5. Zope Products

5.1. Introduction

Zope products extend Zope with new functionality. Products most often provide new addable objects. In this chapter, we are going to look at building products on the file system. Filesystem products require more overhead to build, but offer more power and flexibility, and they can be developed with familiar tools such as text editors and version controlling systems.

Soon we will make the examples referenced in this chapter available for download as an example product. Until that time, you will see references to files in this chapter that are not available yet. This will be made available soon.

5.2. Development Process

This chapter begins with a discussion of how you will develop products. We’ll focus on common engineering tasks that you’ll encounter as you develop products.

5.2.1. Consider Alternatives

Before you jump into the development of a product you should consider the alternatives. Would your problem be better solved with External Methods, or Python Scripts? Products excel at extending Zope with new addable classes of objects. If this does not figure centrally in your solution, you should look elsewhere. Products, like External Methods allow you to write unrestricted Python code on the filesystem.

5.2.2. Starting with Interfaces

The first step in creating a product is to create one or more interfaces which describe the product. See Chapter 2 for more information on interfaces and how to create them.

Creating interfaces before you build an implementation is a good idea since it helps you see your design and assess how well it fulfills your requirements.

Consider this interface for a multiple choice poll component (see Poll.py):

from zope.interface import Interface

class IPoll(Interface):
    """A multiple choice poll"""

    def castVote(index):
        """Votes for a choice"""

    def getTotalVotes():
        """Returns total number of votes cast"""

    def getVotesFor(index):
        """Returns number of votes cast for a given response"""

    def getResponses():
        """Returns the sequence of responses"""

    def getQuestion():
        """Returns the question"""

How you name your interfaces is entirely up to you. Here we’ve decided to use prefix “I” in the name of the interface.

5.2.3. Implementing Interfaces

After you have defined an interface for your product, the next step is to create a prototype in Python that implements your interface.

Here is a prototype of a PollImplemtation class that implements the interface you just examined (see PollImplementation.py):

from poll import Poll

class PollImplementation:
    """A multiple choice poll, implements the Poll interface.

    The poll has a question and a sequence of responses. Votes
    are stored in a dictionary which maps response indexes to a
    number of votes.
    """

    implements(IPoll)

    def __init__(self, question, responses):
        self._question = question
        self._responses = responses
        self._votes = {}
        for i in range(len(responses)):
            self._votes[i] = 0

    def castVote(self, index):
        """Votes for a choice"""
        self._votes[index] = self._votes[index] + 1

    def getTotalVotes(self):
        """Returns total number of votes cast"""
        total = 0
        for v in self._votes.values():
            total = total + v
        return total

    def getVotesFor(self, index):
        """Returns number of votes cast for a given response"""
        return self._votes[index]

    def getResponses(self):
        """Returns the sequence of responses"""
        return tuple(self._responses)

    def getQuestion(self):
        """Returns the question"""
        return self._question

You can use this class interactively and test it. Here’s an example of interactive testing:

>>> from PollImplementation import PollImplementation
>>> p = PollImplementation("What's your favorite color?",
...                        ["Red", "Green", "Blue", "I forget"])
>>> p.getQuestion()
"What's your favorite color?"
>>> p.getResponses()
('Red', 'Green', 'Blue', 'I forget')
>>> p.getVotesFor(0)
0
>>> p.castVote(0)
>>> p.getVotesFor(0)
1
>>> p.castVote(2)
>>> p.getTotalVotes()
2
>>> p.castVote(4)
Traceback (innermost last):
File "<stdin>", line 1, in ?
File "PollImplementation.py", line 23, in castVote
self._votes[index] = self._votes[index] + 1
KeyError: 4

Interactive testing is one of Python’s great features. It lets you experiment with your code in a simple but powerful way.

At this point you can do a fair amount of work, testing and refining your interfaces and classes which implement them. See Chapter 9 for more information on testing.

So far you have learned how to create Python classes that are documented with interfaces, and verified with testing. Next you’ll examine the Zope product architecture. Then you’ll learn how to fit your well crafted Python classes into the product framework.

5.2.4. Building Product Classes

To turn a component into a product you must fulfill many contracts. For the most part these contracts are not yet defined in terms of interfaces. Instead you must subclass from base classes that implement the contracts. This makes building products confusing, and this is an area that we are actively working on improving.

5.2.5. Base Classes

Consider an example product class definition:

from Acquisition import Implicit
from Globals import Persistent
from AccessControl.Role import RoleManager
from OFS.SimpleItem import Item

class PollProduct(Implicit, Persistent, RoleManager, Item):
    """
    Poll product class
    """
    ...

The order of the base classes depends on which classes you want to take precedence over others. Most Zope classes do not define similar names, so you usually don’t need to worry about what order these classes are used in your product. Let’s take a look at each of these base classes.

5.2.5.1. Acquisition.Implicit

This is the normal acquisition base class. See the API Reference for the full details on this class. Many Zope services such as object publishing and security use acquisition, so inheriting from this class is required for products. Actually, you can choose to inherit from Acquisition.Explicit if you prefer, however, it will prevent folks from dynamically binding Python Scripts and DTML Methods to instances of your class. In general you should subclass from Acquisition.Implicit unless you have a good reason not to.

XXX: is this true? I thought that any ExtensionClass.Base can be acquired. The Implicit and Explicit just control how the class can acquire, not how it is acquired.

5.2.5.2. Globals.Persistent

This base class makes instances of your product persistent. For more information on persistence and this class see Chapter 4.

In order to make your poll class persistent you’ll need to make one change. Since _votes is a dictionary this means that it’s a mutable non-persistent sub-object. You’ll need to let the persistence machinery know when you change it:

def castVote(self, index):
    """Votes for a choice"""
    self._votes[index] = self._votes[index] + 1
    self._p_changed = 1

The last line of this method sets the _p_changed attribute to 1. This tells the persistence machinery that this object has changed and should be marked as dirty, meaning that its new state should be written to the database at the conclusion of the current transaction. A more detailed explanation is given in the Persistence chapter of this guide.

5.2.5.3. OFS.SimpleItem.Item

This base class provides your product with the basics needed to work with the Zope management interface. By inheriting from Item your product class gains a whole host of features: the ability to be cut and pasted, capability with management views, WebDAV support, basic FTP support, undo support, ownership support, and traversal controls. It also gives you some standard methods for management views and error display including manage_main(). You also get the getId(), title_or_id(), title_and_id() methods and the this() DTML utility method. Finally this class gives your product basic dtml-tree tag support. Item is really an everything-but-the-kitchen-sink kind of base class.

Item requires that your class and instances have some management interface related attributes.

• meta_type – This attribute should be a short string which is the name of your product class as it appears in the product add list. For example, the poll product class could have a meta_type with value as Poll.
• id or __name__ – All Item instances must have an id string attribute which uniquely identifies the instance within it’s container. As an alternative you may use __name__ instead of id.
• title – All Item instances must have a title string attribute. A title may be an empty string if your instance does not have a title.

In order to make your poll class work correctly as an Item you’ll need to make a few changes. You must add a meta_type class attribute, and you may wish to add an id parameter to the constructor:

class PollProduct(..., Item):

    meta_type = 'Poll'
    ...

    def __init__(self, id, question, responses):
        self.id = id
        self._question = question
        self._responses = responses
        self._votes = {}
        for i in range(len(responses)):
            self._votes[i] = 0

Finally, you should probably place Item last in your list of base classes. The reason for this is that Item provides defaults that other classes such as ObjectManager and PropertyManager override. By placing other base classes before Item you allow them to override methods in Item.

5.2.5.4. AccessControl.Role.RoleManager

This class provides your product with the ability to have its security policies controlled through the web. See Chapter 6 for more information on security policies and this class.

5.2.5.5. OFS.ObjectManager

This base class gives your product the ability to contain other Item instances. In other words, it makes your product class like a Zope folder. This base class is optional. See the API Reference for more details. This base class gives you facilities for adding Zope objects, importing and exporting Zope objects, WebDAV, and FTP. It also gives you the objectIds, objectValues, and objectItems methods.

ObjectManager makes few requirements on classes that subclass it. You can choose to override some of its methods but there is little that you must do.

If you wish to control which types of objects can be contained by instances of your product you can set the meta_types class attribute. This attribute should be a tuple of meta_types. This keeps other types of objects from being created in or pasted into instances of your product. The meta_types attribute is mostly useful when you are creating specialized container products.

5.2.5.6. OFS.PropertyManager

This base class provides your product with the ability to have user-managed instance attributes. See the API Reference for more details. This base class is optional.

Your class may specify that it has one or more predefined properties, by specifying a ‘_properties’ class attribute. For example:

_properties=({'id':'title', 'type': 'string', 'mode': 'w'},
             {'id':'color', 'type': 'string', 'mode': 'w'},
            )

The _properties structure is a sequence of dictionaries, where each dictionary represents a predefined property. Note that if a predefined property is defined in the _properties structure, you must provide an attribute with that name in your class or instance that contains the default value of the predefined property.

Each entry in the _properties structure must have at least an id and a type key. The id key contains the name of the property, and the type key contains a string representing the object’s type. The type string must be one of the values: float, int, long, string, lines, text, date, tokens, selection, or multiple section. For more information on Zope properties see the Zope Book.

For selection and multiple selection properties, you must include an addition item in the property dictionary, select_variable which provides the name of a property or method which returns a list of strings from which the selection(s) can be chosen. For example:

_properties=({'id' : 'favorite_color',
              'type' : 'selection',
              'select_variable' : 'getColors'
             },
            )

Each entry in the _properties structure may optionally provide a mode key, which specifies the mutability of the property. The mode string, if present, must be w, d, or wd.

A w present in the mode string indicates that the value of the property may be changed by the user. A d indicates that the user can delete the property. An empty mode string indicates that the property and its value may be shown in property listings, but that it is read-only and may not be deleted.

Entries in the _properties structure which do not have a mode item are assumed to have the mode wd (writable and deleteable).

5.2.6. Security Declarations

In addition to inheriting from a number of standard base classes, you must declare security information in order to turn your component into a product. See Chapter 6 for more information on security and instructions for declaring security on your components.

Here’s an example of how to declare security on the poll class:

from AccessControl import ClassSecurityInfo

class PollProduct(...):
    ...

    security = ClassSecurityInfo()

    security.declareProtected('Use Poll', 'castVote')
    def castVote(self, index):
        ...

    security.declareProtected('View Poll results', 'getTotalVotes')
    def getTotalVotes(self):
        ...

    security.declareProtected('View Poll results', 'getVotesFor')
    def getVotesFor(self, index):
        ...

    security.declarePublic('getResponses')
    def getResponses(self):
        ...

    security.declarePublic('getQuestion')
    def getQuestion(self):
        ...

For security declarations to be set up Zope requires that you initialize your product class. Here’s how to initialize your poll class:

from Globals import InitializeClass

class PollProduct(...):
   ...

InitializeClass(PollProduct)

5.2.7. Summary

Congratulations, you’ve created a product class. Here it is in all its glory (see examples/PollProduct.py):

from Poll import Poll
from AccessControl import ClassSecurityInfo
from Globals import InitializeClass
from Acquisition import Implicit
from Globals import Persistent
from AccessControl.Role import RoleManager
from OFS.SimpleItem import Item

class PollProduct(Implicit, Persistent, RoleManager, Item):
    """Poll product class, implements Poll interface.

    The poll has a question and a sequence of responses. Votes
    are stored in a dictionary which maps response indexes to a
    number of votes.
    """

    implements(IPoll)

    meta_type = 'Poll'

    security = ClassSecurityInfo()

    def __init__(self, id, question, responses):
        self.id = id
        self._question = question
        self._responses = responses
        self._votes = {}
        for i in range(len(responses)):
            self._votes[i] = 0

    security.declareProtected('Use Poll', 'castVote')
    def castVote(self, index):
        "Votes for a choice"
        self._votes[index] = self._votes[index] + 1
        self._p_changed = 1

    security.declareProtected('View Poll results', 'getTotalVotes')
    def getTotalVotes(self):
        "Returns total number of votes cast"
        total = 0
        for v in self._votes.values():
            total = total + v
        return total

    security.declareProtected('View Poll results', 'getVotesFor')
    def getVotesFor(self, index):
        "Returns number of votes cast for a given response"
        return self._votes[index]

    security.declarePublic('getResponses')
    def getResponses(self):
        "Returns the sequence of responses"
        return tuple(self._responses)

    security.declarePublic('getQuestion')
    def getQuestion(self):
        "Returns the question"
        return self._question

InitializeClass(Poll)

Now it’s time to test your product class in Zope. To do this you must register your product class with Zope.

5.3. Registering Products

Products are Python packages that live in ‘lib/python/Products’. Products are loaded into Zope when Zope starts up. This process is called product initialization. During product initialization, each product is given a chance to register its capabilities with Zope.

5.3.1. Product Initialization

When Zope starts up it imports each product and calls the product’s ‘initialize’ function passing it a registrar object. The ‘initialize’ function uses the registrar to tell Zope about its capabilities. Here is an example ‘__init__.py’ file:

from PollProduct import PollProduct, addForm, addFunction

def initialize(registrar):
    registrar.registerClass(
        PollProduct,
        constructors=(addForm, addFunction),
        )

This function makes one call to the registrar object which registers a class as an addable object. The registrar figures out the name to put in the product add list by looking at the ‘meta_type’ of the class. Zope also deduces a permission based on the class’s meta-type, in this case Add Polls (Zope automatically pluralizes “Poll” by adding an “s”). The ‘constructors’ argument is a tuple of objects consisting of two functions: an add form which is called when a user selects the object from the product add list, and the add method which is the method called by the add form. Note that these functions are protected by the constructor permission.

Note that you cannot restrict which types of containers can contain instances of your classes. In other words, when you register a class, it will appear in the product add list in folders if the user has the constructor permission.

See the API Reference for more information on the ProductRegistrar interface.

5.3.2. Factories and Constructors

Factories allow you to create Zope objects that can be added to folders and other object managers. Factories are discussed in Chapter 12 of the Zope Book. The basic work a factory does is to put a name into the product add list and associate a permission and an action with that name. If you have the required permission then the name will appear in the product add list, and when you select the name from the product add list, the action method will be called.

Products use Zope factory capabilities to allow instances of product classes to be created with the product add list. In the above example of product initialization you saw how a factory is created by the product registrar. Now let’s see how to create the add form and the add list.

The add form is a function that returns an HTML form that allows a users to create an instance of your product class. Typically this form collects that id and title of the instance along with other relevant data. Here’s a very simple add form function for the poll class:

def addForm():
    """Returns an HTML form."""
    return """<html>
    <head><title>Add Poll</title></head>
    <body>
    <form action="addFunction">
    id <input type="type" name="id"><br>
    question <input type="type" name="question"><br>
    responses (one per line)
    <textarea name="responses:lines"></textarea>
    </form>
    </body>
    </html>"""

Notice how the action of the form is addFunction. Also notice how the lines of the response are marshalled into a sequence. See Chapter 2 for more information about argument marshalling and object publishing.

It’s also important to include a HTML head tag in the add form. This is necessary so that Zope can set the base URL to make sure that the relative link to the addFunction works correctly.

The add function will be passed a FactoryDispatcher as its first argument which proxies the location (usually a Folder) where your product was added. The add function may also be passed any form variables which are present in your add form according to normal object publishing rules.

Here’s an add function for your poll class:

def addFunction(dispatcher, id, question, responses):
    """Create a new poll and add it to myself
    """
    p = PollProduct(id, question, responses)
    dispatcher.Destination()._setObject(id, p)

The dispatcher has three methods:

• Destination – The ObjectManager where your product was added.
• DestinationURL – The URL of the ObjectManager where your product was added.
• manage_main – Redirects to a management view of the ObjectManager where your product was added.

Notice how it calls the _setObject() method of the destination ObjectManager class to add the poll to the folder. See the API Reference for more information on the ObjectManager interface.

The add function should also check the validity of its input. For example the add function should complain if the question or response arguments are not of the correct type.

Finally you should recognize that the constructor functions are not methods on your product class. In fact they are called before any instances of your product class are created. The constructor functions are published on the web so they need to have doc strings, and are protected by a permission defined in during product initialization.

5.3.3. Testing

Now you’re ready to register your product with Zope. You need to add the add form and add method to the poll module. Then you should create a Poll directory in your lib/python/Products directory and add the Poll.py, PollProduct.py, and __init__.py files. Then restart Zope.

Now login to Zope as a manager and visit the web management interface. You should see a ‘Poll’ product listed inside the Products folder in the Control_Panel. If Zope had trouble initializing your product you will see a traceback here. Fix your problems, if any and restart Zope. If you are tired of all this restarting, take a look at the Refresh facility covered in Chapter 7.

Now go to the root folder. Select Poll from the product add list. Notice how you are taken to the add form. Provide an id, a question, and a list of responses and click Add. Notice how you get a black screen. This is because your add method does not return anything. Notice also that your poll has a broken icon, and only has the management views. Don’t worry about these problems now, you’ll find out how to fix these problems in the next section.

Now you should build some DTML Methods and Python Scripts to test your poll instance. Here’s a Python Script to figure out voting percentages:

## Script (Python) "getPercentFor"
##parameters=index
##
"""Returns the percentage of the vote given a response index. Note,
this script should be bound a poll by acquisition context."""
poll = context
return float(poll.getVotesFor(index)) / poll.getTotalVotes()

Here’s a DTML Method that displays poll results and allows you to vote:

<dtml-var standard_html_header>

<h2>
  <dtml-var getQuestion>
</h2>

<form> <!-- calls this dtml method -->

<dtml-in getResponses>
  <p>
    <input type="radio" name="index" value="&dtml-sequence-index;">
    <dtml-var sequence-item>
  </p>
</dtml-in>

<input type="submit" value=" Vote ">

</form>

<!-- process form -->

<dtml-if index>
  <dtml-call expr="castVote(index)">
</dtml-if>

<!-- display results -->

<h2>Results</h2>

<p><dtml-var getTotalVotes> votes cast</p>

<dtml-in getResponses>
  <p>
    <dtml-var sequence-item> -
    <dtml-var expr="getPercentFor(_.get('sequence-index'))">%
  </p>
</dtml-in>

<dtml-var standard_html_footer>

To use this DTML Method, call it on your poll instance. Notice how this DTML makes calls to both your poll instance and the getPercentFor Python script.

At this point there’s quite a bit of testing and refinement that you can do. Your main annoyance will be having to restart Zope each time you make a change to your product class (but see Chapter 9 for information on how to avoid all this restarting). If you vastly change your class you may break existing poll instances, and will need to delete them and create new ones. See Chapter 9 for more information on debugging techniques which will come in handy.

5.3.4. Building Management Interfaces

Now that you have a working product let’s see how to beef up its user interface and create online management facilities.

5.3.5. Defining Management Views

All Zope products can be managed through the web. Products have a collection of management tabs or views which allow managers to control different aspects of the product.

A product’s management views are defined in the manage_options class attribute. Here’s an example:

manage_options=(
    {'label' : 'Edit', 'action' : 'editMethod'},
    {'label' : 'View', 'action' : 'viewMethod'},
    )

The manage_options structure is a tuple that contains dictionaries. Each dictionary defines a management view. The view dictionary can have a number of items.

• ‘label’ – This is the name of the management view
• ‘action’ – This is the URL that is called when the view is chosen. Normally this is the name of a method that displays a management view.
• ‘target’ – An optional target frame to display the action. This item is rarely needed.
• ‘help’ – Optional help information associated with the view. You’ll find out more about this option later.

Management views are displayed in the order they are defined. However, only those management views for which the current user has permissions are displayed. This means that different users may see different management views when managing your product.

Normally you will define a couple custom views and reusing some existing views that are defined in your base classes. Here’s an example:

class PollProduct(..., Item):
    ...

    manage_options=(
        {'label' : 'Edit', 'action' : 'editMethod'},
        {'label' : 'Options', 'action' : 'optionsMethod'},
        ) + RoleManager.manage_options + Item.manage_options

This example would include the standard management view defined by RoleManager which is Security and those defined by Item which are Undo and Ownership. You should include these standard management views unless you have good reason not to. If your class has a default view method (index_html) you should also include a View view whose action is an empty string. See Chapter 2 for more information on index_html.

Note: you should not make the View view the first view on your class. The reason is that the first management view is displayed when you click on an object in the Zope management interface. If the View view is displayed first, users will be unable to navigate to the other management views since the view tabs will not be visible.

5.3.6. Creating Management Views

The normal way to create management view methods is to use DTML. You can use the DTMLFile class to create a DTML Method from a file. For example:

from Globals import DTMLFile

class PollProduct(...):
  ...

  editForm = DTMLFile('dtml/edit', globals())
  ...

This creates a DTML Method on your class which is defined in the dtml/edit.dtml file. Notice that you do not have to include the .dtml file extension. Also, don’t worry about the forward slash as a path separator; this convention will work fine on Windows. By convention DTML files are placed in a dtml subdirectory of your product. The globals() argument to the DTMLFile constructor allows it to locate your product directory. If you are running Zope in debug mode then changes to DTML files are reflected right away. In other words you can change the DTML of your product’s views without restarting Zope to see the changes.

DTML class methods are callable directly from the web, just like other methods. So now users can see your edit form by calling the editForm method on instances of your poll class. Typically DTML methods will make calls back to your instance to gather information to display. Alternatively you may decide to wrap your DTML methods with normal methods. This allows you to calculate information needed by your DTML before you call it. This arrangement also ensures that users always access your DTML through your wrapper. Here’s an example:

from Globals import DTMLFile

class PollProduct(...):
  ...

  _editForm = DTMLFile('dtml/edit', globals())

  def editForm(self, ...):
      ...

      return self._editForm(REQUEST, ...)

When creating management views you should include the DTML variables manage_page_header and manage_tabs at the top, and manage_page_footer at the bottom. These variables are acquired by your product and draw a standard management view header, tabs widgets, and footer. The management header also includes CSS information which you can take advantage of if you wish to add CSS style information to your management views. The management CSS information is defined in the lib/python/App/dtml/manage_page_style.css.dtml` file. Here are the CSS classes defined in this file and conventions for their use.

• ‘form-help’ – Explanatory text related to forms. In the future, users may have the option to hide this text.
• ‘std-text’ – Declarative text unrelated to forms. You should rarely use this class.
• ‘form-title’ – Form titles.
• ‘form-label’ – Form labels for required form elements.
• ‘form-optional’ – Form labels for optional form elements.
• ‘form-element’ – Form elements. Note, because of a Netscape bug, you should not use this class on ‘textarea’ elements.
• ‘form-text’ – Declarative text in forms.
• ‘form-mono’ – Fixed width text in forms. You should rarely use this class.

Here’s an example management view for your poll class. It allows you to edit the poll question and responses (see editPollForm.dtml):

<dtml-var manage_page_header>
<dtml-var manage_tabs>

<p class="form-help">
This form allows you to change the poll's question and
responses. <b>Changing a poll's question and responses
will reset the poll's vote tally.</b>.
</p>

<form action="editPoll">
<table>

  <tr valign="top">
    <th class="form-label">Question</th>
    <td><input type="text" name="question" class="form-element"
    value="&dtml-getQuestion;"></td>
  </tr>

  <tr valign="top">
    <th class="form-label">Responses</th>
    <td><textarea name="responses:lines" cols="50" rows="10">
    <dtml-in getResponses>
    <dtml-var sequence-item html_quote>
    </dtml-in>
    </textarea>
    </td>
  </tr>

  <tr>
    <td></td>
    <td><input type="submit" value="Change" class="form-element"></td>
  </tr>

</table>
</form>

<dtml-var manage_page_header>

This DTML method displays an edit form that allows you to change the questions and responses of your poll. Notice how poll properties are HTML quoted either by using html_quote in the dtml-var tag, or by using the dtml-var entity syntax.

Assuming this DTML is stored in a file editPollForm.dtml in your product’s dtml directory, here’s how to define this method on your class:

class PollProduct(...):
    ...

    security.declareProtected('View management screens', 'editPollForm')
    editPollForm = DTML('dtml/editPollForm', globals())

Notice how the edit form is protected by the View management screens permission. This ensures that only managers will be able to call this method.

Notice also that the action of this form is editPoll. Since the poll as it stands doesn’t include any edit methods you must define one to accept the changes. Here’s an editPoll method:

class PollProduct(...):
    ...

    def __init__(self, id, question, responses):
        self.id = id
        self.editPoll(question, response)

    ...

    security.declareProtected('Change Poll', 'editPoll')
    def editPoll(self, question, responses):
        """
        Changes the question and responses.
        """
        self._question = question
        self._responses = responses
        self._votes = {}
        for i in range(len(responses)):
            self._votes[i] = 0

Notice how the __init__ method has been refactored to use the new editPoll method. Also notice how the editPoll method is protected by a new permissions, Change Poll.

There still is a problem with the editPoll method. When you call it from the editPollForm through the web nothing is returned. This is a bad management interface. You want this method to return an HTML response when called from the web, but you do not want it to do this when it is called from __init__. Here’s the solution:

class Poll(...):
    ...

    def editPoll(self, question, responses, REQUEST=None):
        """Changes the question and responses."""
        self._question = question
        self._responses = responses
        self._votes = {}
        for i in range(len(responses)):
            self._votes[i] = 0
        if REQUEST is not None:
            return self.editPollForm(REQUEST,
                manage_tabs_message='Poll question and responses changed.')

If this method is called from the web, then Zope will automatically supply the REQUEST parameter. (See chapter 4 for more information on object publishing). By testing the REQUEST you can find out if your method was called from the web or not. If you were called from the web you return the edit form again.

A management interface convention that you should use is the manage_tab_message DTML variable. If you set this variable when calling a management view, it displays a status message at the top of the page. You should use this to provide feedback to users indicating that their actions have been taken when it is not obvious. For example, if you don’t return a status message from your editPoll method, users may be confused and may not realize that their changes have been made.

Sometimes when displaying management views, the wrong tab will be highlighted. This is because ‘manage_tabs’ can’t figure out from the URL which view should be highlighted. The solution is to set the ‘management_view’ variable to the label of the view that should be highlighted. Here’s an example, using the ‘editPoll’ method:

def editPoll(self, question, responses, REQUEST=None):
    """
    Changes the question and responses.
    """
    self._question = question
    self._responses = responses
    self._votes = {}
    for i in range(len(responses)):
        self._votes[i] = 0
    if REQUEST is not None:
        return self.editPollForm(REQUEST,
            management_view='Edit',
            manage_tabs_message='Poll question and responses changed.')

Now let’s take a look a how to define an icon for your product.

5.3.7. Icons

Zope products are identified in the management interface with icons. An icon should be a 16 by 16 pixel GIF image with a transparent background. Normally icons files are located in a www subdirectory of your product package. To associate an icon with a product class, use the icon parameter to the registerClass method in your product’s constructor. For example:

def initialize(registrar):
    registrar.registerClass(
        PollProduct,
        constructors=(addForm, addFunction),
        icon='www/poll.gif'
        )

Notice how in this example, the icon is identified as being within the product’s www subdirectory.

See the API Reference for more information on the registerClass method of the ProductRegistrar interface.

5.3.8. Online Help

Zope has an online help system that you can use to provide help for your products. Its main features are context-sensitive help and API help. You should provide both for your product.

5.3.9. Context Sensitive Help

To create context sensitive help, create one help file per management view in your product’s help directory. You have a choice of formats including: HTML, DTML, structured text, GIF, JPG, and PNG.

Register your help files at product initialization with the registerHelp() method on the registrar object:

def initialize(registrar):
    ...
    registrar.registerHelp()

This method will take care of locating your help files and creating help topics for each help file. It can recognize these file extensions: .html, .htm, .dtml, .txt, .stx, .gif, .jpg, .png.

If you want more control over how your help topics are created you can use the registerHelpTopic() method which takes an id and a help topic object as arguments. For example:

from mySpecialHelpTopics import MyTopic

def initialize(context):
    ...
    context.registerHelpTopic('myTopic', MyTopic())

Your help topic should adhere to the ‘HelpTopic’ interface. See the API Reference for more details.

The chief way to bind a help topic to a management screen is to include information about the help topic in the class’s manage_options structure. For example:

manage_options = (
    {'label': 'Edit',
     'action': 'editMethod',
     'help': ('productId','topicId')},
    )

The help value should be a tuple with the name of your product’s Python package, and the file name (or other id) of your help topic. Given this information, Zope will automatically draw a Help button on your management screen and link it to your help topic.

To draw a help button on a management screen that is not a view (such as an add form), use the ‘HelpButton’ method of the ‘HelpSys’ object like so:

<dtml-var "HelpSys.HelpButton('productId', 'topicId')">

This will draw a help button linked to the specified help topic. If you prefer to draw your own help button you can use the helpURL method instead like so:

<dtml-var "HelpSys.helpURL(
  topic='productId',
  product='topicId')">

This will give you a URL to the help topic. You can choose to draw whatever sort of button or link you wish.

5.3.10. Other User Interfaces

In addition to providing a through the web management interface your products may also support many other user interfaces. You product might have no web management interfaces, and might be controlled completely through some other network protocol. Zope provides interfaces and support for FTP, WebDAV and XML-RPC. If this isn’t enough you can add other protocols.

5.3.11. FTP and WebDAV Interfaces

Both FTP and WebDAV treat Zope objects like files and directories. See Chapter 3 for more information on FTP and WebDAV.

By simply sub-classing from ‘SimpleItem.Item’ and ‘ObjectManager’ if necessary, you gain basic FTP and WebDAV support. Without any work your objects will appear in FTP directory listings and if your class is an ‘ObjectManager’ its contents will be accessible via FTP and WebDAV. See Chapter 2 for more information on implementing FTP and WebDAV support.

5.3.12. XML-RPC and Network Services

XML-RPC is covered in Chapter 2. All your product’s methods can be accessible via XML-RPC. However, if your are implementing network services, you should explicitly plan one or more methods for use with XML-RPC.

Since XML-RPC allows marshalling of simple strings, lists, and dictionaries, your XML-RPC methods should only accept and return these types. These methods should never accept or return Zope objects. XML-RPC also does not support ‘None’ so you should use zero or something else in place of ‘None’.

Another issue to consider when using XML-RPC is security. Many XML-RPC clients still don’t support HTTP basic authorization. Depending on which XML-RPC clients you anticipate, you may wish to make your XML-RPC methods public and accept authentication credentials as arguments to your methods.

5.3.13. Content Management Framework Interface

The Content Management Framework is an evolving content management extension for Zope. It provides a number of interfaces and conventions for content objects. If you wish to support the CMF you should consult the CMF user interface guidelines and interface documentation.

Supporting the CMF interfaces is not a large burden if you already support the Zope management interface. You should consider supporting the CMF if your product class handles user manageable content such as documents, images, business forms, etc.

5.3.14. Packaging Products

Zope products are normally packaged as tarballs. You should create your product tarball in such a way as to allow it to be unpacked in the Products directory. For example, cd to the Products directory and then issue a tar comand like so:

$ tar zcvf MyProduct-1.0.1.tgz MyProduct

This will create a gzipped tar archive containing your product. You should include your product name and version number in file name of the archive.

See the Poll-1.0.tgz file for an example of a fully packaged Python product.

5.3.15. Product Information Files

Along with your Python and ZPT files you should include some information about your product in its root directory.

• README.txt – Provides basic information about your product.

  Zope will parse this file as StructuredText and make it available on the README view of your product in the control panel.

• VERSION.txt – Contains the name and version of your product on a single line. For example, ‘Multiple Choice Poll 1.1.0’. Zope will display this information as the ‘version’ property of your product in the control panel.

• LICENSE.txt – Contains your product license, or a link to it.

You may also wish to provide additional information. Here are some suggested optional files to include with your product.

• INSTALL.txt – Provides special instructions for installing the product and components on which it depends. This file is only optional if your product does not require more than an ungzip/untar into a Zope installation to work.
• TODO.txt – This file should make clear where this product release needs work, and what the product author intends to do about it.
• CHANGES.txt and HISTORY.txt – ‘CHANGES.txt’ should enumerate changes made in particular product versions from the last release of the product. Optionally, a ‘HISTORY.txt’ file can be used for older changes, while ‘CHANGES.txt’ lists only recent changes.
• DEPENDENCIES.txt – Lists dependencies including required os platform, required Python version, required Zope version, required Python packages, and required Zope products.

5.3.16. Product Directory Layout

By convention your product will contain a number of sub-directories. Some of these directories have already been discussed in this chapter. Here is a summary of them.

• www – Contains your icon & ZPT files.
• help – Contains your help files.
• tests – Contains your unit tests.

It is not necessary to include these directories if your don’t have anything to go in them.

5.4. Evolving Products

As you develop your product classes you will generally make a series of product releases. While you don’t know in advance how your product will change, when it does change there are measures that you can take to minimize problems.

5.4.1. Evolving Classes

Issues can occur when you change your product class because instances of these classes are generally persistent. This means that instances created with an old class will start using a new class. If your class changes drastically this can break existing instances.

The simplest way to handle this situation is to provide class attributes as defaults for newly added attributes. For example if the latest version of your class expects an ‘improved_spam’ instance attribute while earlier versions only sported ‘spam’ attributes, you may wish to define an ‘improved_spam’ class attribute in your new class so your old objects won’t break when they run with your new class. You might set ‘improved_spam’ to None in your class, and in methods where you use this attribute you may have to take into account that it may be None. For example:

class Sandwich(...):

    improved_spam = None
    ...

    def assembleSandwichMeats(self):
        ...
        # test for old sandwich instances
        if self.improved_spam is None:
            self.updateToNewSpam()
        ...

Another solution is to use the standard Python pickling hook ‘__setstate__’, however, this is in general more error prone and complex.

A third option is to create a method to update old instances. Then you can manually call this method on instances to update to them. Note, this won’t work unless the instances function well enough to be accessible via the Zope management screens.

While you are developing a product you won’t have to worry too much about these details, since you can always delete old instances that break with new class definitions. However, once you release your product and other people start using it, then you need to start planning for the eventuality of upgrading.

Another nasty problem that can occur is breakage caused by renaming your product classes. You should avoid this since it breaks all existing instances. If you really must change your class name, provide aliases to it using the old name. You may however, change your class’s base classes without causing these kinds of problems.

5.4.2. Evolving Interfaces

The basic rule of evolving interfaces is don’t do it. While you are working privately you can change your interfaces all you wish. But as soon as you make your interfaces public you should freeze them. The reason is that it is not fair to users of your interfaces to changes them after the fact. An interface is contract. It specifies how to use a component and it specifies how to implement types of components. Both users and developers will have problems if your change the interfaces they are using or implementing.

The general solution is to create simple interfaces in the first place, and create new ones when you need to change an existing interface. If your new interfaces are compatible with your existing interfaces you can indicate this by making your new interfaces extend your old ones. If your new interface replaces an old one but does not extend it you should give it a new name such as, WidgetWithBellsOn. Your components should continue to support the old interface in addition to the new one for a few releases.

5.5. Conclusion

Migrating your components into fully fledged Zope products is a process with a number of steps. There are many details to keep track of. However, if you follow the recipe laid out in this chapter you should have no problems.

Zope products are a powerful framework for building web applications. By creating products you can take advantage of Zope’s features including security, scalability, through the web management, and collaboration.