Martian 0.14

Martian is a Python library that provides infrastructure for declarative configuration of Python code. Martian is especially useful for the construction of frameworks that need to provide a flexible plugin infrastructure. Martian doesn't actually provide infrastructure for plugin registries (except for itself). Many frameworks have their own systems for this, and if you need a generic one, you might want to consider zope.component. Martian just allows you to make the registration of plugins less verbose.

You can see Martian as doing something that you can also solve with metaclasses, with the following advantages:

* the developer of the framework doesn't have to write a lot of ad-hoc metaclasses anymore; instead we offer an infrastructure to make life easier.
* configuration doesn't need to happen at import time, but can happen at program startup time. This also makes configuration more tractable for a developer.
* we don't bother the developer that uses the framework with the surprising behavior that metaclasses sometimes bring. The classes the user has to deal with are normal classes.

Why is this package named martian? In the novel "Stranger in a Strange Land", the verb grok is introduced:

Grok means to understand so thoroughly that the observer becomes a part of the observed -- to merge, blend, intermarry, lose identity in group experience.

In the context of this package, "grokking" stands for the process of deducing declarative configuration actions from Python code. In the novel, grokking is originally a concept that comes from the planet Mars. Martians grok. Since this package helps you grok code, it's called Martian.

Martian provides a framework that allows configuration to be expressed in declarative Python code. These declarations can often be deduced from the structure of the code itself. The idea is to make these declarations so minimal and easy to read that even extensive configuration does not overly burden the programmers working with the code.

The martian package is a spin-off from the Grok project, in the context of which this codebase was first developed. While Grok uses it, the code is completely independent of Grok.

Motivation

"Deducing declarative configuration actions from Python code" - that sounds very abstract. What does it actually mean? What is configuration? What is declarative configuration? In order to explain this, we'll first take a look at configuration.

Larger frameworks often offer a lot of points where you can modify their behavior: ways to combine its own components with components you provide yourself to build a larger application. A framework offers points where it can be configured with plugin code. When you plug some code into a plugin point, it results in the updating of some registry somewhere with the new plugin. When the framework uses a plugin, it will first look it up in the registry. The action of registering some component into a registry can be called configuration.

Let's look at an example framework that offers a plugin point. We introduce a very simple framework for plugging in different template languages, where each template language uses its own extension. You can then supply the framework with the template body and the template extension and some data, and render the template.

Let's look at the framework:

>>> import string
>>> class templating(FakeModule):
...
... class InterpolationTemplate(object):
... "Use %(foo)s for dictionary interpolation."
... def __init__(self, text):
... self.text = text
... def render(self, **kw):
... return self.text % kw
...
... class TemplateStringTemplate(object):
... "PEP 292 string substitutions."
... def __init__(self, text):
... self.template = string.Template(text)
... def render(self, **kw):
... return self.template.substitute(**kw)
...
... # the registry, we plug in the two templating systems right away
... extension_handlers = { '.txt': InterpolationTemplate,
... '.tmpl': TemplateStringTemplate }
...
... def render(data, extension, **kw):
... """Render the template at filepath with arguments.
...
... data - the data in the file
... extension - the extension of the file
... keyword arguments - variables to interpolate
...
... In a real framework you could pass in the file path instead of
... data and extension, but we don't want to open files in our
... example.
...
... Returns the rendered template
... """
... template = extension_handlers[extension](data)
... return template.render(**kw)

Since normally we cannot create modules in a doctest, we have emulated the templating Python module using the FakeModule class. Whenever you see FakeModule subclasses, imagine you're looking at a module definition in a .py file. Now that we have defined a module templating, we also need to be able to import it. Fake modules are always placed automatically into the martiantest.fake namespace so you can import them from there:

>>> from martiantest.fake import templating

Now let's try the render function for the registered template types, to demonstrate that our framework works:

>>> templating.render('Hello %(name)s!', '.txt', name="world")
'Hello world!'
>>> templating.render('Hello ${name}!', '.tmpl', name="universe")
'Hello universe!'

File extensions that we do not recognize cause a KeyError to be raised:

>>> templating.render('Hello', '.silly', name="test")
Traceback (most recent call last):
...
KeyError: '.silly'

We now want to plug into this filehandler framework and provide a handler for .silly files. Since we are writing a plugin, we cannot change the templating module directly. Let's write an extension module instead:

>>> class sillytemplating(FakeModule):
... class SillyTemplate(object):
... "Replace {key} with dictionary values."
... def __init__(self, text):
... self.text = text
... def render(self, **kw):
... text = self.text
... for key, value in kw.items():
... text = text.replace('{%s}' % key, value)
... return text
...
... templating.extension_handlers['.silly'] = SillyTemplate
>>> from martiantest.fake import sillytemplating

In the extension module, we manipulate the extension_handlers dictionary of the templating module (in normal code we'd need to import it first), and plug in our own function. .silly handling works now:

>>> templating.render('Hello {name}!', '.silly', name="galaxy")
'Hello galaxy!'

Above we plug into our extension_handler registry using Python code. Using separate code to manually hook components into registries can get rather cumbersome - each time you write a plugin, you also need to remember you need to register it.

Doing template registration in Python code also poses a maintenance risk. It is tempting to start doing fancy things in Python code such as conditional configuration, making the configuration state of a program hard to understand. Another problem is that doing configuration at import time can also lead to unwanted side effects during import, as well as ordering problems, where you want to import something that really needs configuration state in another module that is imported later. Finally, it can also make code harder to test, as configuration is loaded always when you import the module, even if in your test perhaps you don't want it to be.

Martian provides a framework that allows configuration to be expressed in declarative Python code. Martian is based on the realization that what to configure where can often be deduced from the structure of Python code itself, especially when it can be annotated with additional declarations. The idea is to make it so easy to write and register a plugin so that even extensive configuration does not overly burden the developer.

Configuration actions are executed during a separate phase ("grok time"), not at import time, which makes it easier to reason about and easier to test.

Configuration the Martian Way

Let's now transform the above templating module and the sillytemplating module to use Martian. First we must recognize that every template language is configured to work for a particular extension. With Martian, we annotate the classes themselves with this configuration information. Annotations happen using directives, which look like function calls in the class body.

Let's create an extension directive that can take a single string as an argument, the file extension to register the template class for:

>>> import martian
>>> class extension(martian.Directive):
... scope = martian.CLASS
... store = martian.ONCE
... default = None

We also need a way to easily recognize all template classes. The normal pattern for this in Martian is to use a base class, so let's define a Template base class:

>>> class Template(object):
... pass

We now have enough infrastructure to allow us to change the code to use Martian style base class and annotations:

>>> class templating(FakeModule):
...
... class InterpolationTemplate(Template):
... "Use %(foo)s for dictionary interpolation."
... extension('.txt')
... def __init__(self, text):
... self.text = text
... def render(self, **kw):
... return self.text % kw
...
... class TemplateStringTemplate(Template):
... "PEP 292 string substitutions."
... extension('.tmpl')
... def __init__(self, text):
... self.template = string.Template(text)
... def render(self, **kw):
... return self.template.substitute(**kw)
...
... # the registry, empty to start with
... extension_handlers = {}
...
... def render(data, extension, **kw):
... # this hasn't changed
... template = extension_handlers[extension](data)
... return template.render(**kw)
>>> from martiantest.fake import templating

As you can see, there have been very few changes:

* we made the template classes inherit from Template.
* we use the extension directive in the template classes.
* we stopped pre-filling the extension_handlers dictionary.

So how do we fill the extension_handlers dictionary with the right template languages? Now we can use Martian. We define a grokker for Template that registers the template classes in the extension_handlers registry:

>>> class meta(FakeModule):
... class TemplateGrokker(martian.ClassGrokker):
... martian.component(Template)
... martian.directive(extension)
... def execute(self, class_, extension, **kw):
... templating.extension_handlers[extension] = class_
... return True
>>> from martiantest.fake import meta

What does this do? A ClassGrokker has its execute method called for subclasses of what's indicated by the martian.component directive. You can also declare what directives a ClassGrokker expects on this component by using martian.directive() (the directive directive!) one or more times.

The execute method takes the class to be grokked as the first argument, and the values of the directives used will be passed in as additional parameters into the execute method. The framework can also pass along an arbitrary number of extra keyword arguments during the grokking process, so we need to declare **kw to make sure we can handle these.

All our grokkers will be collected in a special Martian-specific registry:

>>> reg = martian.GrokkerRegistry()

We will need to make sure the system is aware of the TemplateGrokker defined in the meta module first, so let's register it first. We can do this by simply grokking the meta module:

>>> reg.grok('meta', meta)
True

Because TemplateGrokker is now registered, our registry now knows how to grok Template subclasses. Let's grok the templating module:

>>> reg.grok('templating', templating)
True

Let's try the render function of templating again, to demonstrate we have successfully grokked the template classes:

>>> templating.render('Hello %(name)s!', '.txt', name="world")
'Hello world!'
>>> templating.render('Hello ${name}!', '.tmpl', name="universe")
'Hello universe!'

.silly hasn't been registered yet:

>>> templating.render('Hello', '.silly', name="test")
Traceback (most recent call last):
...
KeyError: '.silly'

Let's now register .silly from an extension module:

>>> class sillytemplating(FakeModule):
... class SillyTemplate(Template):
... "Replace {key} with dictionary values."
... extension('.silly')
... def __init__(self, text):
... self.text = text
... def render(self, **kw):
... text = self.text
... for key, value in kw.items():
... text = text.replace('{%s}' % key, value)
... return text
>>> from martiantest.fake import sillytemplating

As you can see, the developer that uses the framework has no need anymore to know about templating.extension_handlers. Instead we can simply grok the module to have SillyTemplate be register appropriately:

>>> reg.grok('sillytemplating', sillytemplating)
True

We can now use the .silly templating engine too:

>>> templating.render('Hello {name}!', '.silly', name="galaxy")
'Hello galaxy!'

Admittedly it is hard to demonstrate Martian well with a small example like this. In the end we have actually written more code than in the basic framework, after all. But even in this small example, the templating and sillytemplating module have become more declarative in nature. The developer that uses the framework will not need to know anymore about things like templating.extension_handlers or an API to register things there. Instead the developer can registering a new template system anywhere, as long as he subclasses from Template, and as long as his code is grokked by the system.

Finally note how Martian was used to define the TemplateGrokker as well. In this way Martian can use itself to extend itself.

Grokking instances

Above we've seen how you can grok classes. Martian also supplies a way to grok instances. This is less common in typical frameworks, and has the drawback that no class-level directives can be used, but can still be useful.

Let's imagine a case where we have a zoo framework with an Animal class, and we want to track instances of it:

>>> class Animal(object):
... def __init__(self, name):
... self.name = name
>>> class zoo(FakeModule):
... horse = Animal('horse')
... chicken = Animal('chicken')
... elephant = Animal('elephant')
... lion = Animal('lion')
... animals = {}
>>> from martiantest.fake import zoo

We define an InstanceGrokker subclass to grok Animal instances:

>>> class meta(FakeModule):
... class AnimalGrokker(martian.InstanceGrokker):
... martian.component(Animal)
... def execute(self, instance, **kw):
... zoo.animals[instance.name] = instance
... return True
>>> from martiantest.fake import meta

Let's create a new registry with the AnimalGrokker in it:

>>> reg = martian.GrokkerRegistry()
>>> reg.grok('meta', meta)
True

We can now grok the zoo module:

>>> reg.grok('zoo', zoo)
True

The animals will now be in the animals dictionary:

>>> sorted(zoo.animals.items())
[('chicken', < Animal object at ... >),
 ('elephant', < Animal object at ... >),
 ('horse', < Animal object at ... >),
 ('lion', < Animal object at ... >)]

More information

For many more details and examples of more kinds of grokkers, please see src/martian/core.txt. For more information on directives see src/martian/directive.txt.


Product's homepage

Requirements:

· Python

What's New in This Release: [ read full changelog ]

· The computation of the default value for a directive can now be defined inside the directive class definition