python-martian 0.12-0ubuntu2 / usr / share / pyshared / martian / core.txt

Martian core
============

A simple framework
------------------

Let's look at a very simple framework where files are handled by their
extensions::

  >>> class filehandler(FakeModule):
  ...   import os
  ...
  ...   def handle_txt(filepath):
  ...     return "Text file"
  ...
  ...   def handle_xml(filepath):
  ...     return "XML file"
  ...
  ...   extension_handlers = { '.txt': handle_txt, '.xml': handle_xml }
  ...
  ...   def handle(filepath):
  ...      name, ext = os.path.splitext(filepath)
  ...      return extension_handlers[ext](filepath)
  >>> from martiantest.fake import filehandler

Now let's try the ``handle`` function for a few file types::

  >>> filehandler.handle('test.txt')
  'Text file'
  >>> filehandler.handle('test2.xml')
  'XML file'

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

  >>> filehandler.handle('image.png')
  Traceback (most recent call last):
  ...
  KeyError: '.png'

We now want to plug into this filehandler framework and provide a
handler for ``.png`` files::

  >>> class pnghandler(FakeModule):
  ...    def handle_png(filepath):
  ...        return "PNG file"
  ...
  ...    filehandler.extension_handlers['.png'] = handle_png
  >>> from martiantest.fake import pnghandler

In the extension module, we manipulate the ``extension_handlers``
dictionary of the ``filehandler`` module and plug in our own
function. PNG handling works now::

  >>> filehandler.handle('image.png')
  'PNG file'

Grokkers that grok
------------------

Let's write a grokker that can grok the file types. A ``Grokker`` is
an object that can *grok* objects - execute configuration actions
pertaining to the grokked object, such as registering it with some
central registry. Different kinds of grokkers can grok different types
of objects (instances, classes, functions).

Let's define a Grokker to help us register the file type handler
functions as seen in our previous example::

  >>> import types
  >>> from zope.interface import implements
  >>> import martian
  >>> class FileTypeGrokker(martian.InstanceGrokker):
  ...   martian.component(types.FunctionType)
  ...
  ...   def grok(self, name, obj, **kw):
  ...     if not name.startswith('handle_'):
  ...       return False
  ...     ext = name.split('_')[1]
  ...     filehandler.extension_handlers['.' + ext] = obj
  ...     return True

This ``InstanceGrokker`` allows us to grok instances of a particular
type (such as functions). We need to define the type of object we're
looking for with the ``martian.component`` directive. In the ``grok``
method, we first make sure we only grok functions that have a name
that starts with ``handle_``. Then we determine the used extension
from the name and register the funcion in the ``extension_handlers``
dictionary of the ``filehandler`` module. We return ``True`` if we
indeed grokked the object.

An instance will provide the IGrokker interface::

  >>> filetype_grokker = FileTypeGrokker()
  >>> from martian.interfaces import IGrokker
  >>> IGrokker.providedBy(filetype_grokker)
  True

Now let's use the grokker to grok a new handle function::

  >>> def handle_jpg(filepath):
  ...   return "JPG file"
  >>> filetype_grokker.grok('handle_jpg', handle_jpg)
  True

After we grokked, we have registered a handler for ``.jpg`` files
(the extension to register under was deduced from the function name)::

  >>> sorted(filehandler.extension_handlers.keys())
  ['.jpg', '.png', '.txt', '.xml']

This means now our ``filehandler.handle`` function is now able to
handle JPG files as well::

  >>> filehandler.handle('image2.jpg')
  'JPG file'

If we try to grok a function that doesn't start with ``handle_`` in its
name, nothing will happen::

  >>> def something(filepath):
  ...   return 'Something'
  >>> filetype_grokker.grok('something', something)
  False
  >>> 'something' in filehandler.extension_handlers
  False

Grokking a module
-----------------

Grokking individual components is useful, but to make Martian really
useful we need to be able to grok whole modules or packages as well.
Let's look at a special grokker that can grok a Python module, the
``ModuleGrokker``.

The idea is that the ``ModuleGrokker`` groks any components in a
module that it recognizes. A ``ModuleGrokker`` does not work alone. It
needs to be supplied with one or more grokkers that can grok the
components to be founded in a module::

  >>> module_grokker = martian.ModuleGrokker()
  >>> module_grokker.register(filetype_grokker)

We now define a module that defines a few filetype handlers to be
grokked::

  >>> class lotsofhandlers(FakeModule):
  ...   def handle_exe(filepath):
  ...     return "EXE file"
  ...
  ...   def handle_ogg(filepath):
  ...     return "OGG file"
  ...
  ...   def handle_svg(filepath):
  ...     return "SVG file"
  >>> from martiantest.fake import lotsofhandlers

Let's grok it::

  >>> module_grokker.grok('lotsofhandlers', lotsofhandlers)
  True

The new registrations are now available::

  >>> sorted(filehandler.extension_handlers.keys())
  ['.exe', '.jpg', '.ogg', '.png', '.svg', '.txt', '.xml']

The system indeed recognizes them now::

  >>> filehandler.handle('test.ogg')
  'OGG file'
  >>> filehandler.handle('test.svg')
  'SVG file'
  >>> filehandler.handle('test.exe')
  'EXE file'

As you can see, with Martian we can now define handlers without ever
having to register them manually. This allows us to rewrite our
original module and take out the manual registrations completely::

  >>> class filehandler(FakeModule):
  ...   import os
  ...
  ...   def handle_txt(filepath):
  ...     return "Text file"
  ...
  ...   def handle_xml(filepath):
  ...     return "XML file"
  ...
  ...   extension_handlers = {}
  ...
  ...   def handle(filepath):
  ...      name, ext = os.path.splitext(filepath)
  ...      return extension_handlers[ext](filepath)
  >>> from martiantest.fake import filehandler

Let's use martian to do the registrations for us::

  >>> module_grokker.grok('filehandler', filehandler)
  True
  >>> filehandler.handle('test.txt')
  'Text file'

InstanceGrokker
---------------

We have seen how to grok module-level functions. Let's now grok some
other kind of instance, a ``Color``::

  >>> class color(FakeModule):
  ...   class Color(object):
  ...     def __init__(self, r, g, b):
  ...       self.r = r
  ...       self.g = g
  ...       self.b = b
  ...     def __repr__(self):
  ...       return '<Color %s %s %s>' % (self.r, self.g, self.b)
  ...   all_colors = {}
  >>> from martiantest.fake import color

We now want a grokker that can recognize colors and put them in the
``all_colors`` dictionary, with the names as the keys, and the color
object as the values. We can use ``InstanceGrokker`` to construct it::

  >>> class ColorGrokker(martian.InstanceGrokker):
  ...   martian.component(color.Color)
  ...   def grok(self, name, obj, **kw):
  ...     color.all_colors[name] = obj
  ...     return True

Let's create ``color_grokker`` and grok a color::

  >>> color_grokker = ColorGrokker()
  >>> black = color.Color(0, 0, 0) # we DO consider black as a color :)
  >>> color_grokker.grok('black', black)
  True

It ends up in the ``all_colors`` dictionary::

  >>> color.all_colors
  {'black': <Color 0 0 0>}

If we put ``color_grokker`` into a ``ModuleGrokker``, we can now grok
multiple colors in a module::

  >>> Color = color.Color
  >>> class colors(FakeModule):
  ...   red = Color(255, 0, 0)
  ...   green = Color(0, 255, 0)
  ...   blue = Color(0, 0, 255)
  ...   white = Color(255, 255, 255)
  >>> from martiantest.fake import colors
  >>> colors_grokker = martian.ModuleGrokker()
  >>> colors_grokker.register(color_grokker)
  >>> colors_grokker.grok('colors', colors)
  True
  >>> sorted(color.all_colors.items())
  [('black', <Color 0 0 0>),
   ('blue', <Color 0 0 255>),
   ('green', <Color 0 255 0>),
   ('red', <Color 255 0 0>),
   ('white', <Color 255 255 255>)]

Subclasses of ``Color`` are also grokked::

  >>> class subcolors(FakeModule):
  ...   class SpecialColor(Color):
  ...     pass
  ...   octarine = SpecialColor(-255, 0, -255)
  >>> from martiantest.fake import subcolors
  >>> colors_grokker.grok('subcolors', subcolors)
  True
  >>> 'octarine' in color.all_colors
  True

MultiInstanceGrokker
--------------------

In the previous section we have created a particular grokker that
looks for instances of a component class, in this case
``Color``. Let's introduce another ``InstanceGrokker`` that looks for
instances of ``Sound``::

  >>> class sound(FakeModule):
  ...   class Sound(object):
  ...     def __init__(self, desc):
  ...       self.desc = desc
  ...     def __repr__(self):
  ...       return '<Sound %s>' % (self.desc)
  ...   all_sounds = {}
  >>> from martiantest.fake import sound

  >>> class SoundGrokker(martian.InstanceGrokker):
  ...   martian.component(sound.Sound)
  ...   def grok(self, name, obj, **kw):
  ...     sound.all_sounds[name] = obj
  ...     return True
  >>> sound_grokker = SoundGrokker()

What if we now want to look for ``Sound`` and ``Color`` instances at
the same time? We have to use the ``color_grokker`` and
``sound_grokker`` at the same time, and we can do this with a
``MultiInstanceGrokker``::

  >>> from martian.core import MultiInstanceGrokker
  >>> multi_grokker = MultiInstanceGrokker()
  >>> multi_grokker.register(color_grokker)
  >>> multi_grokker.register(sound_grokker)

Let's grok a new color with our ``multi_grokker``::

  >>> grey = Color(100, 100, 100)
  >>> multi_grokker.grok('grey', grey)
  True
  >>> 'grey' in color.all_colors
  True

Let's grok a sound with our ``multi_grokker``::

  >>> moo = sound.Sound('Moo!')
  >>> multi_grokker.grok('moo', moo)
  True
  >>> 'moo' in sound.all_sounds
  True

We can also grok other objects, but this will have no effect::

  >>> something_else = object()
  >>> multi_grokker.grok('something_else', something_else)
  False

Let's put our ``multi_grokker`` in a ``ModuleGrokker``. We can do
this by passing it explicitly to the ``ModuleGrokker`` factory::

  >>> module_grokker = martian.ModuleGrokker(grokker=multi_grokker)

We can now grok a module for both ``Color`` and ``Sound`` instances::

  >>> Sound = sound.Sound
  >>> class lightandsound(FakeModule):
  ...   dark_red = Color(150, 0, 0)
  ...   scream = Sound('scream')
  ...   dark_green = Color(0, 150, 0)
  ...   cheer = Sound('cheer')
  >>> from martiantest.fake import lightandsound
  >>> module_grokker.grok('lightandsound', lightandsound)
  True
  >>> 'dark_red' in color.all_colors
  True
  >>> 'dark_green' in color.all_colors
  True
  >>> 'scream' in sound.all_sounds
  True
  >>> 'cheer' in sound.all_sounds
  True

ClassGrokker
------------

Besides instances we can also grok classes. Let's define an
application where we register classes representing animals.  Animals
can be given names using the ``name`` directive::

  >>> from martian.directive import Directive, CLASS, ONCE

  >>> class animal(FakeModule):
  ...   class name(Directive):
  ...     scope = CLASS
  ...     store = ONCE
  ...
  ...   class Animal(object):
  ...     def __repr__(self):
  ...       return '<Animal %s>' % animal.name.bind().get(self)
  ...
  ...   all_animals = {}
  ...   def create_animal(name):
  ...     return all_animals[name]()
  >>> from martiantest.fake import animal

Let's define a grokker that can grok an ``Animal``.  We could either
implement the ``grok`` method as with ``InstanceGrokkers``, or we can
rely on the implementation that the baseclass already provides.  In
the latter case, we just have to declare what directives the grokker
may want to use on the class and the implement the ``execute`` method::

  >>> class AnimalGrokker(martian.ClassGrokker):
  ...   martian.component(animal.Animal)
  ...   martian.directive(animal.name)
  ...   def execute(self, class_, name, **kw):
  ...     animal.all_animals[name] = class_
  ...     return True

Let's test our grokker::

  >>> class Snake(animal.Animal):
  ...   animal.name('snake')
  ...
  >>> animal_grokker = AnimalGrokker()
  >>> animal_grokker.grok('Snake', Snake)
  True
  >>> animal.all_animals.keys()
  ['snake']

We can create a snake now::

  >>> animal.create_animal('snake')
  <Animal snake>

Note that we can supply a different default value for the directive
default::

  >>> class AnimalGrokker(AnimalGrokker):
  ...   martian.directive(animal.name, default='generic animal')
  ...
  >>> class Generic(animal.Animal):
  ...   pass
  ...
  >>> animal_grokker = AnimalGrokker()
  >>> animal_grokker.grok('Generic', Generic)
  True
  >>> sorted(animal.all_animals.keys())
  ['generic animal', 'snake']

Moreover, we can also supply a default factory that may want to
determine a dynamic default value based on the class that's being
grokked.  For instance, let's say the default name of an animal should
the class name converted to lowercase letters::

  >>> def default_animal_name(class_, module, **data):
  ...   return class_.__name__.lower()
  ...
  >>> class AnimalGrokker(AnimalGrokker):
  ...   martian.directive(animal.name, get_default=default_animal_name)
  ...
  >>> class Mouse(animal.Animal):
  ...   pass
  ...
  >>> animal_grokker = AnimalGrokker()
  >>> animal_grokker.grok('Mouse', Mouse)
  True
  >>> sorted(animal.all_animals.keys())
  ['generic animal', 'mouse', 'snake']

Note that these default value factories will also get the data from
all directives that are in front of them in the grokker's directive
list.  For instance, consider the following directive:

  >>> class zoologicalname(animal.name):
  ...   pass
  ...

with the following default rule that takes the regular name as the
default zoological name::

  >>> def default_zoological_name(class_, module, name, **data):
  ...   return name
  ...
  >>> class ZooAnimalGrokker(martian.ClassGrokker):
  ...   martian.component(animal.Animal)
  ...   martian.directive(animal.name, get_default=default_animal_name)
  ...   martian.directive(zoologicalname, get_default=default_zoological_name)
  ...
  ...   def execute(self, class_, name, zoologicalname, **kw):
  ...     print zoologicalname
  ...     return True
  ...
  >>> class Hippopotamus(animal.Animal):
  ...   pass
  ...   # No need to use animal.name(), we'll take the class name as default.
  ...   # The zoological name is the same as well.
  ...

  >>> zoo_animal_grokker = ZooAnimalGrokker()
  >>> zoo_animal_grokker.grok('Hippopotamus', Hippopotamus)
  hippopotamus
  True

If you pass a non-directive to ``martian.directive``, you get an error::

  >>> class Test(martian.ClassGrokker):
  ...    martian.directive('foo')
  Traceback (most recent call last):
  GrokImportError: The 'directive' directive can only be called with a directive.

MethodGrokker
-------------

A special kind of class grokker is the ``MethodGrokker``.  It inspects
the class at hand and calls ``execute`` for each *method* the class
provides.

Consider the following baseclass for circus animals:

  >>> class CircusAnimal(animal.Animal):
  ...   def begin_show(self):
  ...     pass
  ...   def end_show(self):
  ...      pass

Circus animals define lots of methods which we'll collect using this
grokker:

  >>> circus_animals = {}
  >>> from martian import MethodGrokker
  >>> class CircusAnimalGrokker(MethodGrokker):
  ...   martian.component(CircusAnimal)
  ...   def execute(self, class_, method, **kw):
  ...     circus_animals.setdefault(class_.__name__, []).append(method.__name__)
  ...     return True
  ...

Now consider the following circus animals:

  >>> class Monkey(CircusAnimal):
  ...   def climb(self):
  ...     pass
  ...   def _take_dump(self):
  ...     pass
  ...
  >>> class Camel(CircusAnimal):
  ...   def walk(self):
  ...     pass
  ...   def spit(self):
  ...     pass

  >>> circus_animal_grokker = CircusAnimalGrokker()
  >>> circus_animal_grokker.grok('Monkey', Monkey)
  True
  >>> circus_animal_grokker.grok('Camel', Camel)
  True

Let's look at the results:

  >>> for circus_animal, methods in sorted(circus_animals.items()):
  ...     print "%s can %s." % (circus_animal, " and ".join(sorted(methods)))
  ...
  Camel can spit and walk.
  Monkey can climb.

As we see, private methods (those beginning with underscores) have
been ignored.  Furthermore, methods inherited from the component
baseclass (in this case ``CircusAnimal``) have also been ignored.

If we wrote a class without any methods, we would encounter an error:

  >>> class Snail(CircusAnimal):
  ...   pass

  >>> circus_animal_grokker.grok('Snail', Snail)
  Traceback (most recent call last):
    ...
  GrokError: <class 'Snail'> does not define any public
  methods. Please add methods to this class to enable its
  registration.

MultiClassGrokker
-----------------

We now want to be able to grok the following module and have the
``Animal`` subclasses (but not the ``Chair`` class, which is not an
animal) automatically become available::

  >>> class animals(FakeModule):
  ...   class Elephant(animal.Animal):
  ...     animal.name('elephant')
  ...   class Tiger(animal.Animal):
  ...     animal.name('tiger')
  ...   class Lion(animal.Animal):
  ...     animal.name('lion')
  ...   class Chair(object):
  ...     animal.name('chair')
  >>> from martiantest.fake import animals

First we need to wrap our ``AnimalGrokker`` into a ``MultiClassGrokker``::

 >>> from martian.core import MultiClassGrokker
 >>> multi_grokker = MultiClassGrokker()
 >>> multi_grokker.register(animal_grokker)

Now let's wrap it into a ``ModuleGrokker`` and grok the module::

  >>> grokker = martian.ModuleGrokker(grokker=multi_grokker)
  >>> grokker.grok('animals', animals)
  True

The animals (but not anything else) should have become available::

  >>> sorted(animal.all_animals.keys())
  ['elephant', 'generic animal', 'lion', 'mouse', 'snake', 'tiger']

We can create animals using their name now::

  >>> animal.create_animal('elephant')
  <Animal elephant>
  >>> animal.create_animal('tiger')
  <Animal tiger>

MultiGrokker
------------

``MultiInstanceGrokker`` and ``MultiClassGrokker`` can grok instances
and classes respectively, but a ``MultiInstanceGrokker`` won't work
correctly if it runs into a class and vice versa. For that we use a
``MultiGrokker``, which can deal with the full range of objects that
can be grokked, and skips those it doesn't recognize.

Let's fill a ``MultiGrokker`` with a bunch of grokkers::

  >>> from martian import MultiGrokker
  >>> multi = MultiGrokker()
  >>> multi.register(filetype_grokker)
  >>> multi.register(color_grokker)
  >>> multi.register(sound_grokker)
  >>> multi.register(animal_grokker)

Let's try it with some individual objects::

  >>> class Whale(animal.Animal):
  ...    animal.name('whale')
  >>> multi.grok('Whale', Whale)
  True
  >>> 'whale' in animal.all_animals
  True

This should have no effect, but not fail::

  >>> my_whale = Whale()
  >>> multi.grok('my_whale', my_whale)
  False

Grokked by the ColorGrokker::

  >>> multi.grok('dark_grey', Color(50, 50, 50))
  True
  >>> 'dark_grey' in color.all_colors
  True

Grokked by the SoundGrokker::

  >>> multi.grok('music', Sound('music'))
  True
  >>> 'music' in sound.all_sounds
  True

Not grokked::

  >>> class RockMusic(Sound):
  ...   pass
  >>> multi.grok('RockMusic', RockMusic)
  False

Grokked by SoundGrokker::

  >>> multi.grok('rocknroll', RockMusic('rock n roll'))
  True
  >>> 'rocknroll' in sound.all_sounds
  True

Not grokked::

  >>> class Chair(object):
  ...   pass
  >>> multi.grok('Chair', Chair)
  False

Grokked by ``filetype_grokker``::

  >>> def handle_py(filepath):
  ...   return "Python file"
  >>> multi.grok('handle_py', handle_py)
  True
  >>> '.py' in filehandler.extension_handlers
  True

Not grokked:

  >>> def foo():
  ...   pass
  >>> multi.grok('foo', foo)
  False

Not grokked either::

  >>> another = object()
  >>> multi.grok('another', another)
  False

Let's make a module which has a mixture between classes and instances,
some of which can be grokked::

  >>> class mix(FakeModule):
  ...   # grokked by AnimalGrokker
  ...   class Whale(animal.Animal):
  ...      animal.name('whale')
  ...   # not grokked
  ...   my_whale = Whale()
  ...   # grokked by ColorGrokker
  ...   dark_grey = Color(50, 50, 50)
  ...   # grokked by SoundGrokker
  ...   music = Sound('music')
  ...   # not grokked
  ...   class RockMusic(Sound):
  ...      pass
  ...   # grokked by SoundGrokker
  ...   rocknroll = RockMusic('rock n roll')
  ...   # grokked by AnimalGrokker
  ...   class Dragon(animal.Animal):
  ...     animal.name('dragon')
  ...   # not grokked
  ...   class Chair(object):
  ...     pass
  ...   # grokked by filetype_grokker
  ...   def handle_py(filepath):
  ...     return "Python file"
  ...   # not grokked
  ...   def foo():
  ...     pass
  ...   # grokked by AnimalGrokker
  ...   class SpermWhale(Whale):
  ...     animal.name('sperm whale')
  ...   # not grokked
  ...   another = object()
  >>> from martiantest.fake import mix

Let's construct a ``ModuleGrokker`` that can grok this module::

  >>> mix_grokker = martian.ModuleGrokker(grokker=multi)

Note that this is actually equivalent to calling ``ModuleGrokker``
without arguments and then calling ``register`` for the individual
``ClassGrokker`` and ``InstanceGrokker`` objects.

Before we do the grokking, let's clean up our registration
dictionaries::

  >>> filehandler.extension_handlers = {}
  >>> color.all_colors = {}
  >>> sound.all_sounds = {}
  >>> animal.all_animals = {}

Now we grok::

  >>> mix_grokker.grok('mix', mix)
  True
  >>> sorted(filehandler.extension_handlers.keys())
  ['.py']
  >>> sorted(color.all_colors.keys())
  ['dark_grey']
  >>> sorted(sound.all_sounds.keys())
  ['music', 'rocknroll']
  >>> sorted(animal.all_animals.keys())
  ['dragon', 'sperm whale', 'whale']

GlobalGrokker
-------------

Sometimes you want to let a grok action happen for each module. The
grok action could for instance read the globals of a module, or even
static files associated with the module by name. Let's create a module
with some global value::

  >>> class g(FakeModule):
  ...   amount = 50
  >>> from martiantest.fake import g

Now let's create a ``GlobalGrokker`` that reads ``amount`` and stores
it in the ``read_amount`` dictionary::

  >>> read_amount = {}
  >>> from martian import GlobalGrokker
  >>> class AmountGrokker(GlobalGrokker):
  ...   def grok(self, name, module, **kw):
  ...     read_amount[None] = module.amount
  ...     return True

Let's construct a ``ModuleGrokker`` with this ``GlobalGrokker`` registered::

  >>> grokker = martian.ModuleGrokker()
  >>> grokker.register(AmountGrokker())

Now we grok and should pick up the right value::

  >>> grokker.grok('g', g)
  True
  >>> read_amount[None]
  50

Old-style class support
-----------------------

So far we have only grokked either new-style classes or instances of
new-style classes. It is also possible to grok old-style classes and
their instances::

  >>> class oldstyle(FakeModule):
  ...   class Machine:
  ...     pass
  ...   all_machines = {}
  ...   all_machine_instances = {}
  >>> from martiantest.fake import oldstyle

Let's make a grokker for the old style class::

  >>> class MachineGrokker(martian.ClassGrokker):
  ...   martian.component(oldstyle.Machine)
  ...   def grok(self, name, obj, **kw):
  ...     oldstyle.all_machines[name] = obj
  ...     return True

And another grokker for old style instances::

  >>> class MachineInstanceGrokker(martian.InstanceGrokker):
  ...   martian.component(oldstyle.Machine)
  ...   def grok(self, name, obj, **kw):
  ...     oldstyle.all_machine_instances[name] = obj
  ...     return True

The multi grokker should succesfully grok the old-style ``Machine`` class
and instances of it::

  >>> multi = MultiGrokker()
  >>> multi.register(MachineGrokker())
  >>> multi.register(MachineInstanceGrokker())
  >>> class Robot(oldstyle.Machine):
  ...   pass
  >>> multi.grok('Robot', Robot)
  True
  >>> oldstyle.all_machines.keys()
  ['Robot']
  >>> robot = Robot()
  >>> multi.grok('robot', robot)
  True
  >>> oldstyle.all_machine_instances.keys()
  ['robot']

Grokking a package
------------------

A package consists of several sub modules. When grokking a package,
all the files in the package will be grokked. Let's first create a simple
grokker for the ``Animal`` class defined by the package::

  >>> from martian.tests.testpackage import animal
  >>> all_animals = {}
  >>> class AnimalGrokker(martian.ClassGrokker):
  ...   martian.component(animal.Animal)
  ...   def grok(self, name, obj, **kw):
  ...     all_animals[name] = obj
  ...     return True

The grokker will collect animals into the ``all_animals`` dictionary.

Let's register this grokker for a ModuleGrokker::

  >>> module_grokker = martian.ModuleGrokker()
  >>> module_grokker.register(AnimalGrokker())

Now let's grok the whole ``testpackage`` for animals::

  >>> from martian import grok_dotted_name
  >>> grok_dotted_name('martian.tests.testpackage', grokker=module_grokker)

We should now get some animals::

  >>> sorted(all_animals.keys())
  ['Animal', 'Bear', 'Dragon', 'Lizard', 'Python', 'SpermWhale', 'Whale']

Preparation and finalization
----------------------------

Before grokking a module, it may be that we need to do some
preparation. This preparation can include setting up some parameters
to pass along to the grokking process, for instance. We can pass
a ``prepare`` function a the ModuleGrokker::

  >>> class Number(object):
  ...   def __init__(self, nr):
  ...     self.nr = nr
  >>> all_numbers = {}
  >>> class NumberGrokker(martian.InstanceGrokker):
  ...  martian.component(Number)
  ...  def grok(self, name, obj, multiplier, **kw):
  ...    all_numbers[obj.nr] = obj.nr * multiplier
  ...    return True
  >>> def prepare(name, module, kw):
  ...   kw['multiplier'] = 3
  >>> module_grokker = martian.ModuleGrokker(prepare=prepare)
  >>> module_grokker.register(NumberGrokker())

We have created a ``prepare`` function that does one thing: create a
``multiplier`` parameter that is passed along the grokking
process. The ``NumberGrokker`` makes use of this to prepare the
``all_numbers`` dictionary values.

Let's try this with a module::

  >>> class numbers(FakeModule):
  ...   one = Number(1)
  ...   two = Number(2)
  ...   four = Number(4)
  >>> from martiantest.fake import numbers
  >>> module_grokker.grok('numbers', numbers)
  True
  >>> sorted(all_numbers.items())
  [(1, 3), (2, 6), (4, 12)]

You can also optionally register a finalization function, which will
be run at the end of a module grok::

  >>> def finalize(name, module, kw):
  ...     all_numbers['finalized'] = True
  >>> module_grokker = martian.ModuleGrokker(prepare=prepare, finalize=finalize)
  >>> module_grokker.register(NumberGrokker())
  >>> all_numbers = {}
  >>> module_grokker.grok('numbers', numbers)
  True
  >>> 'finalized' in all_numbers
  True

Sanity checking
---------------

Grokkers must return ``True`` if grokking succeeded, or ``False`` if
it didn't. If they return something else (typically ``None`` as the
programmer forgot to), the system will raise an error::

  >>> class BrokenGrokker(martian.InstanceGrokker):
  ...  martian.component(Number)
  ...  def grok(self, name, obj, **kw):
  ...    pass

  >>> module_grokker = martian.ModuleGrokker()
  >>> module_grokker.register(BrokenGrokker())
  >>> module_grokker.grok('numbers', numbers)
  Traceback (most recent call last):
    ...
  GrokError: <BrokenGrokker object at ...> returns None instead of
  True or False.

Let's also try this with a GlobalGrokker::

  >>> class MyGrokker(GlobalGrokker):
  ...   def grok(self, name, module, **kw):
  ...     return "Foo"
  >>> module_grokker = martian.ModuleGrokker()
  >>> module_grokker.register(MyGrokker())
  >>> module_grokker.grok('numbers', numbers)
  Traceback (most recent call last):
    ...
  GrokError: <MyGrokker object at ...> returns 'Foo' instead of True or False.

Meta Grokkers
-------------

Meta grokkers are grokkers that grok grokkers. This mechanism can be
used to extend Martian. Let's register a ``ClassMetaGrokker`` that
looks for subclasses of ``ClassGrokker``::

  >>> from martian.core import MetaGrokker
  >>> class ClassMetaGrokker(MetaGrokker):
  ...   martian.component(martian.ClassGrokker)
  >>> multi_grokker = MultiGrokker()
  >>> multi_grokker.register(ClassMetaGrokker(multi_grokker))

``multi_grokker`` should now grok subclasses of ``ClassGrokker``, such
as ``AnimalGrokker``::

  >>> all_animals = {} # clean out animal registry
  >>> multi_grokker.grok('AnimalGrokker', AnimalGrokker)
  True

Our multi_grokker should now also be able to grok animals::

  >>> class Woodpecker(animal.Animal):
  ...   pass
  >>> multi_grokker.grok('Woodpecker', Woodpecker)
  True

A ``MetaMultiGrokker`` is a ``MultiGrokker`` that comes preconfigured with
grokkers for ``ClassGrokker``, ``InstanceGrokker`` and ``GlobalGrokker``::

  >>> from martian import MetaMultiGrokker
  >>> multi_grokker = MetaMultiGrokker()

It works for ``ClassGrokker``::

  >>> all_animals = {}
  >>> multi_grokker.grok('AnimalGrokker', AnimalGrokker)
  True
  >>> multi_grokker.grok('Woodpecker', Woodpecker)
  True
  >>> all_animals
  {'Woodpecker': <class 'Woodpecker'>}

and for ``InstanceGrokker``::

  >>> color.all_colors = {}
  >>> multi_grokker.grok('ColorGrokker', ColorGrokker)
  True
  >>> multi_grokker.grok('color', Color(255, 0, 0))
  True
  >>> color.all_colors
  {'color': <Color 255 0 0>}

and for ``GlobalGrokker``::

  >>> read_amount = {}
  >>> multi_grokker.grok('AmountGrokker', AmountGrokker)
  True
  >>> grokker.grok('g', g)
  True
  >>> read_amount[None]
  50

We can clear the meta multi grokker::

  >>> multi_grokker.clear()

It won't grok particular classes or instances anymore::

  >>> multi_grokker.grok('Woodpecker', Woodpecker)
  False
  >>> multi_grokker.grok('color', Color(255, 0, 0))
  False

It can still grok grokkers::

  >>> multi_grokker.grok('ColorGrokker', ColorGrokker)
  True

Executing meta grokkers only once
---------------------------------

In case of ``ClassGrokker`` and all other grokkers that are grokked
by meta grokkers, we only want the grokking to occur once even if
the same module (or package) is grokked twice::

  >>> class TestOnce(object):
  ...   pass
  >>> executed = []
  >>> class somemodule(FakeModule):
  ...   class TestGrokker(martian.ClassGrokker):
  ...     martian.component(TestOnce)
  ...     def grok(self, name, obj, **kw):
  ...        executed.append(name)
  ...        return True
  >>> from martiantest.fake import somemodule
  >>> module_grokker = martian.ModuleGrokker(MetaMultiGrokker())

Let's grok the module once::

  >>> module_grokker.grok('somemodule', somemodule)
  True

Let's grok it twice::

  >>> module_grokker.grok('somemodule', somemodule)
  True

Even though we have grokked it twice, it is still only registered once. We
can show this by actually having it grok a ``TestOnce`` subclass::

  >>> class anothermodule(FakeModule):
  ...   class TestSub(TestOnce):
  ...      pass
  >>> from martiantest.fake import anothermodule
  >>> module_grokker.grok('anothermodule', anothermodule)
  True
  >>> executed
  ['TestSub']

This also works for instance grokkers::

  >>> class TestInstanceOnce(object):
  ...   pass
  >>> executed = []
  >>> class somemodule(FakeModule):
  ...   class TestGrokker(martian.InstanceGrokker):
  ...     martian.component(TestInstanceOnce)
  ...     def grok(self, name, obj, **kw):
  ...        executed.append(name)
  ...        return True
  >>> from martiantest.fake import somemodule
  >>> module_grokker.clear()
  >>> module_grokker.grok('somemodule', somemodule) # once
  True
  >>> module_grokker.grok('somemodule', somemodule) # twice
  True
  >>> class anothermodule(FakeModule):
  ...   test = TestInstanceOnce()
  >>> from martiantest.fake import anothermodule
  >>> module_grokker.grok('anothermodule', anothermodule)
  True
  >>> executed
  ['test']

It also works for global grokkers::

  >>> executed = []
  >>> class somemodule(FakeModule):
  ...   class TestGrokker(GlobalGrokker):
  ...     def grok(self, name, obj, **kw):
  ...       executed.append(name)
  ...       return True
  >>> from martiantest.fake import somemodule
  >>> module_grokker.clear()
  >>> module_grokker.grok('somemodule', somemodule) # once
  True
  >>> module_grokker.grok('somemodule', somemodule) # twice
  True

The second grokking will already make ``somemodule`` grokked::

  >>> executed
  ['somemodule']

Now let's grok another module::

  >>> class anothermodule(FakeModule):
  ...   pass
  >>> from martiantest.fake import anothermodule
  >>> module_grokker.grok('anothermodule', anothermodule)
  True
  >>> executed
  ['somemodule', 'anothermodule']

Priority
--------

When grokking a module using a ``ModuleGrokker``, grokker execution
can be determined by their priority. By default, grokkers have a
priority of ``0``. Let's define two base classes, ``A`` and ``B``,
which can be grokked::

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

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

Let's define a special kind of class grokker that records the order in
which names get grokked::

  >>> order = []
  >>> class OrderGrokker(martian.ClassGrokker):
  ...   def grok(self, name, obj, **kw):
  ...     order.append(name)
  ...     return True

Now we define two grokkers for subclasses of ``A`` and ``B``, where
the ``BGrokker`` has a higher priority::

  >>> class AGrokker(OrderGrokker):
  ...   martian.component(A)
  >>> class BGrokker(OrderGrokker):
  ...   martian.component(B)
  ...   martian.priority(10)

Let's register these grokkers::

  >>> multi_grokker = MetaMultiGrokker()
  >>> multi_grokker.grok('AGrokker', AGrokker)
  True
  >>> multi_grokker.grok('BGrokker', BGrokker)
  True

Let's create a module containing ``A`` and ``B`` subclasses::

  >>> class mymodule(FakeModule):
  ...   class ASub(A):
  ...     pass
  ...   class BSub(B):
  ...     pass
  >>> from martiantest.fake import mymodule

We'll grok it::

  >>> module_grokker = martian.ModuleGrokker(multi_grokker)
  >>> module_grokker.grok('mymodule', mymodule)
  True

Since the ``BGrokker`` has a higher priority, we expect the following
order of grokking::

  >>> order
  ['BSub', 'ASub']

This also works for GlobalGrokkers. We will define a GlobalGrokker
that has a higher priority than the default, but lower than B::

  >>> class MyGlobalGrokker(GlobalGrokker):
  ...   martian.priority(5)
  ...   def grok(self, name, obj, **kw):
  ...     order.append(name)
  ...     return True
  >>> multi_grokker.grok('MyGlobalGrokker', MyGlobalGrokker)
  True

We will grok the module again::

  >>> order = []
  >>> module_grokker.grok('mymodule', mymodule)
  True

This time, the global grokker should appear after 'BSub' but before 'ASub'::

  >>> order
  ['BSub', 'mymodule', 'ASub']


Module info
-----------

In addition to the ``name`` and ``object`` positional arguments,
grokkers will get also get a ``module_info`` keyword argument.  It is
an ``IModuleInfo`` object which can be used, for example, to query
module annotations.  Consider the following grokker:

  >>> from martian.error import GrokError
  >>> class AnnotationsGrokker(GlobalGrokker):
  ...   def grok(self, name, module, module_info, **kw):
  ...       ann = module_info.getAnnotation('some.annotation', None)
  ...       if ann is None:
  ...           raise GrokError('Did not find annotation!', module)
  ...       if ann != 'ME GROK SAY HI':
  ...           raise GrokError('Wrong annotation!', module)
  ...       return True

Now let's provide a fake module:

  >>> import new, sys
  >>> annotations = new.module('annotations')
  >>> annotations.__file__ = '/fake/module/annotations.py'
  >>> sys.modules['annotations'] = annotations

Clearly, it can't find the module-level variable yet:

  >>> module_grokker = martian.ModuleGrokker()
  >>> module_grokker.register(AnnotationsGrokker())
  >>> import martian
  >>> martian.grok_dotted_name('annotations', module_grokker)
  Traceback (most recent call last):
  ...
  GrokError: Did not find annotation!

Let's provide the annotation so that the grokker works as expected:

  >>> annotations.__some_annotation__ = 'ME GROK SAY HI'
  >>> martian.grok_dotted_name('annotations', module_grokker)

Finally clean up:

  >>> del sys.modules['annotations']