Plone Deprecation Guide

Introduction

This document describes rationales, configuration and best practices of deprecations in Plone, Zope and Python. It is meant as a styleguide on how to apply deprecations in Plone core packages. It also has a value as a general overview on how to deprecate in Python.

Why Deprecation

At some point we

  • need to get rid of old code,
  • want to unify api style (consistent api),
  • fix typos in namings,
  • move code around (inside package or to another package).

While refactoring code, moving modules, functions, classes and methods is often needed. To not break third party code imports from the old place or usage of old functions/ methods must work for while. Deprecated methods are usually removed with the next major release of Plone. Following the semantic versioning guideline is recommended.

Help Programmers, No annoyance

Deprecation has to support the consumers of the code - the programmers using it. From their point of view, Plone core code is an API to them. Any change is annoying to them anyway, but they feel better if deprecation warnings are telling them what to do.

Deprecations must always log at level warning and have to answers the question:

"Why is the code gone from the old place? What to do instead?"

A short message is enough., i.e.:

  • "Replaced by new API xyz, found at abc.cde".,
  • "Moved to xyz, because of abc.",
  • "Name had a typo, new name is "xyz".

All logging has to be done once, i.e. on first usage or first import. It must not flood the logs.

Use Cases

Renaming
We may want to rename classes, methods, functions or global or class variables in order to get a more consistent api or because of a typo, etc. We never just rename, we always provide a deprecated version logging a verbose deprecation warning with information where to import from in future.
Moving a module, class, function, etc to another place
For some reason, i.e. merging packages, consistent api or resolving cirular import problems, we need to move code around. When imported from the old place it logs a verbose deprecation warning with information where to import from in future.
Deprecation of a whole package

A whole package (folder with __init__.py)

  • all imports still working, logging deprecation warnings on first import
  • ZCML still exists, but is empty (or includes the zcml from the new place if theres no auto import (i.e. for meta.zcml).
Deprecation of a whole python egg
We will provide a last major release with no 'real' code, only backward compatible (bbb) imports of public API are provided. This will be done the way described above for a whole package. The README clearly states why it was moved and where to find the code now.
Deprecation of a GenericSetup profile
They may got renamed for consistency or are superfluos after an update. Code does not need to break to support this.

Enable Deprecation Warnings

Zope

Zope does configure logging and warnings, so the steps below (under section Python) are not needed.

Using plone.recipe.zope2instance add the option deprecation-warnings = on to the buildouts [instance] section.

[buildout]
parts = instance

[instance]
recipe = plone.recipe.zope2instance
...
deprecation-warnings = on
...

This just sets a configuration option in zope.conf.

Without the recipe this can be set manually as well: In zope.conf custom filters for warnings can be defined.

...
<warnfilter>
    action always
    category exceptions.DeprecationWarning
</warnfilter>
...

Python

Enable Warnings

Warnings are written to stderr by default, but DeprecationWarning output is surpressed by default.

Output can be enabled by starting the Python interpreter with the -W [all|module|once] option.

It is possible to enable output in code too:

import warnings
warnings.simplefilter("module")
Configure Logging

Once output is enabled it is possible to redirect warnings to the logger:

import logging
logging.captureWarnings(True)

Running tests

In Plone tests deprecation warnings are not shown by default. The zope.conf setting is not taken into account.

In order to enable deprecation warnings, the Python way with the -W command option must to be used.

Given youre using a modern buildout with virtualenv as recommended, the call looks like so:

./bin/python -W module ./bin/test

Deprecation Best Practice

Vanilla Deprecation Messages

Python offers a built-in DeprecationWarning which can be issued using standard libraries warnings module.

For details read the official documentation about warnings.

In short it works like so

import warnings
warnings.warn('deprecated', DeprecationWarning)

Moving Whole Modules

Given a package old.pkg with a module foo.py need to be moved to a package new.pkg as bar.py.

zope.deprecation offers a helper.

  1. Move the foo.py as bar.py to the new.pkg.
  2. At the old place create a new foo.py and add to it
from zope.deprecation import moved
moved('new.pkg.bar', 'Version 2.0')

Now you can still import the namespace from bar at the old place, but get a deprecation warning:

DeprecationWarning: old.pkg.foo has moved to new.pkg.bar. Import of old.pkg.foo will become unsupported in Version 2.0

Moving Whole Packages

This is the same as moving a module, just create for each module a file.

Deprecating methods and properties

You can use the @deprecate decorator from zope.deprecation to deprecate methods in a module:

from zope.deprecation import deprecate

@deprecate('Old method is no longer supported, use new_method instead.')
def old_method():
    return 'some value'

The deprecated wrapper method is for deprecating properties:

from zope.deprecation import deprecated

foo = None
foo = deprecated(foo, 'foo is no more, use bar instead')

Moving functions and classes

Given we have a Python file at old/foo/bar.py and want to move some classes or functions to new/baz/baaz.py.

Here zope.deferredimport offers a deprecation helper. It also avoids circular imports on initialization time.

import zope.deferredimport
zope.deferredimport.initialize()

zope.deferredimport.deprecated(
    "Import from new.baz.baaz instead",
    SomeOldClass='new.baz:baaz.SomeMovedClass',
    some_old_function='new.baz:baaz.some_moved_function',
)

def some_function_which_is_not_touched_at_all():
    pass

Deprecating a GenericSetup profile

Starting with GenericSetup 1.8.2 (part of Plone > 5.0.2) the post_handler attribute in ZCML can be used to call a function after the profile was applied. We use this feature to issue a warning.

First we register the same profile twice. Under the new name and under the old name:

<genericsetup:registerProfile
    name="default"
    title="My Fance Package"
    directory="profiles/default"
    description="..."
    provides="Products.GenericSetup.interfaces.EXTENSION"
    />

<genericsetup:registerProfile
    name="some_confusing_name"
    title="My Fance Package (deprecated)"
    directory="profiles/some_confusing_name"
    description="... (use profile default instaed)"
    provides="Products.GenericSetup.interfaces.EXTENSION"
    post_handler=".setuphandlers.deprecate_profile_some_confusing_name"
    />

And in setuphandlers.py add a function:

import warnings

def deprecate_profile_some_confusing_name(tool):
    warnings.warn(
        'The profile with id "some_confusing_name" was renamed to "default".',
        DeprecationWarning
    )