Dectate: Advanced Decorator Configuration System

Dectate is a Python library that lets you construct a decorator-based configuration system for frameworks. Configuration is associated with class objects. It supports configuration inheritance and overrides as well as conflict detection.

Using Dectate

Introduction

Dectate is a configuration system that can help you construct Python frameworks. A framework needs to record some information about the functions and classes that the user supplies. We call this process configuration.

Imagine for instance a framework that supports a certain kind of plugins. The user registers each plugin with a decorator:

from framework import plugin

@plugin(name="foo")
def foo_plugin(...):
   ...

Here the framework registers as a plugin the function foo_plugin under the name foo.

You can implement the plugin decorator as follows:

plugins = {}

class plugin(name):
    def __init__(self, name):
        self.name = name

    def __call__(self, f):
       plugins[self.name] = f

In the user application the user makes sure to import all modules that use the plugin decorator. As a result, the plugins dict contains the names as keys and the functions as values. Your framework can then use this information to do whatever you need to do.

There are a lot of examples of code configuration in frameworks. In a web framework for instance the user can declare routes and assemble middleware.

You may be okay constructing a framework with the simple decorator technique described above. But advanced frameworks need a lot more that the basic decorator system described above cannot offer. You may for instance want to allow the user to reuse configuration, override it, do more advanced error checking, and execute configuration in a particular order.

Dectate supports such advanced use cases. It was extracted from the Morepath web framework.

Features

Here are some features of Dectate:

  • Decorator-based configuration – users declare things by using Python decorators on functions and classes: we call these decorators directives, which issue configuration actions.
  • Dectate detects conflicts between configuration actions in user code and reports what pieces of code are in conflict.
  • Users can easily reuse and extend configuration: it’s just Python class inheritance.
  • Users can easily override configurations in subclasses.
  • You can compose configuration actions from other, simpler ones.
  • You can control the order in which configuration actions are executed. This is unrelated to where the user uses the directives in code. You do this by declaring dependencies between types of configuration actions, and by grouping configuration actions together.
  • You can declare exactly what objects are used by a type of configuration action to register the configuration – different types of actions can use different registries.
  • Unlike normal decorators, configuration actions aren’t performed immediately when a module is imported. Instead configuration actions are executed only when the user explicitly commits the configuration. This way, all configuration actions are known when they are performed.
  • Dectate-based decorators always return the function or class object that is decorated unchanged, which makes the code more predictable for a Python programmer – the user can use the decorated function or class directly in their Python code, just like any other.
  • Dectate-based configuration systems are themselves easily extensible with new directives and registries.
  • Dectate-based configuration systems can be queried. Dectate also provides the infrastructure to easily construct command-line tools for querying configuration.

Actions

In Dectate, the simple plugins example above looks like this:

import dectate

class PluginAction(dectate.Action):
    config = {
       'plugins': dict
    }
    def __init__(self, name):
        self.name = name

    def identifier(self, plugins):
        return self.name

    def perform(self, obj, plugins):
        plugins[self.name] = obj

We have formulated a configuration action that affects a plugins dict.

App classes

Configuration in Dectate is associated with special classes which derive from dectate.App. We also associate the action with it as a directive:

class PluginApp(dectate.App):
    plugin = dectate.directive(PluginAction)

Let’s use it now:

@PluginApp.plugin('a')
def f():
    pass # do something interesting

@PluginApp.plugin('b')
def g():
    pass # something else interesting

We have registered the function f on PluginApp. The name argument is 'a'. We’ve registered g under 'b'.

We can now commit the configuration for PluginApp:

dectate.commit(PluginApp)

Once the commit has successfully completed, we can take a look at the configuration:

>>> sorted(PluginApp.config.plugins.items())
[('a', <function f at ...>), ('b', <function g at ...>)]

What are the changes between this and the simple plugins example?

The main difference is that the plugin decorator is associated with a class and so is the resulting configuration, which gets stored as the plugins attribute of dectate.App.config. The other difference is that we provide an identifier method in the action definition. These differences support configuration reuse, conflicts, extension, overrides and isolation.

Reuse

You can reuse configuration by simply subclassing PluginApp:

class SubApp(PluginApp):
   pass

We commit both classes:

dectate.commit(PluginApp, SubApp)

SubClass now contains all the configuration declared for PluginApp:

>>> sorted(SubApp.config.plugins.items())
[('a', <function f at ...>), ('b', <function g at ...>)]

So class inheritance lets us reuse configuration, which allows extension and overrides, which we discuss below.

Conflicts

Consider this example:

class ConflictingApp(PluginApp):
    pass

@ConflictingApp.plugin('foo')
def f():
    pass

@ConflictingApp.plugin('foo')
def g():
    pass

Which function should be registered for foo, f or g? We should refuse to guess and instead raise an error that the configuration is in conflict. This is exactly what Dectate does:

>>> dectate.commit(ConflictingApp)
Traceback (most recent call last):
  ...
ConflictError: Conflict between:
 File "...", line 4
   @ConflictingApp.plugin('foo')
 File "...", line 8
   @ConflictingApp.plugin('foo')

As you can see, Dectate reports the lines in which the conflicting configurations occurs.

How does Dectate know that these configurations are in conflict? This is what the identifier method in our action definition did:

def identifier(self, plugins):
    return self.name

We say here that the configuration is uniquely identified by its name attribute. If two configurations exist with the same name, the configuration is considered to be in conflict.

Extension

When you subclass configuration, you can also extend SubApp with additional configuration actions:

@SubApp.plugin('c')
def h():
    pass # do something interesting

dectate.commit(PluginApp, SubApp)

SubApp now has the additional plugin c:

>>> sorted(SubApp.config.plugins.items())
[('a', <function f at ...>), ('b', <function g at ...>), ('c', <function h at ...>)]

But PluginApp is unaffected:

>>> sorted(PluginApp.config.plugins.items())
[('a', <function f at ...>), ('b', <function g at ...>)]

Overrides

What if you wanted to override a piece of configuration? You can do this in SubApp by simply reusing the same name:

@SubApp.plugin('a')
def x():
    pass

dectate.commit(PluginApp, SubApp)

In SubApp we now have changed the configuration for a to register the function x instead of f. If we had done this for MyApp this would have been a conflict, but doing so in a subclass lets you override configuration instead:

>>> sorted(SubApp.config.plugins.items())
[('a', <function x at ...>), ('b', <function g at ...>), ('c', <function h at ...>)]

But PluginApp still uses f:

>>> sorted(PluginApp.config.plugins.items())
[('a', <function f at ...>), ('b', <function g at ...>)]

Isolation

We have already seen in the inheritance and override examples that PluginApp is isolated from configuration extension and overrides done for SubApp. We can in fact entirely isolate configuration from each other.

We first set up a new base class with a directive, independently from everything before:

class PluginAction2(dectate.Action):
    config = {
       'plugins': dict
    }
    def __init__(self, name):
        self.name = name

    def identifier(self, plugins):
        return self.name

    def perform(self, obj, plugins):
        plugins[self.name] = obj

class BaseApp(dectate.App):
    plugin = dectate.directive(PluginAction2)

We don’t set up any configuration for BaseApp; it’s intended to be part of our framework. Now we create two subclasses:

class OneApp(BaseApp):
    pass

class TwoApp(BaseApp):
    pass

As you can see OneApp and TwoApp are completely isolated from each other; the only thing they share is a common BaseApp.

We register a plugin for OneApp:

@OneApp.plugin('a')
def f():
    pass

This won’t affect TwoApp in any way:

dectate.commit(OneApp, TwoApp)
>>> sorted(OneApp.config.plugins.items())
[('a', <function f at ...>)]
>>> sorted(TwoApp.config.plugins.items())
[]

OneApp and TwoApp are isolated, so configurations are independent, and cannot conflict or override.

The Anatomy of an Action

Let’s consider the plugin action in detail:

class PluginAction(dectate.Action):
    config = {
       'plugins': dict
    }
    def __init__(self, name):
        self.name = name

    def identifier(self, plugins):
        return self.name

    def perform(self, obj, plugins):
        plugins[self.name] = obj

What is going on here?

  • We implement a custom class called PluginAction that inherits from dectate.Action.
  • config (dectate.Action.config) specifies that this directive has a configuration effect on plugins. We declare that plugins is created using the dict factory, so our registry is a plain dictionary. You provide any factory function you like here.
  • __init__ specifies the parameters the directive should take and how to store them on the action object. You can use default parameters and such, but otherwise __init__ should be very simple and not do any registration or validation. That logic should be in perform.
  • identifier (dectate.Action.identifier()) takes the configuration objects specified by config as keyword arguments. It returns an immutable that is unique for this action. This is used to detect conflicts and determine how configurations override each other.
  • perform (dectate.Action.perform()) takes obj, which is the function or class that the decorator is used on, and the arguments specified in config. It should use obj and the information on self to configure the configuration objects. In this case we store obj under the key self.name in the plugins dict.

We then associate the action with a class as a directive:

class PluginApp(dectate.App):
    plugin = dectate.directive(PluginAction)

Once we have declared the directive for our framework we can tell programmers to use it.

Directives have absolutely no effect until commit is called, which we do with dectate.commit. This performs the actions and we can then find the result PluginApp.config (dectate.App.config).

The results are in PluginApp.config.plugins as we set this up with config in our PluginAction.

Depends

In some cases you want to make sure that one type of directive has been executed before the other – the configuration of the second type of directive depends on the former. You can make sure this happens by using the depends (dectate.Action.depends) class attribute.

First we set up a FooAction that registers into a foos dict:

class FooAction(dectate.Action):
    config = {
       'foos': dict
    }
    def __init__(self, name):
        self.name = name

    def identifier(self, foos):
        return self.name

    def perform(self, obj, foos):
        foos[self.name] = obj

Now we create a BarAction directive that depends on FooAction and uses information in the foos dict:

class BarAction(dectate.Action):
    depends = [FooAction]

    config = {
       'foos': dict,  # also use the foos dict
       'bars': list
    }
    def __init__(self, name):
        self.name = name

    def identifier(self, foos, bars):
        return self.name

    def perform(self, obj, foos, bars):
        in_foo = self.name in foos
        bars.append((self.name, obj, in_foo))

In order to use them we need to hook up the actions as directives onto an app class:

class DependsApp(dectate.App):
    foo = dectate.directive(FooAction)
    bar = dectate.directive(BarAction)

Using depends we have ensured that BarAction actions are performed after FooAction action, no matter what order we use them:

@DependsApp.bar('a')
def f():
    pass

@DependsApp.bar('b')
def g():
    pass

@DependsApp.foo('a')
def x():
    pass

dectate.commit(DependsApp)

We expect in_foo to be True for a but to be False for b:

>>> DependsApp.config.bars
[('a', <function f at ...>, True), ('b', <function g at ...>, False)]

config dependencies

In the example above, the items in bars depend on the items in foos and we’ve implemented this dependency in the perform of BarAction.

We can instead make the configuration object for the BarAction depend on foos. This way BarAction does not need to know about foos. You can declare a dependency between config objects with the factory_arguments attribute of the config factory. Any config object that is created in earlier dependencies of this action, or in the action itself, can be listed in factory_arguments. The key and value in factory_arguments have to match the key and value in config of that earlier action.

First we create a FooAction that sets up a foos config item as before:

class FooAction(dectate.Action):
    config = {
       'foos': dict
    }
    def __init__(self, name):
        self.name = name

    def identifier(self, foos):
        return self.name

    def perform(self, obj, foos):
        foos[self.name] = obj

Now we create a Bar class that also depends on the foos dict by listing it in factory_arguments:

class Bar(object):
    factory_arguments = {
        'foos': dict
    }

    def __init__(self, foos):
        self.foos = foos
        self.l = []

    def add(self, name, obj):
        in_foo = name in self.foos
        self.l.append((name, obj, in_foo))

We create a BarAction that depends on the FooAction (so that foos is created first) and that uses the Bar factory:

class BarAction(dectate.Action):
   depends = [FooAction]

   config = {
      'bar': Bar
   }

   def __init__(self, name):
       self.name = name

   def identifier(self, bar):
       return self.name

   def perform(self, obj, bar):
       bar.add(self.name, obj)

And we set them up as directives:

class ConfigDependsApp(dectate.App):
    foo = dectate.directive(FooAction)
    bar = dectate.directive(BarAction)

When we use our directives:

@ConfigDependsApp.bar('a')
def f():
    pass

@ConfigDependsApp.bar('b')
def g():
    pass

@ConfigDependsApp.foo('a')
def x():
    pass

dectate.commit(ConfigDependsApp)

we get the same result as before:

>>> ConfigDependsApp.config.bar.l
[('a', <function f at ...>, True), ('b', <function g at ...>, False)]

app_class_arg

In some cases what you want to configure is not on in the config object (app_class.config), but is associated with the app class in another way. You can get the app class passed in as an argument to dectate.Action.perform(), dectate.Action.identifier(), and so on by setting the special app_class_arg class attribute:

class PluginAction(dectate.Action):
    config = {
       'plugins': dict
    }
    app_class_arg = True

    def __init__(self, name):
        self.name = name

    def identifier(self, plugins, app_class):
        return self.name

    def perform(self, obj, plugins, app_class):
        plugins[self.name] = obj
        app_class.touched = True

class MyApp(dectate.App):
   plugin_with_app_class = dectate.directive(PluginAction)

When we now perform this directive:

@MyApp.plugin_with_app_class('a')
def f():
    pass # do something interesting

dectate.commit(MyApp)

We can see the app class was indeed affected:

>>> MyApp.touched
True

You can also use app_class_arg on a factory so that Dectate passes in the app_class factory argument.

before and after

It can be useful to do some additional setup just before all actions of a certain type are performed, or just afterwards. You can do this using before (dectate.Action.before()) and after (dectate.Action.after()) static methods on the Action class:

class FooAction(dectate.Action):
    config = {
       'foos': list
    }
    def __init__(self, name):
        self.name = name

    @staticmethod
    def before(foos):
        print("before:", foos)

    @staticmethod
    def after(foos):
        print("after:", foos)

    def identifier(self, foos):
        return self.name

    def perform(self, obj, foos):
        foos.append((self.name, obj))

class BeforeAfterApp(dectate.App):
    foo = dectate.directive(FooAction)

@BeforeAfterApp.foo('a')
def f():
    pass

@BeforeAfterApp.foo('b')
def g():
    pass

This executes before just before a and b are configured, and then executes after:

>>> dectate.commit(BeforeAfterApp)
before: []
after: [('a', <function f at ...>), ('b', <function g at ...>)]

grouping actions

Different actions normally don’t conflict with each other. It can be useful to group different actions together in a group so that they do affect each other. You can do this with the group_class (dectate.Action.group_class) class attribute. Grouped classes share their config and their before and after methods.

class FooAction(dectate.Action):
    config = {
       'foos': list
    }
    def __init__(self, name):
        self.name = name

    def identifier(self, foos):
        return self.name

    def perform(self, obj, foos):
        foos.append((self.name, obj))

We now create a BarAction that groups with FooAction:

class BarAction(dectate.Action):
    group_class = FooAction

    def __init__(self, name):
        self.name = name

    def identifier(self, foos):
        return self.name

    def perform(self, obj, foos):
        foos.append((self.name, obj))

class GroupApp(dectate.App):
    foo = dectate.directive(FooAction)
    bar = dectate.directive(BarAction)

It reuses the config from FooAction. This means that foo and bar can be in conflict:

@GroupApp.foo('a')
def f():
    pass

@GroupApp.bar('a')
def g():
    pass
>>> dectate.commit(GroupApp)
Traceback (most recent call last):
  ...
ConflictError: Conflict between:
  File "...", line 8
    @GroupApp.bar('a')

Additional discriminators

In some cases an action should conflict with multiple other actions all at once. You can take care of this with the discriminators (dectate.Action.discriminators()) method on your action:

class FooAction(dectate.Action):
    config = {
       'foos': dict
    }
    def __init__(self, name, extras):
        self.name = name
        self.extras = extras

    def identifier(self, foos):
        return self.name

    def discriminators(self, foos):
        return self.extras

    def perform(self, obj, foos):
        foos[self.name] = obj


class DiscriminatorsApp(dectate.App):
    foo = dectate.directive(FooAction)

An action now conflicts with an action of the same name and with any action that is in the extra list:

# example
@DiscriminatorsApp.foo('a', ['b', 'c'])
def f():
    pass

@DiscriminatorsApp.foo('b', [])
def g():
    pass

And then:

>>> dectate.commit(DiscriminatorsApp)
Traceback (most recent call last):
  ...
ConflictError: Conflict between:
  File "...", line 2:
    @DiscriminatorsApp.foo('a', ['b', 'c'])
  File "...", line 6
    @DiscriminatorsApp.foo('b', [])

Composite actions

When you can define an action entirely in terms of other actions, you can subclass dectate.Composite.

First we define a normal SubAction to use in the composite action later:

class SubAction(dectate.Action):
    config = {
        'my': list
    }

    def __init__(self, name):
        self.name = name

    def identifier(self, my):
        return self.name

    def perform(self, obj, my):
        my.append((self.name, obj))

Now we can define a special dectate.Composite subclass that uses SubAction in an actions (dectate.Composite.actions()) method:

class CompositeAction(dectate.Composite):
    def __init__(self, names):
        self.names = names

    def actions(self, obj):
        return [(SubAction(name), obj) for name in self.names]

class CompositeApp(dectate.App):
    _sub = dectate.directive(SubAction)
    composite = dectate.directive(CompositeAction)

Note that even though _sub is not intended to be a public part of the API we still need to include it in our dectate.App subclass, as Dectate does need to know it exists.

We can now use it:

@CompositeApp.composite(['a', 'b', 'c'])
def f():
    pass

dectate.commit(CompositeApp)

And SubAction is performed three times as a result:

>>> CompositeApp.config.my
[('a', <function f at ...>), ('b', <function f at ...>), ('c', <function f at ...>)]

with statement

Sometimes you want to issue a lot of similar actions at once. You can use the with statement to do so with less repetition:

class FooAction(dectate.Action):
    config = {
        'my': list
    }

    def __init__(self, a, b):
        self.a = a
        self.b = b

    def identifier(self, my):
        return (self.a, self.b)

    def perform(self, obj, my):
        my.append((self.a, self.b, obj))


class WithApp(dectate.App):
    foo = dectate.directive(FooAction)

Instead of this:

class VerboseWithApp(WithApp):
    pass

@VerboseWithApp.foo('a', 'x')
def f():
   pass

@VerboseWithApp.foo('a', 'y')
def g():
   pass

@VerboseWithApp.foo('a', 'z')
def h():
   pass

You can instead write:

class SuccinctWithApp(WithApp):
    pass

with SuccinctWithApp.foo('a') as foo:
    @foo('x')
    def f():
        pass

    @foo('y')
    def g():
        pass

    @foo('z')
    def h():
        pass

And this has the same configuration effect:

>>> dectate.commit(VerboseWithApp, SuccinctWithApp)
>>> VerboseWithApp.config.my
[('a', 'x', <function f at ...>), ('a', 'y', <function g at ...>), ('a', 'z', <function h at ...>)]
>>> SuccinctWithApp.config.my
[('a', 'x', <function f at ...>), ('a', 'y', <function g at ...>), ('a', 'z', <function h at ...>)]

importing recursively

When you use dectate-based decorators across a package, it can be useful to just import all modules in it at once. This way the user cannot forget to import a module with decorators in it.

Dectate itself does not offer this facility, but you can use the importscan library to do this recursive import. Simply do something like:

import my_package

importscan.scan(my_package, ignore=['.tests'])

This imports every module in my_package, except for the tests sub package.

logging

Dectate logs information about the performed actions as debug log messages. By default this goes to the dectate.directive.<directive_name> log. You can use the standard Python logging module function to make this information go to a log file.

If you want to override the name of the log you can set logger_name (dectate.App.logger_name) on the app class:

class MorepathApp(dectate.App):
   logger_name = 'morepath.directive'

querying

Dectate keeps a database of committed actions that can be queried by using dectate.Query.

Here is an example of a query for all the plugin actions on PluginApp:

q = dectate.Query('plugin')

We can now run the query:

>>> list(q(PluginApp))
[(<PluginAction ...>, <function f ...>),
 (<PluginAction ...>, <function g ...>)]

We can also filter the query for attributes of the action:

>>> list(q.filter(name='a')(PluginApp))
[(<PluginAction object ...>, <function f ...>)]

Sometimes the attribute on the action is not the same as the name you may want to use in the filter. You can use dectate.Action.filter_name to create a mapping to the correct attribute.

By default the filter does an equality comparison. You can define your own comparison function for an attribute using dectate.Action.filter_compare.

If you want to allow a query on a Composite action you need to give it some help by defining xs:attr:dectate.Composite.query_classes.

query tool

Dectate also includes a command-line tool that lets you issue queries. You need to configure it for your application. For instance, in the module main.py of your project:

import dectate

def query_tool():
    # make sure to scan or import everything needed at this point
    dectate.query_tool(SomeApp.commit())

In this function you should commit any dectate.App subclasses your application normally uses, and then provide an iterable of them to dectate.query_tool(). These are the applications that are queried by default if you don’t specify --app. We do it all in one here as we can get the app class that were committed from the result of App.commit().

Then in setup.py of your project:

entry_points={
    'console_scripts': [
        'decq = query.main:query_tool',
    ]
},

When you re-install this project you have a command-line tool called decq that lets you issues queries. For instance, this query returns all uses of directive foo in the apps you provided to query_tool:

$ decq foo
App: <class 'query.a.App'>
  File ".../query/b.py", line 4
  @App.foo(name='alpha')

  File ".../query/b.py", line 9
  @App.foo(name='beta')

  File ".../query/b.py", line 14
  @App.foo(name='gamma')

  File ".../query/c.py", line 4
  @App.foo(name='lah')

App: <class 'query.a.Other'>
  File ".../query/b.py", line 19
  @Other.foo(name='alpha')

And this query filters by name:

$ decq foo name=alpha
App: <class 'query.a.App'>
  File ".../query/b.py", line 4
  @App.foo(name='alpha')

App: <class 'query.a.Other'>
  File ".../query/b.py", line 19
  @Other.foo(name='alpha')

You can also explicit provide the app classes to query with the --app option; the default list of app classes is ignored in this case:

$ bin/decq --app query.a.App foo name=alpha
App: <class 'query.a.App'>
  File ".../query/b.py", line 4
  @App.foo(name='alpha')

You need to give --app a dotted name of the dectate.App subclass to query. You can repeat the --app option to query multiple apps.

Not all things you would wish to query on are string attributes. You can provide a conversion function that takes the string input and converts it to the underlying object you want to compare to using dectate.Action.filter_convert.

A working example is in scenarios/query of the Dectate project.

Sphinx Extension

If you use Sphinx to document your project and you use the sphinx.ext.autodoc extension to document your API, you need to install a Sphinx extension so that directives are documented properly. In your Sphinx conf.py add 'dectate.sphinxext' to the extensions list.

__main__ and conflicts

In certain scenarios where you run your code like this:

$ python app.py

and you use __name__ == '__main__' to determine whether the module should run:

if __name__ == '__main__':
    import another_module
    dectate.commit(App)

you might get a ConflictError from Dectate that looks somewhat like this:

Traceback (most recent call last):
 ...
dectate.error.ConflictError: Conflict between:
  File "/path/to/app.py", line 6
    @App.foo(name='a')
  File "app.py", line 6
    @App.foo(name='a')

The same line shows up on both sides of the configuration conflict, but the path is absolute on one side and relative on the other.

This happens because in some scenarios involving __main__, Python imports a module twice (more about this). Dectate refuses to operate in this case until you change your imports so that this doesn’t happen anymore.

How to avoid this scenario? If you use setuptools automatic script creation this problem is avoided entirely.

If you want to use the if __name__ == '__main__' system, keep your main module tiny and just import the main function you want to run from elsewhere.

So, Dectate warns you if you do it wrong, so don’t worry about it.

API

dectate.commit(*apps)

Commit one or more app classes

A commit causes the configuration actions to be performed. The resulting configuration information is stored under the .config class attribute of each App subclass supplied.

This function may safely be invoked multiple times – each time the known configuration is recommitted.

Parameters:*apps – one or more App subclasses to perform configuration actions on.
dectate.topological_sort(l, get_depends)

Topological sort

Given a list of items that depend on each other, sort so that dependencies come before the dependent items. Dependency graph must be a DAG.

Parameters:
  • l – a list of items to sort
  • get_depends – a function that given an item gives other items that this item depends on. This item will be sorted after the items it depends on.
Returns:

the list sorted topologically.

class dectate.App

A configurable application object.

Subclass this in your framework and add directives using the App.directive() decorator.

Set the logger_name class attribute to the logging prefix that Dectate should log to. By default it is "dectate.directive".

classmethod clean()

A method that sets or restores the state of the class.

Normally Dectate only sets up configuration into the config attribute, but in some cases you may touch other aspects of the class during configuration time. You can override this classmethod to set up the state of the class in its pristine condition.

classmethod commit()

Commit this class and any depending on it.

This is intended to be overridden by subclasses if committing the class also commits other classes automatically, such as in the case in Morepath when one app is mounted into another. In such case it should return an iterable of all committed classes.

Returns:an iterable of committed classes
classmethod is_committed()

True if this app class was ever committed.

Returns:bool that is True when the app was committed before.
config = <dectate.app.Config object>

Config object that contains the configuration after commit.

This is installed when the class object is initialized, so during import-time when you use the class statement and subclass dectate.App, but is only filled after you commit the configuration.

This keeps the final configuration result after commit. It is a very dumb object that has no methods and is just a container for attributes that contain the real configuration.

dectate = <dectate.config.Configurable object>

A dectate Configurable instance is installed here.

This is installed when the class object is initialized, so during import-time when you use the class statement and subclass dectate.App.

This keeps tracks of the registrations done by using directives as long as committed configurations.

logger_name = 'dectate.directive'

The prefix to use for directive debug logging.

class dectate.Action

A configuration action.

Base class of configuration actions.

A configuration action is performed for an object (typically a function or a class object) and affects one or more configuration objects.

Actions can conflict with each other based on their identifier and discriminators. Actions can override each other based on their identifier. Actions can only be in conflict with actions of the same action class or actions with the same action_group.

static after(**kw)

Do setup just after actions in a group are performed.

Can be implemented as a static method by the Action subclass.

Parameters:**kw – a dictionary of configuration objects as specified by the config class attribute.
static before(**kw)

Do setup just before actions in a group are performed.

Can be implemented as a static method by the Action subclass.

Parameters:**kw – a dictionary of configuration objects as specified by the config class attribute.
discriminators(**kw)

Returns an iterable of immutables to detect conflicts.

Can be implemented by the Action subclass.

Used for additional configuration conflict detection.

Parameters:**kw – a dictionary of configuration objects as specified by the config class attribute.
Returns:an iterable of immutable values.
filter_get_value(name)

A function to get the filter value.

Takes two arguments, action and name. Should return the value on the filter.

This function is called if the name cannot be determined by looking for the attribute directly using Action.filter_name.

The function should return NOT_FOUND if no value with that name can be found.

For example if the filter values are stored on key_dict:

def filter_get_value(self, name):
    return self.key_dict.get(name, dectate.NOT_FOUND)
Parameters:name – the name of the filter.
Returns:the value to filter on.
get_value_for_filter(name)

Get value. Takes into account filter_name, filter_get_value

Used by the query system. You can override it if your action has a different way storing values altogether.

Parameters:name – the filter name to get the value for.
Returns:the value to filter on.
identifier(**kw)

Returns an immutable that uniquely identifies this config.

Needs to be implemented by the Action subclass.

Used for overrides and conflict detection.

If two actions in the same group have the same identifier in the same configurable, those two actions are in conflict and a ConflictError is raised during commit().

If an action in an extending configurable has the same identifier as the configurable being extended, that action overrides the original one in the extending configurable.

Parameters:**kw – a dictionary of configuration objects as specified by the config class attribute.
Returns:an immutable value uniquely identifying this action.
perform(obj, **kw)

Do whatever configuration is needed for obj.

Needs to be implemented by the Action subclass.

Raise a DirectiveError to indicate that the action cannot be performed due to incorrect configuration.

Parameters:
  • obj – the object that the action should be performed for. Typically a function or a class object.
  • **kw – a dictionary of configuration objects as specified by the config class attribute.
app_class_arg = False

Pass in app class as argument.

In addition to the arguments defined in Action.config, pass in the app class itself as an argument into Action.identifier(), Action.discriminators(), Action.perform(), and Action.before() and Action.after().

code_info

Info about where in the source code the action was invoked.

Is an instance of CodeInfo.

Can be None if action does not have an associated directive but was created manually.

config = {}

Describe configuration.

A dict mapping configuration names to factory functions. The resulting configuration objects are passed into Action.identifier(), Action.discriminators(), Action.perform(), and Action.before() and Action.after().

After commit completes, the configured objects are found as attributes on App.config.

depends = []

List of other action classes to be executed before this one.

The depends class attribute contains a list of other action classes that need to be executed before this one is. Actions which depend on another will be executed after those actions are executed.

Omit if you don’t care about the order.

filter_compare = {}

Map of names used in query filter to comparison functions.

If for instance you want to be able check whether the value of model on the action is a subclass of the value provided in the filter, you can provide it here:

filter_compare = {
    'model': issubclass
}

The default filter compare is an equality comparison.

filter_convert = {}

Map of names to convert functions.

The query tool that can be generated for a Dectate-based application uses this information to parse filter input into actual objects. If omitted it defaults to passing through the string unchanged.

A conversion function takes a string as input and outputs a Python object. The conversion function may raise ValueError if the conversion failed.

A useful conversion function is provided that can be used to refer to an object in a module using a dotted name: convert_dotted_name().

filter_name = {}

Map of names used in query filter to attribute names.

If for instance you want to be able to filter the attribute _foo using foo in the query, you can map foo to _foo:

filter_name = {
   'foo': '_foo'
}

If a filter name is omitted the filter name is assumed to be the same as the attribute name.

group_class = None

Action class to group with.

This class attribute can be supplied with the class of another action that this action should be grouped with. Only actions in the same group can be in conflict. Actions in the same group share the config and before and after of the action class indicated by group_class.

By default an action only groups with others of its same class.

class dectate.Composite

A composite configuration action.

Base class of composite actions.

Composite actions are very simple: implement the action method and return a iterable of actions in there.

actions(obj)

Specify a iterable of actions to perform for obj.

The iteratable should yield action, obj tuples, where action is an instance of class Action or Composite and obj is the object to perform the action with.

Needs to be implemented by the Composite subclass.

Parameters:obj – the obj that the composite action was performed on.
Returns:iterable of action, obj tuples.
code_info

Info about where in the source code the action was invoked.

Is an instance of CodeInfo.

Can be None if action does not have an associated directive but was created manually.

filter_convert = {}

Map of names to convert functions.

The query tool that can be generated for a Dectate-based application uses this information to parse filter input into actual objects. If omitted it defaults to passing through the string unchanged.

A conversion function takes a string as input and outputs a Python object. The conversion function may raise ValueError if the conversion failed.

A useful conversion function is provided that can be used to refer to an object in a module using a dotted name: convert_dotted_name().

query_classes = []

A list of actual action classes that this composite can generate.

This is to allow the querying of composites. If the list if empty (the default) the query system refuses to query the composite. Note that if actions of the same action class can also be generated in another way they are in the same query result.

class dectate.Query(*action_classes)

An object representing a query.

A query can be chained with Query.filter(), Query.attrs(), Query.obj().

Param:*action_classes: one or more action classes to query for. Can be instances of Action or Composite. Can also be strings indicating directive names, in which case they are looked up on the app class before execution.
attrs(*names)

Extract attributes from resulting actions.

The list of attribute names indicates which keys to include in the dictionary. Obeys Action.filter_name and Action.filter_get_value.

Param:*names: list of names to extract.
Returns:iterable of dictionaries.
filter(**kw)

Filter this query by keyword arguments.

The keyword arguments are matched with attributes on the action. Action.filter_name is used to map keyword name to attribute name, by default they are the same. Action.filter_get_value() can also be implemented for more complicated attribute access as a fallback.

By default the keyword argument values are matched by equality, but you can override this using Action.filter_compare.

Can be chained again with a new filter.

Parameters:**kw – keyword arguments to match against.
Returns:iterable of (action, obj).
obj()

Get objects from results.

Throws away actions in the results and return an iterable of objects.

Returns:iterable of decorated objects.
dectate.directive(action_factory)

Create a classmethod to hook action to application class.

You pass in a dectate.Action or a dectate.Composite subclass and can attach the result as a class method to an dectate.App subclass:

class FooAction(dectate.Action):
    ...

class MyApp(dectate.App):
    my_directive = dectate.directive(MyAction)

Alternatively you can also define the direction inline using this as a decorator:

class MyApp(dectate.App):
    @directive
    class my_directive(dectate.Action):
        ...
Parameters:action_factory – an action class to use as the directive.
Returns:a class method that represents the directive.
dectate.query_tool(app_classes)

Command-line query tool for dectate.

Uses command-line arguments to do the query and prints the results.

usage: decq [-h] [–app APP] directive <filter>

Query all directives named foo in given app classes:

$ decq foo

Query directives foo with name attribute set to alpha:

$ decq foo name=alpha

Query directives foo specifically in given app:

$ decq --app=myproject.App foo
Parameters:app_classes – a list of App subclasses to query by default.
dectate.query_app(app_class, directive, **filters)

Query a single app with raw filters.

This function is especially useful for writing unit tests that test the conversion behavior.

Parameters:
  • app_class – a App subclass to query.
  • directive – name of directive to query.
  • **filters – raw (unconverted) filter values.
Returns:

iterable of action, obj tuples.

dectate.convert_dotted_name(s)

Convert input string to an object in a module.

Takes a dotted name: pkg.module.attr gets attr from module module which is in package pkg.

To refer to builtin objects such as int or object, in Python 2 prefix with __builtin__., so __builtin__.int or __builtin__.None. In Python 3 use builtins. as the prefix, so builtins.int and builtins.None.

Raises ValueError if it cannot be imported.

dectate.convert_bool(s)

Convert input string to boolean.

Input string must either be True or False.

dectate.NOT_FOUND = <NOT_FOUND>

Sentinel value returned if filter value cannot be found on action.

class dectate.CodeInfo(path, lineno, sourceline)

Information about where code was invoked.

The path attribute gives the path to the Python module that the code was invoked in.

The lineno attribute gives the linenumber in that file.

The sourceline attribute contains the actual source line that did the invocation.

exception dectate.ConfigError

Raised when configuration is bad.

exception dectate.ConflictError(actions)

Bases: dectate.error.ConfigError

Raised when there is a conflict in configuration.

Describes where in the code directives are in conflict.

exception dectate.DirectiveError

Bases: dectate.error.ConfigError

Can be raised by user when there directive cannot be performed.

Raise it in Action.perform() with a message describing what the problem is:

raise DirectiveError("name should be a string, not None")

This is automatically converted by Dectate to a DirectiveReportError.

exception dectate.DirectiveReportError(message, code_info)

Bases: dectate.error.ConfigError

Raised when there’s a problem with a directive.

Describes where in the code the problem occurred.

exception dectate.TopologicalSortError

Bases: exceptions.ValueError

Raised if dependencies cannot be sorted topologically.

This is due to circular dependencies.

Developing Dectate

Install Dectate for development

Clone Dectate from github:

$ git clone git@github.com:morepath/dectate.git

If this doesn’t work and you get an error ‘Permission denied (publickey)’, you need to upload your ssh public key to github.

Then go to the dectate directory:

$ cd dectate

Make sure you have virtualenv installed.

Create a new virtualenv for Python 3 inside the dectate directory:

$ virtualenv -p python3 env/py3

Activate the virtualenv:

$ source env/py3/bin/activate

Make sure you have recent setuptools and pip installed:

$ pip install -U setuptools pip

Install the various dependencies and development tools from develop_requirements.txt:

$ pip install -Ur develop_requirements.txt

For upgrading the requirements just run the command again.

If you want to test Dectate with Python 2.7 as well you can create a second virtualenv for it:

$ virtualenv -p python2.7 env/py27

You can then activate it:

$ source env/py27/bin/activate

Then uprade setuptools and pip and install the develop requirements as described above.

Note

The following commands work only if you have the virtualenv activated.

Running the tests

You can run the tests using py.test:

$ py.test

To generate test coverage information as HTML do:

$ py.test --cov --cov-report html

You can then point your web browser to the htmlcov/index.html file in the project directory and click on modules to see detailed coverage information.

Running the documentation tests

The documentation contains code. To check these code snippets, you can run this code using this command:

(py3) $ sphinx-build -b doctest doc doc/build/doctest

Or alternatively if you have Make installed:

(py3) $ cd doc
(py3) $ make doctest

Or from the Dectate project directory:

(py3) $ make -C doc doctest

Since the sample code in the documentation is maintained in Python 3 syntax, we do not support running the doctests with Python 2.7.

Building the HTML documentation

To build the HTML documentation (output in doc/build/html), run:

$ sphinx-build doc doc/build/html

Or alternatively if you have Make installed:

$ cd doc
$ make html

Or from the Dectate project directory:

$ make -C doc html

Various checking tools

flake8 is a tool that can do various checks for common Python mistakes using pyflakes, check for PEP8 style compliance and can do cyclomatic complexity checking. To do pyflakes and pep8 checking do:

$ flake8 dectate

To also show cyclomatic complexity, use this command:

$ flake8 --max-complexity=10 dectate

Tox

With tox you can test Morepath under different Python environments.

We have Travis continuous integration installed on Morepath’s github repository and it runs the same tox tests after each checkin.

First you should install all Python versions which you want to test. The versions which are not installed will be skipped. You should at least install Python 3.5 which is required by flake8, coverage and doctests and Python 2.7 for testing Morepath with Python 2.

One tool you can use to install multiple versions of Python is pyenv.

To find out which test environments are defined for Morepath in tox.ini run:

$ tox -l

You can run all tox tests with:

$ tox

You can also specify a test environment to run e.g.:

$ tox -e py35
$ tox -e pep8
$ tox -e docs

History of Dectate

Dectate was extracted from Morepath and then extensively refactored and cleaned up. It is authored by me, Martijn Faassen.

In the beginning (around 2001) there was zope.configuration, part of the Zope 3 project. It features declarative XML configuration with conflict detection and overrides to assemble pieces of Python code.

In 2006, I helped create the Grok project. This did away with the XML based configuration and instead used Python code. This in turn then drove zope.configuration. Grok did not use Python decorators but instead used specially annotated Python classes, which were recursively scanned from modules. Grok’s configuration system was spun off as the Martian library.

Chris McDonough was then inspired by Martian to create Venusian, a deferred decorator execution system. It is like Martian in that it imports Python modules recursively in order to find configuration.

I created the Morepath web framework, which uses decorators for configuration throughout and used Venusian. Morepath grew a configuration subsystem where configuration is associated with classes, and uses class inheritance to power configuration reuse and overrides. This configuration subsystem started to get a bit messy as requirements grew.

So in 2016 I extracted the configuration system from Morepath into its own library, Dectate. This allowed me to extensively refactor the code for clarity and features. Dectate does not use Venusian for configuration. Dectate still defers the execution of configuration actions to an explicit commit phase, so that conflict detection and overrides and such can take place.

CHANGES

0.14 (unreleased)

  • Nothing changed yet.

0.13 (2016-12-23)

  • Add a Sentinel class, used for the NOT_FOUND object.
  • Upload universal wheels to pypi during release.
  • make directive_name property available on Directive instances.

0.12 (2016-10-04)

  • Breaking changes: previously you defined new directives using the App.directive directive. This would lead to import confusion: you have to import the modules that define directives before you can actually use them, even though you’ve already imported your app class.

    In this version of Dectate we’ve changed the way you define directives. Instead of:

    class MyApp(dectate.App):
        pass
    
    @MyApp.directive('foo')
    class FooAction(dectate.Action):
       ...
    

    You now write this:

    class FooAction(directive.Action)
        ...
    
    class MyApp(dectate.App):
        foo = directive(FooAction)
    

    So, you define the directives directly on the app class that needs them.

    Uses of private_action_class should be replaced by an underscored directive definition:

    class MyApp(dectate.App):
       _my_private_thing = directive(PrivateAction)
    
  • Use the same Git ignore file used in other Morepath projects.

  • If you set the app_class_arg class attribute to True on an action, then an app_class is passed along to perform, identifier, etc. This way you can affect the app class directly instead of just its underlying configuration in the config attribute.

  • Similarly if you set the app_class_arg attribute True on a factory class, it is passed in.

  • Add a clean method to the App class. You can override this to introduce your own cleanup policy for aspects of the class that are not contained in the config attribute.

  • We now use virtualenv and pip instead of buildout to set up the development environment. The development documentation has been updated accordingly.

  • Include doctests in Tox and Travis.

0.11 (2016-07-18)

  • Removed: autocommit was removed from the Dectate API. Rely on the commit class method of the App class instead for a more explicit alternative.
  • Removed: auto_query_tool was removed from the Dectate API. Use query_tool(App.commit()) instead.
  • Fix repr of directives so that you can at least see their name.
  • the execution order of filters is now reproducible, to ensure consistent test coverage reports.
  • Use abstract base classes from the standard library for the Action and Composite classes.
  • Use feature detection instead of version detection to ensure Python 2/3 compatibility.
  • Increased test coverage.
  • Set up Travis CI and Coverall as continuous integration services for quality assurance purposes.
  • Add support for Python 3.3 and 3.5.
  • Make Python 3.5 the default testing environment.

0.10.2 (2016-04-26)

  • If nothing is found for an app in the query tool, don’t mention it in the output so as to avoid cluttering the results.
  • Fix a major bug in the query tool where if an app resulted in no results, any subsequent apps weren’t even searched.

0.10.1 (2016-04-26)

  • Create proper deprecation warnings instead of plain warnings for autocommit and auto_query_tool.

0.10 (2016-04-25)

  • Deprecated The autocommit function is deprecated. Rely on the commit class method of the App class instead for a more explicit alternative.
  • Deprecated The auto_query_tool function is deprecated. Rely on dectate.query_tool(MyApp.commit()) instead. Since the commit method returns an iterable of App classes that are required to commit the app class it is invoked on, this returns the right information.
  • topological_sort function is exposed as the public API.
  • A commit class method on App classes.
  • Report on inconsistent uses of factories between different directives’ config settings as well as factory_arguments for registries. This prevents bugs where a new directive introduces the wrong factory for an existing directive.
  • Expanded internals documentation.

0.9.1 (2016-04-19)

  • Fix a subtle bug introduced in the last release. If factory_arguments were in use with a config name only created in that context, it was not properly cleaned up, which in some cases can make a commit of a subclass get the same config object as that of the base class.

0.9 (2016-04-19)

  • Change the behavior of query_tool so that if it cannot find an action class for the directive name the query result is empty instead of making this an error. This makes auto_query_tool work better.
  • Introduce auto_query_tool which uses the automatically found app classes as the default app classes to query.
  • Fix tests that use __builtin__ that were failing on Python 3.
  • Dependencies only listed in factory_arguments are also created during config creation.

0.8 (2016-04-12)

  • Document how to refer to builtins in Python 3.
  • Expose is_committed method on App subclasses.

0.7 (2016-04-11)

  • Fix a few documentation issues.
  • Expose convert_dotted_name and document it.
  • Implement new convert_bool.
  • Allow use of directive name instead of Action subclass as argument to Query.
  • A query_app function which is especially helpful when writing tests for the query tool – it takes unconverted filter arguments.
  • Use newer version of with_metaclass from six.
  • Expose NOT_FOUND and document it.
  • Introduce a new filter_get_value method you can implement if the normal attribute getting and filter_name are not enough.

0.6 (2016-04-06)

  • Introduce a query system for actions and a command-line tool that lets you query actions.

0.5 (2016-04-04)

  • Breaking change The signature of commit has changed. Just pass in one or more arguments you want to commit instead of a list. See #8.

0.4 (2016-04-01)

  • Expose code_info attribute for action. The path in particular can be useful in implementing a directive such as Morepath’s template_directory. Expose it for composite too.
  • Report a few more errors; you cannot use config, before or after after in an action class if group_class is set.
  • Raise a DirectiveReportError if a DirectiveError is raised in a composite actions method.

0.3 (2016-03-30)

  • Document importscan package that can be used in combination with this one.
  • Introduced factory_arguments feature on config factories, which can be used to create dependency relationships between configuration.
  • Fix a bug where config items were not always properly reused. Now only the first one in the action class dependency order is used, and it is not recreated.

0.2 (2016-03-29)

  • Remove clear_autocommit as it was useless during testing anyway. In tests just use explicit commit.
  • Add a dectate.sphinxext module that can be plugged into Sphinx so that directives are documented properly.
  • Document how Dectate deals with double imports.

0.1 (2016-03-29)

  • Initial public release.

Indices and tables