Writing a test may take extra time, but some great benefits usually reward this effort:

  • a test helps to understand the code: a user or a new developer can easily understand the code behavior through testing. The test name should clearly describe what the code is checking, and the test itself shows how the code should be handled.
  • checking the good behavior of the written code. It benefits both the developer and the user of a smart contract: both want to be sure the smart contract behaves as it should.
  • It also makes the code robust for future modifications, i.e., code refactoring or new features. The tests make sure that the behavior has not changed, and if it did, it would clearly outline where.
  • Tests can easily be automated: they can be included in a CI/CD, which will run them after any push.

CI/CD (Continuous Integration / Continuous Delivery) is a method to frequently deliver apps to customers by introducing automation into the stages of app development. The main concepts attributed to CI/CD are continuous integration, continuous delivery, and continuous deployment.

There are different types of tests, as described below in the automated testing strategy.

FIGURE 1: Pyramide of Tests

Unit tests are the base of the pyramid and therefore the most important part.

Unit testing is performed at a very fine granularity by fully verifying the behavior of a portion of code or by partially isolating it from its dependencies. It is therefore simple to write and maintain them. In contrast, an integration test aims to verify that several components work well together.

To go further, take a look at Test-Driven Development (TDD), which is a development method emphasizing the writing of automated tests as a tool to guide the implementation of features.

LIGO does not yet provide a testing framework but one is currently in development and should be available soon. PyTezos makes it possible to test contracts written in different languages.

This chapter details how to write Michelson code unit tests in Python.

Two modules are needed:

  • the standard unittest module: used to write and run unit tests in Python
  • the PyTezos module: used to call the entrypoints of a smart contract, without deploying the smart contract. This module is an interesting option because it is well-maintained and easy to use.

The execution time or CPU time of a given task is defined as the time spent by the system executing that task. Cost can be seen as the complexity and time required for the developer to write a test.

PyTezos Installation


  • Python 3
  • Text editor
  • Cryptographic dependencies, detailed here link.


Creation of a virtual environment

A virtual environment is a self-contained Python installation, separated from the global Python installation. It contains its own modules. Hence, it is most useful when a specific version of a module is needed, without affecting the other modules.

Run this command to create a virtual environment:

python3 -m venv /path/to/env

Activating the environment

source /path/to/env/bin/activate

Installation of the necessary python libraries

Installation of PyTezos:

(venv) pip install pytezos

Verification of the installation:

(venv) python -c "import pytezos"

If the command returns nothing, then the installation is successful.

You can find the official documentation here and all the versions on the project's Github in the Releases part.

The PyTezos version used for the following example is pytezos==3.1.0. You can check the version of your package with the pip freeze command. You can install a specific version if needed with pip install pytezos=={pytezos_version}.

Unittest (Python library)

Unittest is the standard unit testing framework for Python. Before writing a test for smart contracts with PyTezos, you need to know how to use the test library.

Let's see how Unittest works through a simple example.

Consider a python file with two functions: multiply and divide

def multiply(x, y):
return x * y

def divide(x, y):
if y != 0:
return x / y

To test these two functions, let's create a new test file beginning by test_ :

In this file you will need to:

  • import the unittest module
import unittest
  • import the file with the functions you want to test: import calculator
import unittest

import calculator
  • Create a test class that inherits from unittest.TestCase. You can name this class whatever you want but try to keep it descriptive.
import unittest
from unittestExample import calculator

class TestCalculator(unittest.TestCase):
pass #keyword which tells the class to do nothing
  • Write your tests. Keep in mind these few basic rules:
    • A test must check only one behavior at a time.
    • One test = one method.
    • No magic number: all the values used must be declared in variables, with explicit names.
    • There can be no useless variables to pass the test. If a variable can be removed without making the test fail, it must be removed.
    • The method name must be explicit. Anyone should understand what the test takes as input, what behavior is been checked and what result is expected.
    • A test can be divided into three parts (as implemented in the tests below):
      • GIVEN: input declarations, expected results
      • WHEN: the tested method is called with the declared inputs
      • THEN: assertions checks

With unittest, a test method must include test_, otherwise it will not be considered as a test.

class TestCalculator(unittest.TestCase):

def test_multiplying_10_and_5_should_return_(self):
x = 10
y = 5
expected_multiplication_result = 50
result = calculator.multiply(x, y)

self.assertEqual(result, expected_multiplication_result)

def test_dividing_7_and_3_should_return_a_floating_number_2_33333333(self):
x = 7
y = 3
expected_division_result = 2.3333333333333335

result = calculator.divide(x, y)

self.assertEqual(result, expected_division_result)

At the beginning of this chapter, several benefits of unit testing were raised. This simple test-suite gives a good example:

  • Reading the name of the test must give a clear idea about its code implementation. For instance, the second test shows that it is not a Euclidean division that is been implemented
  • The division method could be the Euclidean division. The test checks that it returns a float number and not an int. If another developer was to change it into an Euclidean division, the test would fail and instantly warn the developer of a breaking change.

Note that class names are by convention in CamelCase and test method names are in snake_case.

You can run your tests in the command line as follows:

python -m unittest test_calculator

This should return:

Ran 2 tests in 0.002s


Note that the command has executed all the tests in the file, but that it is only possible to execute some specific tests.

Indeed, the unittest module can be used from the command line to execute tests from modules, classes or even individual test methods:

python -m unittest test_calculator
python -m unittest test_calculator.TestCalculator
python -m unittest test_calculator.TestCalculator.test_sub

Testing a compiled smart contract with PyTezos

PyTezos library is a Python toolset for the Tezos blockchain. It includes work with keys, signatures, contracts, operations, RPC query builder, and a high-level interface for smart contract interaction.

This module is very interesting for for unit-testing smart-contracts:

  • It can directly test the Michelson code with a Python script without having to deploy it on a testnet.
  • Since it does not have to wait for transactions confirmations, the tests are fast to run.
  • It can simulate any execution context (sender, amount, storage, etc.)
  • It gives total control over the storage: each test has its own initial storage, and the output storage can be checked.
  • The tests are independent from one another. If the tests were run on a deployed smart contract, the initial storage of a test would be the output storage of the previous one.

In this section, we will test the entrypoints of a Michelson script for a smart contract that is compiled but not deployed.

For this we will need:

  • The Unittest library, which is the standard framework for writing tests in Python.
  • Two very useful classes from PyTezos:
    • ContractInterface allowing to interface with the entrypoints of a contract and interact with them.
    • MichelsonRuntimeError allowing to handle errors raised during execution.

To write the tests we will use the following template:

from unittest import TestCase, skip
from pytezos import MichelsonRuntimeError, ContractInterface

path_to_michelson_contract = "/path/to/"

class TestContract(TestCase):

def setUpClass(cls):
cls.myContract = ContractInterface.create_from(path_to_michelson_contract)

def test_description_1(self):

def test_description_2(self):

@skip("test 3 is not launched")
def test_description_3(self):

PyTezos requires the path to a file containing the Michelson code in path_to_michelson_contract.

To compile LIGO code into Michelson:

ligo compile contract file.ligo >

Remember to recompile after any modification of the contract.

Equivalence between Michelson types and Python

PyTezos allows you to interpret Michelson code, here are the equivalences:

List, Set[]
mutez, tezInterpreted as an Integer using .interpret()

For example if you want to check the content of a list in the storage you would write:

self.assertEqual(my_list_from_the_storage, [expected_value1, expected_value2, ...])

The way to get my_list_from_the_storage is detailed below in the examples.

Counter Contract Example

Here is an example of a Counter contract that handles an integer of counter value, an administrator address as storage and allows the administrator only to increment or decrement this counter.

type indiceStorage is record[
counter : int;
administrator : address;

type indiceEntrypoints is Increment of int | Decrement of int

type indiceFullReturn is list(operation) * indiceStorage

function increment(const param : int; const s : indiceStorage) : indiceFullReturn is
if Tezos.get_source () =/= s.administrator
then (failwith("administrator not recognized") : indiceFullReturn)
else ((nil : list (operation)), s with record [counter=s.counter + param])

function decrement(const param : int; const s : indiceStorage) : indiceFullReturn is
if Tezos.get_source () =/= s.administrator
then (failwith("administrator not recognized") : indiceFullReturn)
else ((nil : list (operation)), s with record [counter=s.counter - param])

function main(const ep : indiceEntrypoints; const store : indiceStorage) : indiceFullReturn is
case ep of [
| Increment(p) -> increment(p, store)
| Decrement(p) -> decrement(p, store)

Let's compile this contract and save the result in a Michelson file

ligo compile contract counter.ligo >

The file should contain:

{ parameter (or (int %decrement) (int %increment)) ;
storage (pair (address %administrator) (int %counter)) ;
code { UNPAIR ;
{ SWAP ;
DUG 2 ;
IF { DROP 2 ; PUSH string "administrator not recognized" ; FAILWITH }
{ SWAP ;
DUG 2 ;
NIL operation ;
PAIR } }
{ SWAP ;
DUG 2 ;
IF { DROP 2 ; PUSH string "administrator not recognized" ; FAILWITH }
{ SWAP ;
DUG 2 ;
NIL operation ;
PAIR } } } }

First, let's test the increment entrypoint if the user is the administrator.

Note that only the administrator is allowed to modify the storage and if anyone else tries to do it then the contract will return an error.

if Tezos.get_source () =/= s.administrator
then (failwith("administrator not recognized") : indiceFullReturn)

Test increment entrypoint

For instance, let's write a test that verifies that the storage is incremented by 5 when the administrator performs the Increment(5) action.

  • For this test, a faucet address tz1 has been assigned to the administrator, you can fund an address here.
  • In the setUpClass method we load the contract from the Michelson source code, stored in the file, with the ContractInterface.from_file() method.
  • Note that the name of the test accurately describes the test's intention.
from unittest import TestCase
from pytezos import MichelsonRuntimeError, ContractInterface

path_to_michelson_contract = ""
administrator = "tz1L738ifd66ah69PrmKAZzckvvHnbcSeqjf"

class TestCounterContract(TestCase):

def setUpClass(cls):
cls.counterContract = ContractInterface.from_file(path_to_michelson_contract)

def test_storage_counter_is_incremented_by_5_if_the_source_is_the_administrator(self):
storage = {"administrator": administrator, "counter": 0}
value = 5

result = self.counterContract.increment(value).interpret(storage=storage, source=administrator)

self.assertEqual(["counter"], 5)


  • That the storage has been initialized as a dictionary {} to respect the equivalence with the Michelson format.
  • The incremented value is an int.


  • From our contract self.counterContract we can call an entrypoint and its parameter with .increment(value).
  • Then we can add a context with .interpret() to specify the storage and the source (the original address that is sending the transaction).

THEN Finally we can check that the storage counter has been incremented by 5 with self.assertEqual(<actual_value>, <expected_value>).

Let's run the test:

python -m unittest test_counter.TestCounterContract.test_storage_counter_is_incremented_by_5_if_the_source_is_the_administrator
Ran 1 test in 0.011s


Test unauthorized user (MichelsonRuntimeError)

Now let's imagine that someone other than the administrator tries to modify the storage by incrementing or decrementing it.

Let's write a test to make sure that this user gets rejected.

First, let's add a new user alice at the beginning of the file with a different address from the administrator.

administrator = "tz1L738ifd66ah69PrmKAZzckvvHnbcSeqjf"
alice = "tz1LFuHW4Z9zsCwg1cgGTKU12WZAs27ZD14v"

Then following the first test, let's write the new test:

def test_should_failed_if_the_source_is_not_the_administrator(self):
with self.assertRaises(MichelsonRuntimeError) as administrator_error:
storage = {"administrator": administrator, "counter": 0}
value = 5

self.counterContract.increment(value).interpret(storage=storage, source=alice)

error_message = str(administrator_error.exception.args[-1].strip("\\").strip("'"))
self.assertEqual("administrator not recognized", error_message)
  • The line with self.assertRaises(MichelsonRuntimeError) as administrator_error: retrieves and stores the error in the administrator_error variable.
  • The administrator's address is still defined in the initial state of storage, but this time alice's address is specified in the context's source variable: source=alice.
  • Finally, the string message from the error is stored in the variable error_message and is compared with the original message written in the failwith() of the LIGO code.
python -m unittest test_counter.TestCounterContract.test_should_failed_if_the_source_is_not_the_administrator
Ran 1 test in 0.011s


Now it's your turn to write tests! Try to do the same thing with the entrypoint decrement. The goal of writing tests is to have optimal coverage of the whole code. This allows you to have a robust and high-quality code. Futur developers working on your code will thank you very much for that.