python-martian 0.14-0ubuntu1 / usr / share / pyshared / martian / directive.txt

Directives
==========

When grokking a class, the grokking procedure can be informed by
directives, on a class, or a module. If a directive is absent, the
system falls back to a default. Here we introduce a general way to
define these directives, and how to use them to retrieve information
for a class for use during the grokking procedure.

A simple directive
------------------

We define a simple directive that sets a description::

  >>> from martian import Directive, CLASS, ONCE
  >>> class description(Directive):
  ...     scope = CLASS
  ...     store = ONCE
  ...     default = u''

The name of the directive is ``description``. We specify that the
directive can only be used in the scope of a class. We also specify it
can only be used a single time. Finally we define the default in case
the directive is absent (the empty string).

Now we look at the directive in action::

  >>> class Foo(object):
  ...    description(u"This is a description")

After setting the description, we bind the directive and obtain a
bound directive object.  This object has means for retrieving the data
set by the directive, in particular the ``get`` method::

  >>> description.bind().get(Foo)
  u'This is a description'

Directives in different namespaces get stored differently. We'll
define a similar directive in another namespace::

  >>> class description2(description):
  ...     pass

  >>> class Foo(object):
  ...     description(u"Description1")
  ...     description2(u"Description2")
  >>> description.bind().get(Foo)
  u'Description1'
  >>> description2.bind().get(Foo)
  u'Description2'

If we check the value of a class without the directive, we see the
default value for that directive, this case the empty unicode string::

  >>> class Foo(object):
  ...     pass
  >>> description.bind().get(Foo)
  u''

In certain cases we need to set a value on a component as if the directive was
actually used::

  >>> description.set(Foo, u'value as set')
  >>> description.bind().get(Foo)
  u'value as set'

Subclasses of the original class will inherit the properties set by the
directive:

  >>> class Foo(object):
  ...     description('This is a foo.')
  ...
  >>> class Bar(Foo):
  ...     pass
  ...
  >>> description.bind().get(Bar)
  'This is a foo.'

When we use the directive outside of class scope, we get an error
message::

  >>> description('Description')
  Traceback (most recent call last):
    ...
  GrokImportError: The 'description' directive can only be used on class level.

In particular, we cannot use it in a module::

  >>> class testmodule(FakeModule):
  ...   fake_module = True
  ...   description("Description")
  Traceback (most recent call last):
    ...
  GrokImportError: The 'description' directive can only be used on class level.

We cannot use the directive twice in the class scope. If we do so, we
get an error message as well::

  >>> class Foo(object):
  ...   description(u"Description1")
  ...   description(u"Description2")
  Traceback (most recent call last):
    ...
  GrokImportError: The 'description' directive can only be called once per class.

We cannot call the directive with no argument either::

  >>> class Foo(object):
  ...   description()
  Traceback (most recent call last):
    ...
  TypeError: description takes exactly 1 argument (0 given)

Class and module scope
----------------------

We define a ``layer`` directive that can be used in class and module
scope both::

  >>> from martian import CLASS_OR_MODULE
  >>> class layer(Directive):
  ...     scope = CLASS_OR_MODULE
  ...     store = ONCE

By default, the ``default`` property is None which is why we can omit
specifying it here.

This directive has been declared ``CLASS_OR_MODULE``, so you will
always have to pass a module to the directive. Since we don't have a
module yet we'll simply create a dummy, empty, fallback module::

  >>> dummy = object()

We can use this directive now on a class::

  >>> class Foo(object):
  ...   layer('Test')
  >>> layer.bind().get(Foo, dummy)
  'Test'

The defaulting to ``None`` works::

  >>> class Foo(object):
  ...   pass
  >>> layer.bind().get(Foo, dummy) is None
  True

We can also use it in a module::

  >>> class testmodule(FakeModule):
  ...    layer('Test2')
  ...    class Foo(object):
  ...       pass
  >>> from martiantest.fake import testmodule

When we now try to access ``layer`` on ``Foo``, we find the
module-level default which we just set. We pass the module as the
second argument to the ``get`` method to have it fall back on this::

  >>> layer.bind().get(testmodule.Foo, testmodule)
  'Test2'

Let's look at a module where the directive is not used::

  >>> class testmodule(FakeModule):
  ...   class Foo(object):
  ...      pass
  >>> from martiantest.fake import testmodule

In this case, the value cannot be found so the system falls back on
the default, ``None``::

  >>> layer.bind().get(testmodule.Foo, testmodule) is None
  True




Like with CLASS scope directive where values set are inherited by subclasses,
values set on a class or module level are inherited too, even if the subclass
is defined another module::

  >>> class testmodule_a(FakeModule):
  ...   layer('Value set on baseclass module')
  ...   class FooA(object):
  ...      pass
  >>> from martiantest.fake import testmodule_a
  >>>
  >>> class testmodule_b(FakeModule):
  ...   class FooB(testmodule_a.FooA):
  ...      pass
  >>> from martiantest.fake import testmodule_b

On the baseclass::

  >>> layer.bind().get(testmodule_a.FooA)
  'Value set on baseclass module'

Inherited by the subclass::

  >>> layer.bind().get(testmodule_b.FooB)
  'Value set on baseclass module'

Whenever there's a directive set on the baseclass' module, it will take
precedence like with "normal" inheritance::

  >>> class testmodule_c(FakeModule):
  ...   layer('Value set on subclass module')
  ...   class FooC(testmodule_a.FooA):
  ...      pass
  >>> from martiantest.fake import testmodule_c

  >>> layer.bind().get(testmodule_c.FooC)
  'Value set on subclass module'




Let's now look at this using a directive with CLASS scope only::

  >>> class layer2(Directive):
  ...     scope = CLASS
  ...     store = ONCE

Inheritance in combination with module scope
--------------------------------------------

Let's look at how our layer directive can inherit in combination with
directive use on module scope.

First we define a module which defines a class that gets the ``Test``
layer through the use of the layer directive on module scope::

  >>> class inheritmodule1(FakeModule):
  ...   layer('Test')
  ...   class Foo(object):
  ...      pass
  >>> from martiantest.fake import inheritmodule1

Now we define another module that has a class that inherits from ``Foo``::

  >>> class inheritmodule2(FakeModule):
  ...   class Bar(inheritmodule1.Foo):
  ...      pass
  >>> from martiantest.fake import inheritmodule2

We will now see that ``Bar`` has inherited the layer ``Test``::

  >>> layer.bind().get(inheritmodule2.Bar, inheritmodule2)
  'Test'

Let's try it with another level of inheritance::

  >>> class inheritmodule3(FakeModule):
  ...   class Baz(inheritmodule2.Bar):
  ...       pass
  >>> from martiantest.fake import inheritmodule3

The layer should still be inherited::

  >>> layer.bind().get(inheritmodule3.Baz, inheritmodule3)
  'Test'

Let's override now by having an explicit layer directive on the class
that subclasses ``Foo``::

  >>> class inheritmodule4(FakeModule):
  ...   class OverrideFoo(inheritmodule1.Foo):
  ...      layer('AnotherTest')
  >>> from martiantest.fake import inheritmodule4

  >>> layer.bind().get(inheritmodule4.OverrideFoo, inheritmodule4)
  'AnotherTest'

Let's override now by having an explicit layer directive on
module-level instead::

  >>> class inheritmodule5(FakeModule):
  ...   layer('AnotherTest')
  ...   class OverrideFoo(inheritmodule1.Foo):
  ...      pass
  >>> from martiantest.fake import inheritmodule5

  >>> layer.bind().get(inheritmodule5.OverrideFoo, inheritmodule5)
  'AnotherTest'

Inheritance with module scope also works for old-style classes::

  >>> class oldstyle1(FakeModule):
  ...    layer('one')
  ...    class Base:
  ...       pass
  >>> from martiantest.fake import oldstyle1
  >>> class oldstyle2(FakeModule):
  ...    class Sub(oldstyle1.Base):
  ...       pass
  >>> from martiantest.fake import oldstyle2
  >>> layer.bind().get(oldstyle2.Sub)
  'one'

Using a directive multiple times
--------------------------------

A directive can be configured to allow it to be called multiple times
in the same scope::

  >>> from martian import MultipleTimesDirective
  >>> class multi(MultipleTimesDirective):
  ...     scope = CLASS

We can now use the directive multiple times without any errors::

  >>> class Foo(object):
  ...   multi(u"Once")
  ...   multi(u"Twice")

We can now retrieve the value and we'll get a list::

  >>> multi.bind().get(Foo)
  [u'Once', u'Twice']

The default value for a MultipleTimesDirective is an empty list::

  >>> class Bar(object):
  ...   pass
  >>> multi.bind().get(Bar)
  []

Whenever the directive is used on a sub class of a component, the values set by
directives on the base classes are combined::

  >>> class Qux(Foo):
  ...     multi(u'Triple')
  ...
  >>> multi.bind().get(Qux)
  [u'Once', u'Twice', u'Triple']

You can also create a directive that ignores the values on the base classes::

  >>> from martian import MULTIPLE_NOBASE
  >>> class multi(Directive):
  ...     scope = CLASS
  ...     store = MULTIPLE_NOBASE

  >>> class Foo(object):
  ...     multi(u"Once")
  ...     multi(u"Twice")
  >>> multi.bind().get(Foo)
  [u'Once', u'Twice']

  >>> class Qux(Foo):
  ...     multi(u'Triple')
  ...     multi(u'More')
  >>> multi.bind().get(Qux)
  [u'Triple', u'More']

Using a directive multiple times, as a dictionary
-------------------------------------------------

A directive can be configured to allow it to be called multiple times in the
same scope. In this case the factory method should be overridden to return a
key-value pair::

  >>> from martian import DICT
  >>> class multi(Directive):
  ...     scope = CLASS
  ...     store = DICT
  ...     def factory(self, value):
  ...         return value.lower(), value

We can now use the directive multiple times without any errors::

  >>> class Bar(object):
  ...   multi(u"Once")
  ...   multi(u"Twice")

We can now retrieve the value and we'll get a to the items::

  >>> d = multi.bind().get(Bar)
  >>> print sorted(d.items())
  [(u'once', u'Once'), (u'twice', u'Twice')]

When the factory method does not return a key-value pair, an error is raised::

  >>> class wrongmulti(Directive):
  ...     scope = CLASS
  ...     store = DICT
  ...     def factory(self, value):
  ...         return None

  >>> class Baz(object):
  ...   wrongmulti(u"Once")
  Traceback (most recent call last):
  ...
  GrokImportError: The factory method for the 'wrongmulti' directive should
  return a key-value pair.

  >>> class wrongmulti2(Directive):
  ...     scope = CLASS
  ...     store = DICT
  ...     def factory(self, value):
  ...         return value, value, value

  >>> class Baz(object):
  ...   wrongmulti2(u"Once")
  Traceback (most recent call last):
  ...
  GrokImportError: The factory method for the 'wrongmulti2' directive should
  return a key-value pair.

Like with MULTIPLE store, values set by directives using the DICT store are
combined::

  >>> class multi(Directive):
  ...     scope = CLASS
  ...     store = DICT
  ...     def factory(self, value, andanother):
  ...         return value, andanother
  ...
  >>> class Frepple(object):
  ...   multi(1, 'AAA')
  ...   multi(2, 'BBB')
  ...
  >>> class Fropple(Frepple):
  ...   multi(1, 'CCC')
  ...   multi(3, 'DDD')
  ...   multi(4, 'EEE')

  >>> d = multi.bind().get(Fropple)
  >>> print sorted(d.items())
  [(1, 'CCC'), (2, 'BBB'), (3, 'DDD'), (4, 'EEE')]

Using MULTIPLE and DICT can also work on a module level, even though
inheritance has no meaning there::

  >>> from martian import MODULE
  >>> class multi(MultipleTimesDirective):
  ...     scope = MODULE
  ...
  >>> multi.__module__ = 'somethingelse'
  >>> class module_with_directive(FakeModule):
  ...     fake_module = True
  ...
  ...     multi('One')
  ...     multi('Two')
  ...
  >>> from martiantest.fake import module_with_directive
  >>> print multi.bind().get(module_with_directive)
  ['One', 'Two']

  >>> from martian import MODULE
  >>> class multi(Directive):
  ...     scope = MODULE
  ...     store = DICT
  ...     def factory(self, value, andanother):
  ...         return value, andanother
  ...
  >>> multi.__module__ = 'somethingelse'
  >>> class module_with_directive(FakeModule):
  ...     fake_module = True
  ...
  ...     multi(1, 'One')
  ...     multi(2, 'Two')
  ...
  >>> from martiantest.fake import module_with_directive
  >>> d = multi.bind().get(module_with_directive)
  >>> print sorted(d.items())
  [(1, 'One'), (2, 'Two')]

Directives on an interface
--------------------------

When you use ``zope.interface.Interface`` to define a new interface using
the class statement, in fact a special interface instance is created, not a
class. To let the directive store a value on an interface, we need to
use a special storage (``martian.ONCE_IFACE``)::

  >>> from martian import ONCE_IFACE
  >>> class skin(Directive):
  ...    scope = CLASS
  ...    store = ONCE_IFACE

Note that we still indicate the ``CLASS`` scope for this kind of
directive. At some point we may introduce a special scope for
directives on interfaces.

Let's try the directive. We shouldn't get an error::

  >>> class once_iface(FakeModule):
  ...   from zope.interface import Interface
  ...   class TestIface(Interface):
  ...     skin('Foo')
  >>> from martiantest.fake import once_iface

We can now retrieve the value::

  >>> skin.bind().get(once_iface.TestIface)
  'Foo'

Computed defaults
-----------------

Often instead of just supplying the system with a default, we want to
compute the default in some way. The computation of the default can be defined
in the directive class, by the ``get_default`` classmethod.

We define the ``name`` directive, which if not present, will compute its value
from the name of class, lower-cased.

  >>> class name(Directive):
  ...     scope = CLASS
  ...     store = ONCE
  ...
  ...     @classmethod
  ...     def get_default(cls, component, module=None, **data):
  ...         return component.__name__.lower()
  ...
  >>> bound_name = name.bind()
  >>>
  >>> class Foo(object):
  ...   name('bar')
  ...
  >>> bound_name.get(Foo)
  'bar'

  >>> class Foo2(object):
  ...   pass
  ...
  >>> bound_name.get(Foo2)
  'foo2'

To override the default-default behaviour you can pass a function when binding
the directive. In this case, we do not want the default behaviour of using the
lower cased class name, but have it upper cased::

  >>> def default_name_uppercase(component, module, **data):
  ...     return component.__name__.upper()
  ...
  >>> bound_name = name.bind(get_default=default_name_uppercase)

  >>> class Foo(object):
  ...   name('bar')

  >>> bound_name.get(Foo)
  'bar'

  >>> class Foo2(object):
  ...   pass
  >>> bound_name.get(Foo2)
  'FOO2'

Let's test this with a deeper inheritance hierarchy. Explicit
directives should always trump computed defaults::

  >>> class Subclass(Foo):
  ...   pass
  >>> bound_name.get(Subclass)
  'bar'

Now let's look at a hierarchy in which the explicit rule should
apply::

  >>> bound_name2 = name.bind()
  >>> class Alpha(object):
  ...    pass
  >>> class Beta(Alpha):
  ...    pass
  >>> bound_name2.get(Alpha)
  'alpha'
  >>> bound_name2.get(Beta)
  'beta'

We will now define a default rule that only triggers for particular
components in the inheritance chain, but returns ``UNKNOWN``
otherwise. This can be useful if the default rule is dependent on
outside information. In Grok for instance, the default rule for
``grok.context`` will look for a class that implements ``IContext`` in
the same module, and ``grok.templatedir`` will look for a directory
with templates with a name based on the name of the module with
``_templates`` appended.

This rule returns a value only if the module name includes the number
``1``, and will return ``UNKNOWN`` otherwise::

  >>> import martian
  >>> def default_name_lowercase3(component, module, **data):
  ...     if '1' in module.__name__:
  ...         return component.__name__.lower()
  ...     return martian.UNKNOWN

  >>> bound_name3 = name.bind(get_default=default_name_lowercase3)

This won't trigger for this module, as it doesn't have the character
``1`` in it::

  >>> class testmodule(FakeModule):
  ...    class Foo(object):
  ...       pass
  >>> from martiantest.fake import testmodule
  >>> bound_name3.get(testmodule.Foo, testmodule) is martian.UNKNOWN
  True

Now we define a module which does have ``1`` in it, so the rule should
be triggered::

  >>> class testmodule1(FakeModule):
  ...    class Foo(object):
  ...       pass
  >>> from martiantest.fake import testmodule1
  >>> bound_name3.get(testmodule1.Foo, testmodule1)
  'foo'

This also works with inheritance::

  >>> class testmodule2(FakeModule):
  ...   class Bar(testmodule1.Foo):
  ...      pass
  >>> from martiantest.fake import testmodule2
  >>> bound_name3.get(testmodule2.Bar, testmodule2)
  'foo'

Module-level explicit directives always trump computed defaults as
well.  The ``layer`` directive is ``CLASS_OR_MODULE`` scope. Let's set
up a hierarchy of modules and classes using ``layer`` to demonstrate
this::

  >>> class inheritmodule1(FakeModule):
  ...   layer('Test')
  ...   class Foo(object):
  ...      pass
  >>> from martiantest.fake import inheritmodule1
  >>> class inheritmodule2(FakeModule):
  ...   class Bar(inheritmodule1.Foo):
  ...      pass
  >>> from martiantest.fake import inheritmodule2

We define a way to compute the default in which we compute a string
based on the module name and the class name, so we can later check
whether the right module and class was passed to compute the default::

  >>> def immediate_get_default(component, module, **data):
  ...   return "computed: %s %s" % (module.__name__, component.__name__)

We don't expect the default rule to kick in as we can find an
explicitly set value::

  >>> layer.bind(get_default=immediate_get_default).get(inheritmodule2.Bar,
  ...   inheritmodule2)
  'Test'

Let's now consider a case where we have inheritance without explicit
use of the ``layer`` directive::

  >>> class inheritmodule1(FakeModule):
  ...   class Foo(object):
  ...      pass
  >>> from martiantest.fake import inheritmodule1
  >>> class inheritmodule2(FakeModule):
  ...   class Bar(inheritmodule1.Foo):
  ...      pass
  >>> from martiantest.fake import inheritmodule2

We expect to receive the computed default for ``Bar``, as
``immediate_get_default`` immediately returns a result for any
component::

  >>> layer.bind(get_default=immediate_get_default).get(inheritmodule2.Bar,
  ...   inheritmodule2)
  'computed: martiantest.fake.inheritmodule2 Bar'

Let's try the default rule that triggers upon seeing ``1`` in the
module name again, this time for the ``CLASS_OR_MODULE`` scope
directive ``layer``::

  >>> def picky_get_default(component, module, **data):
  ...     if '1' in module.__name__:
  ...         return "computed: %s %s" % (module.__name__, component.__name__)
  ...     return martian.UNKNOWN

Since only the ``Foo`` class is in a module with the character ``1``
in it (``inheritmodule1``), we will get the result for ``Foo`` (and
its module)::

  >>> layer.bind(get_default=picky_get_default).get(inheritmodule2.Bar,
  ...   inheritmodule2)
  'computed: martiantest.fake.inheritmodule1 Foo'

We will get the same result if we ask ``Foo`` directly::

  >>> layer.bind(get_default=picky_get_default).get(inheritmodule1.Foo,
  ...   inheritmodule1)
  'computed: martiantest.fake.inheritmodule1 Foo'

If we have a hierarchy that never has a module with the character
``1`` in it, we will receive ``UNKNOWN`` (and the grokkker that uses
this directive should raise an error)::

  >>> class inheritmodule3(FakeModule):
  ...   class Foo(object):
  ...      pass
  >>> from martiantest.fake import inheritmodule3
  >>> layer.bind(get_default=picky_get_default).get(inheritmodule3.Foo,
  ...   inheritmodule3) is martian.UNKNOWN
  True

Raising errors in a computed default
------------------------------------

Let's define a simple directive with a default rule that always raises
a ``GrokError`` if the class name doesn't start with an upper case
letter::

  >>> from martian.error import GrokError
  >>> class name(Directive):
  ...     scope = CLASS
  ...     store = ONCE
  ...
  ...     @classmethod
  ...     def get_default(cls, component, module, **data):
  ...         if component.__name__[0].isupper():
  ...              return component.__name__
  ...         raise GrokError(
  ...              "Component %r has a name that doesn't start with upper "
  ...              "case letter." % component, component)

Let's test it::

  >>> class A(object):
  ...   pass
  >>> class b(object):
  ...   pass
  >>> name.bind().get(A)
  'A'
  >>> name.bind().get(b)
  Traceback (most recent call last):
    ...
  GrokError: Component <class 'b'> has a name that doesn't start with upper
  case letter.

Instead of raising ``GrokError`` we can also raise ``UnknownError`` in
a computed default. This has the same meaning as returning
``UNKNOWN``, except that the error information is recorded and the
default rule is tried again on the base class in the mro chain. If the
default rule has the error raised or ``UNKNOWN`` value returned in
each step of the chain, the first ``UnknownError`` that was raised is
converted into a ``GrokError``.

This makes it possible for the default logic to raise specific errors
for the most specific class if the implicit rule failed to apply on
the class and any of its bases. This is used for instance in the
implementation of the ``get_default`` rule for ``grok.context``.

Let's test this behavior with a rule that raises an ``UnknownError``
if there is no ``foo`` attribute on the class::

  >>> from martian import UnknownError
  >>> class name(Directive):
  ...     scope = CLASS
  ...     store = ONCE
  ...
  ...     @classmethod
  ...     def get_default(cls, component, module, **data):
  ...         if not component.__dict__.has_key('foo'):
  ...             raise UnknownError(
  ...                 "Foo cannot be found for class %s" % component.__name__,
  ...                 component)
  ...         return "Found for class %s" % component.__name__

Let's try it on a simple class first::

  >>> class Test:
  ...   pass
  >>> name.bind().get(Test)
  Traceback (most recent call last):
    ...
  GrokError: Foo cannot be found for class Test

Let's try it on a new style class::

  >>> class Test(object):
  ...   pass
  >>> name.bind().get(Test)
  Traceback (most recent call last):
    ...
  GrokError: Foo cannot be found for class Test

Let's try it on a class where there is some inheritance::

  >>> class Test1(object):
  ...    pass
  >>> class Test2(Test1):
  ...    pass
  >>> name.bind().get(Test2)
  Traceback (most recent call last):
    ...
  GrokError: Foo cannot be found for class Test2

As you can see the error message will apply to the most specific
class, ``Test2``, even though of course the error will also occur for
the base class, ``Test1``.

Let's now demonstrate an inheritance scenario where the error does
not occur, because the get_default rule will succeed at some point
during the inheritance chain::

  >>> class Test1(object):
  ...    foo = 1
  >>> class Test2(Test1):
  ...    pass
  >>> name.bind().get(Test2)
  'Found for class Test1'

Computed defaults for instances
-------------------------------

In some cases directives are used to retrieve values from instances instead of
from their classes::

  >>> class name(Directive):
  ...     scope = CLASS
  ...     store = ONCE
  >>> class instancemodule(FakeModule):
  ...   class Hoi(object):
  ...     name('Test')
  ...   class NoDirectiveOnThis(object):
  ...     pass
  ...   hoi = Hoi()
  ...   no_directive_on_this = NoDirectiveOnThis()
  >>> from martiantest.fake import instancemodule

Let's try to use the directive::

  >>> name.bind().get(instancemodule.hoi, instancemodule)
  'Test'

If no directive was used on the class, we will get ``None``, the
default default value::

  >>> (name.bind().get(instancemodule.no_directive_on_this, instancemodule) is
  ...  None)
  True

Let's try it with a computed value now::

  >>> def get_default(component, module, **data):
  ...   return "The default"

  >>> name.bind(get_default=get_default).get(instancemodule.no_directive_on_this)
  'The default'

Computed default for old-style classes
--------------------------------------

We should also test old-style classes with ``CLASS`` scope directives in
combination with computed defaults::

  >>> class layer2(Directive):
  ...     scope = CLASS
  ...     store = ONCE
  ...     @classmethod
  ...     def get_default(cls, component, module=None, **data):
  ...         if '1' in module.__name__:
  ...             return 'we found it'
  ...         return martian.UNKNOWN
  ...
  >>> class oldstyle1(FakeModule):
  ...    class Base:
  ...       pass
  >>> from martiantest.fake import oldstyle1
  >>> class oldstyle2(FakeModule):
  ...    class Sub(oldstyle1.Base):
  ...       pass
  >>> from martiantest.fake import oldstyle2
  >>> layer2.bind().get(oldstyle2.Sub)
  'we found it'

And let's try it with a ``CLASS_OR_MODULE`` scope directive too::

  >>> class layer3(Directive):
  ...     scope = CLASS_OR_MODULE
  ...     store = ONCE
  ...     @classmethod
  ...     def get_default(cls, component, module=None, **data):
  ...         if '1' in module.__name__:
  ...             return 'we found it'
  ...         return martian.UNKNOWN
  >>> layer3.bind().get(oldstyle2.Sub)
  'we found it'

A marker directive
------------------

Another type of directive is a marker directive. This directive takes
no arguments at all, but when used it marks the context::

  >>> from martian import MarkerDirective
  >>> class mark(MarkerDirective):
  ...     scope = CLASS

  >>> class Foo(object):
  ...     mark()

Class ``Foo`` is now marked::

  >>> mark.bind().get(Foo)
  True

When we have a class that isn't marked, we get the default value, ``False``::

  >>> class Bar(object):
  ...    pass
  >>> mark.bind().get(Bar)
  False

If we pass in an argument, we get an error::

  >>> class Bar(object):
  ...   mark("An argument")
  Traceback (most recent call last):
    ...
  TypeError: mark takes no arguments (1 given)


Validation
----------

A directive can be supplied with a validation method. The validation method
checks whether the value passed in is allowed. It should raise
``GrokImportError`` if the value cannot be validated, together with a
description of why not.

First we define our own validation function. A validation function
takes two arguments:

* the name of the directive we're validating for

* the value we need to validate

The name can be used to format the exception properly.

We'll define a validation method that only expects integer numbers::

  >>> from martian.error import GrokImportError
  >>> class number(Directive):
  ...     scope = CLASS
  ...     store = ONCE
  ...     def validate(self, value):
  ...         if type(value) is not int:
  ...             raise GrokImportError(
  ...                 "The '%s' directive can only be called with an integer." %
  ...                 self.name)

  >>> class Foo(object):
  ...    number(3)

  >>> class Foo(object):
  ...    number("This shouldn't work")
  Traceback (most recent call last):
    ...
  GrokImportError: The 'number' directive can only be called with an integer.

Some built-in validation functions
----------------------------------

Let's look at some built-in validation functions.

The ``validateText`` function determines whether a string
is unicode or plain ascii::

  >>> from martian import validateText
  >>> class title(Directive):
  ...     scope = CLASS
  ...     store = ONCE
  ...     default = u''
  ...     validate = validateText

When we pass ascii text into the directive, there is no error::

  >>> class Foo(object):
  ...    title('Some ascii text')

We can also pass in a unicode string without error::

  >>> class Foo(object):
  ...    title(u'Some unicode text')

Let's now try it with something that's not text at all, such as a number.
This fails::

  >>> class Foo(object):
  ...    title(123)
  Traceback (most recent call last):
    ...
  GrokImportError: The 'title' directive can only be called with unicode or ASCII.

It's not allowed to call the direct with a non-ascii encoded string::

  >>> class Foo(object):
  ...   title(u'è'.encode('latin-1'))
  Traceback (most recent call last):
    ...
  GrokImportError: The 'title' directive can only be called with unicode or ASCII.

 >>> class Foo(object):
 ...   title(u'è'.encode('UTF-8'))
 Traceback (most recent call last):
   ...
 GrokImportError: The 'title' directive can only be called with unicode or ASCII.

The ``validateInterfaceOrClass`` function only accepts class or
interface objects::

  >>> from martian import validateInterfaceOrClass
  >>> class klass(Directive):
  ...     scope = CLASS
  ...     store = ONCE
  ...     validate = validateInterfaceOrClass

It works with interfaces and classes::

  >>> class Bar(object):
  ...    pass
  >>> class Foo(object):
  ...    klass(Bar)

  >>> from zope.interface import Interface
  >>> class IBar(Interface):
  ...    pass
  >>> class Foo(object):
  ...    klass(IBar)

It won't work with other things::

  >>> class Foo(object):
  ...   klass(Bar())
  Traceback (most recent call last):
    ...
  GrokImportError: The 'klass' directive can only be called with a class or an interface.

  >>> class Foo(object):
  ...   klass(1)
  Traceback (most recent call last):
    ...
  GrokImportError: The 'klass' directive can only be called with a class or an interface.

The ``validateInterface`` validator only accepts an interface::

  >>> from martian import validateInterface
  >>> class iface(Directive):
  ...     scope = CLASS
  ...     store = ONCE
  ...     validate = validateInterface

Let's try it::

  >>> class Foo(object):
  ...    iface(IBar)

It won't work with classes or other things::

  >>> class Foo(object):
  ...   iface(Bar)
  Traceback (most recent call last):
    ...
  GrokImportError: The 'iface' directive can only be called with an interface.

  >>> class Foo(object):
  ...   iface(1)
  Traceback (most recent call last):
    ...
  GrokImportError: The 'iface' directive can only be called with an interface.

The ``validateClass`` validator only accepts a class::

  >>> from martian import validateClass
  >>> class klass(Directive):
  ...    scope = CLASS
  ...    store = ONCE
  ...    validate = validateClass

  >>> class Foo(object):
  ...    klass(Bar)

But it won't work with an interface or other things::

  >>> class Foo(object):
  ...    klass(IBar)
  Traceback (most recent call last):
    ...
  GrokImportError: The 'klass' directive can only be called with a class.

  >>> class Foo(object):
  ...    klass(Bar())
  Traceback (most recent call last):
    ...
  GrokImportError: The 'klass' directive can only be called with a class.

Declaring base classes
----------------------

There's a special directive called ``martian.baseclass`` which lets
you declare that a certain class is the base class for a series of
other components.  This property should not be inherited by those
components.  Consider the following base class:

  >>> import martian
  >>> class MyBase(object):
  ...     martian.baseclass()

As you would expect, the directive will correctly identify this class as a
baseclass:

  >>> martian.baseclass.bind().get(MyBase)
  True

But, if we create a subclass of this base class, the subclass won't inherit
that property, unlike with a regular directive:

  >>> class SubClass(MyBase):
  ...     pass
  ...
  >>> martian.baseclass.bind().get(SubClass)
  False

Naturally, the directive will also report a false answer if the class doesn't
inherit from a base class at all and hasn't been marked with the directive:

  >>> class NoBase(object):
  ...     pass
  ...
  >>> martian.baseclass.bind().get(NoBase)
  False

Base classes influence computed directives: a directive computation
will not happen on a base class (or in base classes of it).

Let's define a directive with the computed rule that it will look for
an object ``Context`` in the module it finds itself in and return its
``value`` attribute::

  >>> class info(Directive):
  ...    scope = CLASS
  ...    store = ONCE
  ...    @classmethod
  ...    def get_default(cls, component, module, **data):
  ...        context = getattr(module, 'Context', None)
  ...        if context is None:
  ...            return martian.UNKNOWN
  ...        return context.value

Let use this rule with an example where no baseclass is declared
first::

  >>> class basemodule(FakeModule):
  ...    class Context(object):
  ...       value = 1
  ...    class A(object):
  ...       pass
  >>> from martiantest.fake import basemodule
  >>> class submodule(FakeModule):
  ...    class B(basemodule.A):
  ...        pass
  >>> from martiantest.fake import submodule
  >>> info.bind().get(submodule.B)
  1

Now let's apply the rule where ``A`` is declared to be a
baseclass. Since ``A`` is a base class, the computed default will not
take effect::

  >>> class basemodule2(FakeModule):
  ...    class Context(object):
  ...       value = 1
  ...    class A(object):
  ...       martian.baseclass()
  >>> from martiantest.fake import basemodule2
  >>> class submodule2(FakeModule):
  ...    class B(basemodule2.A):
  ...        pass
  >>> from martiantest.fake import submodule2
  >>> info.bind().get(submodule2.B) is martian.UNKNOWN
  True

If we change the default rule so we use ``UnknownError`` we see the same
behavior, except an error message is raised::

  >>> def get_default(component, module, **data):
  ...     context = getattr(module, 'Context', None)
  ...     if context is None:
  ...         raise UnknownError("No Context object found!", component)
  ...     return context.value

It will work for the case where no baseclass is defined, as the rule
can take effect then::

  >>> info.bind(get_default=get_default).get(submodule.B)
  1

But we will get a ``GrokError`` when a baseclass is in play::

  >>> info.bind(get_default=get_default).get(submodule2.B)
  Traceback (most recent call last):
    ...
  GrokError: No Context object found!