来自zope模式的循环导入引用

Question

我遇到了一个与这个SO问题非常相似的问题，但我尝试应用之前的答案没有成功，有人建议我将其作为一个新问题开始：

在下面的代码中，我定义了几个getChoice（）函数，我认为这些函数会推迟循环引用，但没有！？请问这里怎么了？

# ns.content/ns/content/foo.py
from zope import schema
from plone.directives import form
from z3c.relationfield.schema import Relation, RelationChoice
from plone.formwidget.contenttree import ObjPathSourceBinder
class IFoo(form.Schema):
    def getBarChoices():
        # avoiding circular refs...
        from bar import IBar
        return ObjPathSourceBinder(object_provides=IBar.__identifier__)
    barChoices = getBarChoices()
    form.widget(bar=AutocompleteFieldWidget)
    bar = Relation(source= barChoices,required=False)
# ns.content/ns/content/bar.py
from zope import schema
from plone.directives import form
from z3c.relationfield.schema import Relation, RelationChoice
from plone.formwidget.contenttree import ObjPathSourceBinder
class IBar(form.Schema):
    def getFooChoices():
        # avoiding circular refs...
        from foo import IFoo
        return ObjPathSourceBinder(object_provides=IFoo.__identifier__)
    fooChoices = getFooChoices()
    form.widget(foo=AutocompleteFieldWidget)
    foo = Relation(source= fooChoices,required=False)
resultingError = """
  File ".../buildout-cache/eggs/martian-0.11.3-py2.7.egg/martian/scan.py", line 217, in resolve
    __import__(used)
  File ".../zeocluster/src/ns.content/ns/content/bar.py", line 32, in <module>
    class IBar(form.Schema):
  File ".../zeocluster/src/ns.content/ns/content/bar.py", line 48, in IBar
    fooChoices = getFooChoices()
  File ".../zeocluster/src/ns.content/ns/content/bar.py", line 38, in getFooChoices
    from ns.content.foo import IFoo
  File ".../zeocluster/src/ns.content/ns/content/foo.py", line 33, in <module>
    class IFoo(form.Schema):
  File ".../zeocluster/src/ns.content/ns/content/foo.py", line 73, in IFoo
    barChoices = getBarChoices()
  File ".../zeocluster/src/ns.content/ns/content/foo.py", line 39, in getBarChoices
    from ns.content.bar import IBar
zope.configuration.xmlconfig.ZopeXMLConfigurationError: File ".../zeocluster/parts/client1/etc/site.zcml", line 16.2-16.23
    ZopeXMLConfigurationError: File ".../buildout-cache/eggs/Products.CMFPlone-4.2.0.1-py2.7.egg/Products/CMFPlone/configure.zcml", line 102.4-106.10
    ZopeXMLConfigurationError: File ".../zeocluster/src/ns.content/ns/content/configure.zcml", line 18.2-18.27
    ImportError: cannot import name IBar
"""

Answer 1

定义类IFoo时，您在定义时调用getBarChoices()。因此，在解析foo.py时将执行from bar import IBar，从而导致循环导入。

在我看来，你基本上有两个选择：

1）使用字符串作为object_provides的标识符

无论如何，您已经通过使用IFoo.__identifier__做到了这一点，但如果您将其设置为静态而非动态，则将消除循环依赖：

source = ObjPathSourceBinder(object_provides='ns.content.bar.IBar')
bar = Relation(source=source,required=False)

无需在foo.py中导入IBar。这有一个明显的缺点，即IBar的位置现在在代码中是硬编码的，因此无论何时更改IBar的名称或位置，都需要更新其在foo.py中的虚线名称。

2）标记接口

另一种选择是让IFoo和IBar实现您保存在第三个文件（例如ns/content/interfaces.py）中的标记接口。这样你就可以做一些类似的事情

接口.py

from zope.interface import Interface
class IBarMarker(Interface):
    """Marker interface for IBar objects.
    """
class IFooMarker(Interface):
    """Marker interface for IFoo objects.
    """

foo.py

from zope.interface import directlyProvides
from plone.directives import form
from plone.formwidget.contenttree import ObjPathSourceBinder
from plone.formwidget.autocomplete import AutocompleteFieldWidget
from z3c.relationfield.schema import RelationChoice
from ns.content.interfaces import IBarMarker
from ns.content.interfaces import IFooMarker

class IFoo(form.Schema):
    directlyProvides(IFooMarker)
    form.widget(bar=AutocompleteFieldWidget)
    bar = RelationChoice(source=ObjPathSourceBinder(
                            object_provides=IBarMarker.__identifier__),
                         required=False)

bar.py

class IBar(form.Schema):
    directlyProvides(IBarMarker)
    form.widget(foo=AutocompleteFieldWidget)
    foo = RelationChoice(source=ObjPathSourceBinder(
                            object_provides=IFooMarker.__identifier__),
                         required=False)