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Standards

The Charm development best practices are incorporated by reference.

Definitions

Breaking change

A breaking change is any change that results in a deliverable that can not be swaped by its previous version seamlessly. In the context of charms, the following are considered breaking changes:

  • Dropping or renaming a configuration or integration
  • Adding a new required configuration or integration
  • Removing or changing an existing action (e.g., introducing a new required parameter)
  • Introducing or modifying the structure of a secret
  • Changing to an incompatible workload version
  • Introducing a requirement on a juju version which was not previously required

Tool

A tool is a project the team works on which isn't a charm, such as a GitHub action.

Unit Tests

These are tests that cover charm/ service functions to ensure that given a specific context and mocked interfaces, the function returns the expected output. Tests could cover more than one function if some of them don't include business logic. These tests shouldn't be functional, they just ensure that the code is doing what it's supposed to do. A test that requires too many mocks indicates the design needs to be improved to reduce coupling.

Integration Tests

Integration tests ensure that the charm integrated with its dependencies will behave properly. It doesn't test the code in a production like environment, meaning that we don't connect to environments with equivalent resources and production like datasets. The integration tests will spawn specific dependencies that could differ from the production one (e.g using localstack instead of openstack, sqlite instead of a production grade database, ...). These tests can be functional (they ensure that the features provided by the charm are working as intended) or focus on checking an abstracted interface. They don't necessarily need to ensure that the API/Service/CLI they're interacting with are working as intended.

End To End Tests

These are simulated user scenarios running on an environment as close as possible to production. They ususally run on a staging environment with preexisting data, and interact with the charm as a user would (ideally using the juju cli). These are functional tests and ensure that the charm is working as intended in the condition close to production in order to detect issues related to this environment (ressources, pre-existing condition, migrations, ...).

Smoke Tests

These are functional tests for business critical use cases used to ensure that the most critical features are still working after a production deployment. The aim is to ensure that a deployment didn't impact business continuity and detect potential defects immediately after a production release.

Programming Languages and Frameworks

If we develop in many different programming languages and frameworks, the maintenance cost increases, only those with knowledge of a given programming language or framework can work on code bases in that programming language and best practices and standards are difficult to set across the team.

We are a team primarily focused on developing charms, and we should follow the recommended tools and best practices of the ecosystem we work within (and help to improve them where appropriate). As a result, the primary programming language we work in is Python, and the primary framework we use to develop charms with is the Operator Framework.

We will also be working on older charms which may be written in older frameworks such as the Reactive Framework. In some cases we'll work on converting those to the Operator Framework but for small bug fixes or changes this may not be the case.

As a team we'll also be exposed to a much lesser extent to other programming languages as a result of working with specific applications that we deploy on Juju, or via small changes to Juju itself, which is written in Go. We also encourage exploration and experimentation in a range of programming languages depending on the interest of the individual as part of using Canonical's annual training budget.

Charm Ubuntu and Python Version

Using inconsistent minor Python version for development, CI and production means that, even if tests pass during development and CI, the charm may not work in production.

When charms are running in production, the version of Python available to them is dictated by the version of Ubuntu that it is running on. This is configured in the charmcraft.yaml file under the bases.build-on and bases.run-on keys. The CI should be configured to use the same version of Ubuntu as configured under the charmcraft.yaml bases.run-on key. It is recommended that local development be done on the version of Python that is shipped with the Ubuntu version defined in charmcraft.yaml bases.run-on. The unit and integration tests should be run on the same minor Python version as is shipped with the the OS as configured under the charmcraft.yaml bases.run-on key. With tox, for Ubuntu 22.04, this can be done using:

[testenv]
basepython = python3.10

This ensures that the tests are run on the same Python version as the charm will be running in production, catching any issues related to mismatched Python versions.

Downloading Binaries

Downloading binaries is only permitted from what are classed as trusted sources. These are:

These sources are considered trusted because we are confident that we understand the way in which they're built, and the security commitments for packages/snaps produced by them. At any point we can rerun our builds and know that we will pull in the latest versions of these artefacts, and that in the case of the first two they will include security updates that Canonical stands behind.

Downloading binaries from any other sources and including them in our charms or ROCKs exposes our end users to potential security issues either now or in the future. We don't know the manner in which the binaries were built and don't have confidence that they will be updated in the future in case of security vulnerabilities that affect them.

As such, we should ensure that we’re only downloading from either the trusted sources above, or we're downloading the source instead, verifying that download where possible, and then building that code from source on infrastructure we trust (e.g. Launchpad builders or GitHub self-hosted runners).

Any exception to this should be specifically noted/documented and approved by IS Charms managers.

CI-CD

The team maintains a number of repositories which are generally quite similar. If each of the repositories maintains its own workflows, it is difficult to introduce new tools across the repositories owned by the team. Additionally, if shared workflows are unreliable, the team is unable to consistently merge changes and have confidence in the CI-CD.

Use operator-workflows for the CI-CD of any repositories owned by the team. Even if the repository is not a charm, such as a GitHub action, use it to the extent possible. If a repository has unique requirements for certain workflows, use the operator-workflows as much as possible. Consider proposing changes to operator-workflows instead of doing something custom in a repository. Also consider whether to make partial use of workflows in operator-workflows and add anything that can't be done with operator-workflows to the individual repository, if the CI-CD requirements are unique to a repository.

For any changes to operator-workflows, add tests just like we expect for any other repository.

Using shared workflows will make it easy to evolve the workflows used by all our repositories and roll out new tooling and checks across them. It will also avoid duplication in many repositories.

Adding tests to operator-workflows will ensure stability of the workflows and provide examples for how to use them.

Random Values

While creating tests, sometimes you need to assign values to variables or parameters in order to simulate a user behavior, for example. In this case, instead of using constants or fixed values, consider using random ones generated by secrets.token_hex().

Reasons:

  • If you use the same fixed values in your tests every time, your tests may pass even if there are underlying issues with your code. This can lead to false positives and make it difficult to identify and fix real issues in your code.
  • Using random values generated by secrets.token_hex() can help to prevent collisions or conflicts between test data.
  • In the case of sensitive data, if you use fixed values in your tests, there is a risk that may be exposed or leaked, especially if your tests are run in a shared environment.

Example:

from secrets import token_hex

email = token_hex(16)

File Encoding

If file encoding is not specified when interacting with a file, the default value for the operating sytem is used. The default varies across operating systems reducing the portability of code that does not specify a encoding explicitly. See: https://peps.python.org/pep-0597/

For any file operations, specify the utf-8 encoding where possible. For example:

with open(..., encoding="utf-8") as file:
    ...

from pathlib import Path

Path(...).read_text(encoding="utf-8")
Path(...).write_text(..., encoding="utf-8")

This ensures that the code we write is portable across operating systems.

Repository Setup

The repositories that store the source code for our charms are critical to the ongoing development of our charms. They also enforce team policies around code review and ensure business continuity. If the repository is setup poorly, it exposes our team and Canonical to operational risks.

  • GitHub should be used for charm source code and issue tracking.
  • The repository should be publicly accessible.
  • The is-charms team is added as maintainers.
  • Management and director of the team that owns the charm are added as admins on the repository.
  • Branches are auto-deleted after merging.
  • The only option for merging PRs is using a squash commit.
  • The default branch is called main.
  • The default branch is protected and can only be changed using PRs.
  • The number of approvers for PRs is 2.
  • Approvals are not dismissed on new commits.
  • PRs can only be merged if all checks pass.
  • Bypassing of the rules is disabled.
  • Commits must be signed.
  • Automated secret scanning must be enabled.

The above configuration ensures our team processes around changes are enforced and provides access to the repository even if some team members are unavailable.

The repository will contain a CODEOWNERS file in its root to automatically add the is-charms team as reviewer

*       @canonical/is-charms

PR comments and requests for changes

The team uses Github for reviewing changes in the codebase and integrating them within our projects. A reviewer can comment and give feedback that potentially leads to new changes in the submitted code. Github allows you to request changes on a PR, meaning it cannot be merged until the changes are accepted. As the team is distributed, requesting changes as a default slows down merges and requires the reviewer to approve again. Even with sufficient approvals, a PR cannot be merged unless the requestor accepts the changes, which might be trivial.

Our team favors commenting in the usual case and resorting to requesting changes only if there is something problematic. It is adviseable to comment and let the engineer take action rather than request changes and block the PR. The preferred way is to comment instead. Note that committing new changes will reset approvals either way and approvals need to be collected again, which is the expected behavior and part of our workflow. Please note we expect the engineer that raised the PR to deal with the comments in good faith, i.e., not just mark them as resolved when they are not really resolved for the purpose of being able to merge the PR. The engineer that raised the PR is responsible for closing the comments once they are tackled.

With commenting rather than requesting changes, an engineer gives feedback and still allows the team (based on approvals) to decide the way forward - a change might make it in a different PR or the change might not be desireable in the end. The best approach is having as much feedback from as many engineers as possible to be able to reach a sound decision.

Failing Status Checks

The team uses GitHub actions to run CI which includes automated status checks to verify the code works as expected. If PRs are merged with failing status checks, there are potential bugs in the code which may lead to downtime or other operational issues or a shared tool not working as expected.

PRs can only be merged if all status checks pass. Some exceptions could include:

  • If the team has recently taken over a new charm and changes are urgently needed and the test suit of the existing charm is failing for spurious reasons.
  • A tool we depend on is not working, no previous working version is available and a change is urgently needed to fix a critical issue in production.

Even in the above cases it is not clear whether it is reasonable to merge a PR with failing checks due to the high risks of ignoring failing checks that have been adopted by the team. Judgement is required in these cases weighing the risks of introducing bugs with the urgency and impact of the underlying need to land the change.

Alternatives to merging the PR with failing status checks include:

  • Change the code to fix the problem.
  • Disable the status check (e.g., mark the test as xfail). This should not be done lightly as the value the status check provides to the team is lost.
  • Wait for an upstream fix for the issue.

If it is deemed that a change should land despite a failing status check, the following artifacts should be added to the PR:

  • If the status check can be run in another way, e.g. locally, a copy of the run (e.g., a copy of the terminal output) including the git commit SHA which is being approved and the status check passing
  • The reason why the PR needs to be merged without the status check passing (e.g., because the fix is needed in production and the status check is failing due to problems with GitHub).

One of the repository admins should then be asked to review the artifacts and merge the PR after completing the review. The PR should be otherwise ready to be merged (e.g., has been approved).

Resolving the underlying reason the status check is failing should be high priority so that the team can rely on the automation again.

This will ensure that we minimise the number of bugs in our code and tooling.

Test Structure

Tests that are difficult to understand are of lower value because if they fail it is difficult to understand why they fail.

The docstring of a test has 3 sections, arrange, act and assert. Arrange explains the pre-conditions required for the test, act explains what steps the test performs and assert explains what the state must be after all actions are complete. If the description for a section is longer than one line, any additional lines are indented by the default indentation of the file. The test code is separated into blocks for each section. For example:

def test_something():
    """
    arrange: given 2 numbers
    act: when they are added
    assert: the result must be the sum of the 2 numbers. If a description spans
        multiple lines, lines after the first should be indented by the default
        spacing of the file to indicate that the description continues.
    """
    num_a = 1
    num_b = 2

    result = num_a + num_b

    assert result == 3

Whilst this standard is usually valuable, there are cases where it imposes unreasonable constraints. An example is complex functional tests that check multiple interactions that require individual arrange, act, assert blocks because they, for example, build on each other. In those cases, apply judgement keeping in mind the option of breaking up the test into multiple tests.

This structure makes it easy to understand what is required before test execution, how the test works and what it checks for in the end.

Test Exception Raised

Testing an exception being raised might not be enough as a type of exception can be raised due to multiple reasons. E.g., a function could raise ValueError for different arguments.

The property of the exception being raised should be asserted against as well. A common test would be the string representation of the exception. Using the following pytest example:

def something(a: int, b: bool):
  """Do something."""
  if a >= 5:
    raise ValueError("Argument a must be less than 5")
  if b:
    raise ValueError("Some other error message")
  # Rest of the function...


def test_something():
  """Test the something function."""
  with pytest.raises(ValueError) as err:
      something(a=10, b=False)
  assert "Argument a" in str(err.value)
  assert "less than 5" in str(err.value)

In the above example, the something function could have thrown a different ValueError related to the argument b. Testing the string representation of the ValueError ensures the proper type of error is being tested against.

Test Fixture

When declaring fixture functions and requesting them in the same Python module, there is a risk of accidentally using the fixture function declared in the global scope instead of requesting it in the test case function parameters.

To prevent this problem, it is recommended to add a "fixture" prefix or suffix to the fixture function name and use the name argument in the pytest.fixture decorator to name the fixture.

Here's an example of using the name argument in pytest.fixture:

import pytest

@pytest.fixture(name="app")
def app_fixture():
    return "app"

def test_my_fixture(app):
    assert app == "app"

Test Coverage

Unit tests check whether the intended functionality has been implemented. This is valuable to reduce bugs and checking whether any changes to the code break any features. The benefits of testing are roughly proportional to the test coverage percentage, the lower the coverage the higher the chances of bugs and the higher the risk of code changes.

The team has a coverage percentage which is the maximum of 85% and the current percentage on the default branch, usually main. Any code that is already covered on the default branch should not cease to be covered by new commits to main, such as through a pull request. Any coverage exclusion should have an explanatory comment, such as:

# Exclude from coverage since unit tests should not run as __main__
if __name__ == "__main__":  # pragma: no cover
   ...

To enforce this, the pyproject.toml file should include the following configuration:

[tool.coverage.report]
fail_under = <maximum of coverage % on main and 85%>

This value should be updated in each PR to reflect any increase in coverage compared to the main branch as a result of the PR.

This ensures a high coverage minimum and no coverage regression.

Type Hints

Python is a dynamic programming language that does not require type declarations. Without type information, arguments might be passed to functions that are not of the expected type (e.g., passing None where it is not expected), which leads to more bugs. It also makes it more difficult to know what functions accept as input and return as output.

Except when impractical, declare type hints on function parameters, return values and class and instance variables. Examples of when type hints might be impractical (not an exhaustive list):

  • dictionaries with many nested dictionaries,
  • decorator functions,
  • when making small changes or
  • contributions to projects not owned by the team.

To leverage the power of type hints, the following configuration snippet should be added to pyproject.toml. This helps the user during the linting process by ensuring that all functions, including tests, have type definitions and checks for any typing issues even if a function does not have explicit type hints on it.

[tool.mypy]
check_untyped_defs = true
disallow_untyped_defs = true

[[tool.mypy.overrides]]
module = "tests.*"
disallow_untyped_defs = false

The type hints should be checked with mypy. More information on type hints can be found here: PEP 484.

This will help users know what functions expect as parameters and return and catches more bugs earlier.

Handling Typing Issues with python-libjuju

In tests and elsewhere when interacting with python-libjuju, it is a frequent requirement to check whether certain attributes are None. Doing this in many tests reduces the readability of the code.

Instead of putting assert ops_test.model in individual tests, write a fixture:

@pytest_asyncio.fixture(scope="module", name="model")
async def model_fixture(ops_test: pytest_operator.plugin.OpsTest) -> ops.model.Model:
    """The current test model."""
    assert ops_test.model
    return ops_test.model

Instead of putting assert hasattr(app, "units"), write a fixture:

@pytest_asyncio.fixture(scope="function", name="units")
async def units_fixture(app: ops.model.Application) -> list[ops.model.Unit]:
    """The current test unit."""
    assert hasattr(app, "units")
    return app.units

This reduces code duplication which increases the readability of the tests.

Static Code Analysis

There are many potential problems with code that can be spotted based on analysing source code without executing it, such as mismatches in type expectations. Additionally, code formatting discussions during PRs can be cumbersome and take up a lot of time.

The following automated static code analysis tools should be used locally and enforced through the CI system:

  • black for code formatting
  • isort for import sorting
    • line length of 99
    • black profile
  • flake8 for pythonic code style
    • refer to indico pyproject.toml
    • use the following additional plugins:
      • flake8-docstrings
      • flake8-docstrings-complete
      • flake8-test-docs
      • flake8-copyright
      • flake8-builtins
      • pyproject-flake8
      • pep8-naming for additional configurations
  • bandit for security checks
  • codespell for spelling problems
  • woke for inclusive language
  • prettier for JSON and YAML formatting
  • mypy for type checks
  • pylint for further python code style checks

Note:

  • Disabling checks should be the last resort, alternatives such as refactoring the code should be considered first. For example, instead of disabling the too-many-arguments pylint rule, consider grouping the arguments, e.g., using a typing.NamedTuple.

  • When disabling a rule, if the tool allows for it, use the name of the rule rather than the code. For example, for pylint, always use the name of the rule (like too-many-arguments) rather than the code. This makes it easier for readers to know which rule is being disabled and potentially why.

  • If a rule is disabled, a comment should to be included above the line disabling the rule explaining why the rule is disabled. This will mean that future readers don't have to guess why it was disabled and can also consider whether the disable can be removed. For rules tied to import, it may be better to comment on each usage rather than on the import. Note that comments for import must be added at the start of an import section to prevent the formatter from messing up the import sections:

    # Comment explaining why subprocess is imported.
import logging
import os
import subprocess  # nosec B404
import time

  • Disabling should be done as specifically as possible. That means, disabling the narrowest rule possible on the narrowest section of code. For example, instead of disabling a rule entirely, disable it on a file. Instead of disabling a rule for a file, disable it for just a line of code. The preferred way is to disable on a line of code:

    <code>  # disable rule

    If a rule needs to be disabled for a section, re-enable it as soon as possible:

    # disable rule
<code>
# enable rule

This ensures consistency across our projects, catches many potential bugs before code is deployed and simplifies PRs.

Source Code Documentation

  • This section of the documentation applies to charm repositories.

In order to make PRs easier to review, lazydocs is used to generate source code documentation from Python docstrings. It is recommended to install pre-commit hooks to run tox src-docs testing environment.

The contents of tox src-docs testing environment are as follows:

[testenv:src-docs]
allowlist_externals=sh
description = Generate documentation for src
deps =
    lazydocs
    -r{toxinidir}/requirements.txt
commands =
    ; can't run lazydocs directly due to needing to run it on src/* which
    ; produces an invocation error in tox
    sh generate-src-docs.sh

The environment above requires generate-src-docs.sh to be present at the root directory. The contents of generate-src-docs.sh are as follows:

#!/usr/bin/env bash

# Copyright 2023 Canonical Ltd.
# See LICENSE file for licensing details.

rm -rf src-docs # remove deleted class docs that persist
lazydocs --no-watermark --output-path src-docs src/*

The entire workflow can be triggered via git pre-commit hook. Run the following command at the root directory of the git repository to install the pre-commit hook.

echo -e "tox -e src-docs\ngit add src-docs\n" > .git/hooks/pre-commit
chmod +x .git/hooks/pre-commit

On a successful commit, the markdown documentation is generated under src-docs directory.

Function and Method Ordering

Without a logical order, it can be difficult to follow modules and classes as the number of functions or methods on them grow increasing the maintenance burden of the code.

Functions should be ordered according to the "step-down" rule. This means that a module should be readable from top to bottom, with functions ordered by level of abstraction, from general to specific. A calling function should always be above the called function. Functions should also be grouped together logically. If functions have a similar purpose, they should be grouped together.

On classes, the __init__ method should come first followed by any other factory methods, such as from_charm. The rest of the methods on a class should be ordered similar to the guidance for function ordering on modules.

This will make it easier for readers to understand the code reducing the maintenance cost.

Non Compliant Code

Standards and best practices evolve over time which means that code already written may not comply with a new standard. If this is wide spread, it can lead to a culture of ignoring the standard and can degreade the value of team standards.

At a minimum, any code changed in a pull request complies with team standards. The team values initiative updating code to comply with new standards and recognises that this needs to be balanced with other priorities, such as delivering new features of fixing bugs. If possible, standards should be enforced automatically by the CI system.

This ensures that:

  1. code under active development is likely to be compliant with team standards,
  2. less time is spent updating code that doesn't need to be updated frequently to new standards,
  3. it encourages the team to go out of their way to implement any new standards as individuals see value in doing so and
  4. compliance is enforced using CI where possible.

Publishing to a channel

Charms published to tracks different from latest should guarantee seemlessly upgrades between revisions, that is, revisions should not introduce breaking changes.