2. Change Management in Iris from the User’s perspective

As Iris changes, user code will need revising from time to time to keep it working, or to maintain best practice. At the very least, you are advised to review existing code to ensure it functions correctly with new releases.

Here, we define ways to make this as easy as possible.

To reduce code maintenance problems to an absolute minimum, Iris applies defined change management procedures to ensure that :

  • you can be confident that your code will still work with a future release
  • you will be aware of future incompatibility problems in advance
  • you can defer making code compatibility changes for some time, until it suits you

2.1. Key principles you can rely on

Iris code editions are published as defined version releases, with a given major and minor version number in the version name, “major.minor.xxx”, as explained in the releases section below.

  • Code that currently works should still work, and have the same results and effects, in any subsequent sub-release with the same major release number.
  • The only time we will make changes that can break existing code is at a major release.
  • At a major release, code that works and emits no deprecation warnings in the latest previous (minor) release should still work, and have the same results and effects.

What can possibly go wrong ?

If your code produces deprecation warnings, then it could behave differently, or fail, at the next major release.

2.2. User Actions : How you should respond to changes and releases

Checklist :

  • when a new testing or candidate version is announced

    if convenient, test your working legacy code against it and report any problems.

  • when a new minor version is released

    • review the ‘Whats New’ documentation to see if it introduces any deprecations that may affect you.
    • run your working legacy code and check for any deprecation warnings, indicating that modifications may be necessary at some point
    • when convenient :
      • review existing code for use of deprecated features
      • rewrite code to replace deprecated features
  • when a new major version is announced

    ensure your code runs, without producing deprecation warnings, in the previous minor release

  • when a new major version is released

    check for new deprecation warnings, as for a minor release

2.3. Details

The Iris change process aims to minimise the negative effects of change, by providing :

  • defined processes for release and change management
  • release versioning
  • backwards code compatibility through minor version releases
  • a way to ensure compatibility with a new major version release
  • deprecation notices and warnings to highlight all impending changes

Our practices are intended be compatible with the principles defined in the SemVer project .

Key concepts covered here:

2.3.1. Backwards compatibility

“Backwards-compatible” changes are those that leave any existing valid API usages unchanged (see terminology below). Only such changes may be included in minor releases.

The following are examples of backward-compatible changes :

  • changes to documentation
  • adding to a module : new submodules, functions, classes or properties
  • adding to a class : new methods or properties
  • adding to a function or method : new optional arguments or keywords

The following are examples of non- backward-compatible changes :

  • removing (which includes renaming) any public module or submodule

  • removing any public component
    : a module, class, method, function or

    data object property of a public API component

  • removing any property of a public object

  • removing an argument or keyword from a method or function

  • adding a required argument to a method or function

  • removing a keyword (even one that has no effect)

  • changing the effect of any particular combination of arguments and/or keywords

Note that it is also possible to modify the behaviour of an existing usage by making it depend on a newly-defined external control variable. This is effectively a change to the ‘default behaviour’ of a specific usage. Although this seems similar to adding a keyword, the cases where the new behaviour operates and where it does not are not distinguishable by a different code usage, which makes this somewhat dangerous. We do use this type of change, but any behaviour ‘mode’ controls of this sort are usually added as part of the iris.Future definition. See Usage of iris.FUTURE, below. Terminology : API, features, usages and behaviours

The API is the components of the iris module and its submodules which are “public” : In Python, by convention, this normally means everything whose name does not have a single leading underscore “_”. This applies to all public modules and their properties : submodules, contained public classes, data and properties, functions and methods. An exception is when a module or class defines an ‘__all__’ property : In that case, the public aspects are just the ones listed there.

Note: these are standard Python conventions, not specific to Iris. See: PEP8.

The Iris project considers all public API features as “supported”, which means that we will not change or remove them without deprecation, and will undertake to fix any bugs discovered. We do however make an important exception for the content of the ‘experimental’ module : APIs in iris.experimental module are published for initial evaluation and feedback, and can be revised or removed without warning at a subsequent release.

A “feature” of the API includes public objects as above, but may also be used more loosely to indicate a class or mode of behaviour, for example when a keyword has a specific value, like “interpolate(mode=’linear’)”.

A “usage” is any code referring to public API elements, for example :

  • print(iris.thing)
  • iris.submodule.call(arg1)
  • iris.module.call(arg1, arg2, *more_args)
  • iris.module.call(arg1, arg2, control=3)
  • x = iris.module.class(arg, key=4)

A “behaviour” is whatever Iris does when you invoke a particular API usage, encompassing both returned values and any side effects such as code state changes or data written to files.

As the above code examples are all public feature usages, they should therefore continue to work, with the same behaviours, at least until the next major version release.


What is the “same” behaviour, for backwards-compatibility purposes ?

Unfortunately, the guarantee to preserve “what Iris does” within a major version cannot ensure totally identical and repeatable behaviour for any possible usage, because this can also depend on the exact installed versions of all dependencies (i.e. the other Python modules and system libraries that Iris uses).

See Versions of Installed Dependencies.

Minor-release code changes are backwards-compatible, meaning that they must result in “the same” actions from Iris. Ultimately, however, this is only a statement of intent, as we need freedom to modify code with “no change” effects, yet any change to Iris code or dependencies could alter total behaviour in some respects : For instance, it could take more or less time or memory, produce results in a different sequence, or perform a calculation slightly differently (with possible small differences in floating point results).

As behaviour can only be tested in specific ways on a specific installation, so any given user installation may experience changes in behaviour, though hopefully always slight, with a minor release change. This applies to changes in the Iris minor release version, or a different version of some dependency; or other changes to the operating system environment.

2.3.2. Releases and Versions

Iris releases have a unique identifying version string, in the form “<major>.<minor>.<micro><extension>”, available to code as iris.__version__ .

This contains major and minor release numbers. The numbering and meaning of these are defined, following the SemVer project.

The essential aspects of the “<major>.<minor>.<micro><extension>” arrangement are :

  • “<major>”, “<minor>” and “<micro>” are all integers, thus version 2.12 is later than 2.2 (i.e. it is “two point twelve”, not “two point one two”).
  • “<major>.<minor>” denote the software release version.
  • A non-zero “<micro>” denotes a bugfix version, thus a release “X.Y.0” may be followed by “X.Y.1”, “X.Y.2” etc, which only differ by containing bugfixes. Any bugfix release supercedes its predecessors, and does not change any (valid) APIs or behaviour : hence, it is always advised to replace a given version with its latest bugfix successor, and there should be no reason not to.
  • “<extension>” is blank for formal releases. It used to indicate provisional software for testing : The version string in a development code branch is always labelled “-DEV”, and release candidates for testing during the release process are labelled “-rc1”, “-rc2” etc. For development code, the version number is that of the next release, which this code version is progressing towards, e.g. “1.2-DEV” for all code branches since the 1.1 release and intended for release in “1.2”.


Our use of “-<extension>” is typical, but does not follow strict SemVer principles.

The code for a specific release is identified by a git tag which is the version string : see Developer’s Guide section on releases. Major and Minor Releases

The term “release” refers both to a specific state of the Iris code, which we have assigned a given version string, and the act of defining it (i.e. we “release a release”).

According to SemVer principles, changes that alter the behaviour of existing code can only be made at a major release, i.e. when “X.0” is released following the last previous “(X-1).Y.Z”.

Minor releases, by contrast, consist of bugfixes, new features, and deprecations : Any valid exisiting code should be unaffected by these, so it will still run with the same results.

At a major release, only deprecated behaviours and APIs can be changed or removed.

2.3.3. Deprecations

A deprecation is issued when we decide that an existing feature needs to be removed or modified : We add notices to the documentation, and issue a runtime “Deprecation Warning” whenever the feature is used.

For a wider perspective, see : https://en.wikipedia.org/wiki/Deprecation. For the developer view of this, see Developer’s Guide section on deprecations.

Deprecation manages incompatible changes in a strictly controlled way. This allows APIs to evolve to the most effective form, even when that means that existing code could behave differently or fail : This is important because the freedom to remove features helps prevent the API becoming progressively cluttered, and modifying existing behaviours allows us to use the most natural code syntax for the most used features.

We can only remove features or change behaviours at a major release. Thus, we first deprecate the feature in a minor release, to provide adequate warning that existing code may need to be modified.

When we make a release that introduces a deprecation :

In most cases, we also provide detailed advice in the documentation and/or warning messages on how to replace existing usage with a ‘new’ way of doing things. In all cases, we must provide a transitional period where both old and new features are available :

  • the ‘old’ style works as it did before
  • any usage of the ‘old’ features will emit a warnings.WarningMessage message, noting that the feature is deprecated and what to use instead
  • the ‘new’ style can be adopted as soon as convenient

This is to warn users :

  • not to use the deprecated features in any new code, and
  • eventually to rewrite old code to use the newer or better alternatives Deprecated features support through the Release cycle

The whole point of a deprecation is that the feature continues to work, but with a warning, for some time before an unavoidable change occurs. When a version that introduces a deprecation is released, the effects are as follows:

  • code that may be affected by the proposed change will result in deprecation warnings
  • code that currently works will, however, continue to work unchanged, at least until the next major release
  • you can avoid all deprecation warnings by suitable changes to your code
  • code which uses no deprecated features, and thus produces no deprecation warnings, will continue to work unchanged even at a major release
  • code that generates deprecation warnings may cease to work at the next major release. Future options, iris.FUTURE

A special approach is needed where the replacement behaviour is not controlled by a distinct API usage.

When we extend an API, or add a new feature, we usually add a new method or keyword. In those cases, code using the new feature is clearly distinct from any previous (valid) usage, so this is relatively simple to manage. However, sometimes we really need to change the way an API works, without modifying or extending (i.e. complicating) the existing user interface. In that case, existing user code might sometimes have different behaviour with the new release, which we obviously need to avoid.

For example :

We might decide there is a more useful way of loading cubes from files of a particular input data format.

  • the user code usage is simply by calls to “iris.load”
  • the change is not a bugfix, as the old way isn’t actually “wrong”
  • we don’t want to add an extra keyword into all the relevant calls
  • we don’t see a longterm future for the existing behaviour : we expect everyone to adopt the new interpretation, eventually

For changes of this sort, the release will define a new boolean property of the iris.FUTURE object, as a control to select between the ‘old’ and ‘new’ behaviours, with values False=’old’ and True=’new’. See iris.Future for examples.

In these cases, as any “deprecated usage” is not clearly distinguishable in the form of the user code, it is especially important to take note of any deprecation messages appearing when legacy code runs.

Sequence of changes to `iris.FUTURE`

To allow user code to avoid unexpected any behavioural changes, the iris.Future controls follow a special management cycle, as follows (see also the relevant Developer Guide section):

At (minor) release “<X>.<Y>..”:
  • Changes to API:
    • the new behaviour is made available, alongside the old one
    • a new future option iris.FUTURE.<new_enable> is provided to switch between them.
    • the new option defaults to iris.FUTURE.<new_enable>=False, meaning the ‘old’ behaviour is the default.
    • when any relevant API call is made that invokes the old behaviour, a deprecation warning is emitted.
  • User actions:

    • If your code encounters the new deprecation warning, you should try enabling the new control option, and make any necessary rewrites to make it work. This will stop the deprecation warning appearing.
    • If you encounter problems making your code work with the new behaviour, and don’t have time to fix them, you should make your code explicitly turn off the option for now, i.e. :: iris.FUTURE.<new_enable> = False. This locks you into the old behaviour, but your code will continue to work, even beyond the next major release when the default behaviour will change (see on).
At (major) release “<X+1>.0...”:
  • Changes to API:
    • the control default is changed to iris.FUTURE.<new_enable>=True
    • the control property is itself deprecated, so that assigning to it now results in a deprecation warning.
    • when any affected API call is made, a deprecation warning is (still) emitted, if the old behaviour is in force. The “old” behaviour is, however, still available and functional.
  • User actions:

    • If your code is already using the “new” behaviour, it will now work without needing to set the Future option. You should remove the code which enables the option, as this will now emit a deprecation message. In the next major release, this would cause an error.
    • If your code is explicitly turning the option off, it will continue to work in the same way at this point, but obviously time is runnning out.
    • If your code is still using the old behaviour and not setting the control option at all, its behaviour might now have changed unexpectedly and you should review this.
At (major) release “<X+2>...”:
  • Changes to API:
    • the control property is removed
    • the “old” behaviour is removed Versions of Installed Dependencies

The dependencies of Iris (required and optional) are defined in Installing Iris.

Iris does not specify exact required versions for its dependencies, but it may specify a minimum acceptable version number. Iris is normally expected to be compatible with any version up to the latest current release.

When a new release of a dependency is found to cause problems, Iris may define the supported version more precisely, but this would be a temporary fix which should be removed in a later release.