4. Object Publishing

4.1. Introduction

Zope puts your objects on the web. This is called object publishing. One of Zope’s unique characteristics is the way it allows you to walk up to your objects and call methods on them with simple URLs. In addition to HTTP, Zope makes your objects available to other network protocols including FTP, WebDAV and XML-RPC.

In this chapter you’ll find out exactly how Zope publishes objects. You’ll learn all you need to know in order to design your objects for web publishing.

4.2. HTTP Publishing

When you contact Zope with a web browser, your browser sends an HTTP request to Zope’s web server. After the request is completely received, it is processed by ‘ZPublisher’, which is Zope’s object publisher. ‘ZPublisher’ is a kind of light-weight ORB (Object Request Broker). It takes the request and locates an object to handle the request. The publisher uses the request URL as a map to locate the published object. Finding an object to handle the request is called traversal, since the publisher moves from object to object as it looks for the right one. Once the published object is found, the publisher calls a method on the published object, passing it parameters as necessary. The publisher uses information in the request to determine which method to call, and what parameters to pass. The process of extracting parameters from the request is called argument marshalling. The published object then returns a response, which is passed back to Zope’s web server. The web server, then passes the response back to your web browser.

The publishing process is summarized in [2-1]

2.1 Object publishing

Typically the published object is a persistent object that the published module loads from the ZODB. See Chapter 4 for more information on the ZODB.

This chapter will cover all the steps of object publishing in detail. To summarize, object publishing consists of the main steps:

1. The client sends a request to the publisher
2. The publisher locates the published object using the request URL as a map.
3. The publisher calls the published object with arguments from the request.
4. The publisher interprets and returns the results to the client.

The chapter will also cover all the technical details, special cases and extra-steps that this list glosses over.

4.3. URL Traversal

Traversal is the process the publisher uses to locate the published object. Typically the publisher locates the published object by walking along the URL. Take for example a collection of objects:

class Classification:
    ...

class Animal:
    ...

    def screech(self, ...):
        ...

vertebrates=Classification(...)
vertebrates.mammals=Classification(...)
vertebrates.reptiles=Classification(...)
vertebrates.mammals.monkey=Animal(...)
vertebrates.mammals.dog=Animal(...)
vertebrates.reptiles.lizard=Animal(...)

This collection of objects forms an object hierarchy. Using Zope you can publish objects with URLs. For example, the URL ‘http://zope/vertebrates/mammals/monkey/screech‘, will traverse the object hierarchy, find the ‘monkey’ object and call its ‘screech’ method.

2.2 Traversal path through an object hierarchy

The publisher starts from the root object and takes each step in the URL as a key to locate the next object. It moves to the next object and continues to move from object to object using the URL as a guide.

Typically the next object is a sub-object of the current object that is named by the path segment. So in the example above, when the publisher gets to the ‘vertebrates’ object, the next path segment is “mammals”, and this tells the publisher to look for a sub-object of the current object with that name. Traversal stops when Zope comes to the end of the URL. If the final object is found, then it is published, otherwise an error is returned.

Now let’s take a more rigorous look at traversal.

4.4. Traversal Interfaces

Zope defines interfaces for publishable objects, and publishable modules.

When you are developing for Zope you almost always use the ‘Zope’ package as your published module. However, if you are using ‘ZPublisher’ outside of Zope you’ll be interested in the published module interface.

4.5. Publishable Object Requirements

Zope has few restrictions on publishable objects. The basic rule is that the object must have a doc string. This requirement goes for method objects too.

Another requirement is that a publishable object must not have a name that begin with an underscore. These two restrictions are designed to keep private objects from being published.

Finally, published objects cannot be Python module objects.

4.6. Traversal Methods

During traversal, ‘ZPublisher’ cuts the URL into path elements delimited by slashes, and uses each path element to traverse from the current object to the next object. ‘ZPublisher’ locates the next object in one of three ways:

1. Using ‘__bobo_traverse__’
2. Using ‘getattr’
3. Using dictionary access.

First the publisher attempts to call the traversal hook method, ‘__bobo_traverse__’. If the current object has this method it is called with the request and the current path element. The method should return the next object or ‘None’ to indicate that a next object can’t be found. You can also return a tuple of objects from ‘__bobo_traverse__’ indicating a sequence of sub-objects. This allows you to add additional parent objects into the request. This is almost never necessary.

Here’s an example of how to use ‘__bobo_traverse__’:

def __bobo_traverse__(self, request, key):
    # if there is a special cookie set, return special
    # subobjects, otherwise return normal subobjects

    if request.cookies.has_key('special'):
        # return a subobject from the special dict
        return self.special_subobjects.get(key, None)

    # otherwise return a subobject from the normal dict
    return self.normal_subobjects.get(key, None)

This example shows how you can examine the request during the traversal process.

If the current object does not define a ‘__bobo_traverse__’ method, then the next object is searched for using ‘getattr’. This locates sub-objects in the normal Python sense.

If the next object can’t be found with ‘getattr’, ‘ZPublisher’ calls on the current object as though it were a dictionary. Note: the path element will be a string, not an integer, so you cannot traverse sequences using index numbers in the URL.

For example, suppose ‘a’ is the current object, and ‘next’ is the name of the path element. Here are the three things that ‘ZPublisher’ will try in order to find the next object:

1. ‘a.__bobo_traverse__(“next”)’
2. ‘a.next’
3. ‘a[“next”]’

4.7. Publishing Methods

Once the published object is located with traversal, Zope publishes it in one of three possible ways.

• Calling the published object – If the published object is a function or method or other callable object, the publisher calls it. Later in the chapter you’ll find out how the publisher figures out what arguments to pass when calling.
• Calling the default method – If the published object is not callable, the publisher uses the default method. For HTTP ‘GET’ and ‘POST’ requests the default method is ‘index_html’. For other HTTP requests such as ‘PUT’ the publisher looks for a method named by the HTTP method. So for an HTTP ‘HEAD’ request, the publisher would call the ‘HEAD’ method on the published object.
• Stringifying the published object – If the published object isn’t callable, and doesn’t have a default method, the publisher publishes it using the Python ‘str’ function to turn it into a string.

After the response method has been determined and called, the publisher must interpret the results.

4.8. Character Encodings for Responses

If the published method returns an object of type ‘string’, a plain 8-bit character string, the publisher will use it directly as the body of the response.

Things are different if the published method returns a unicode string, because the publisher has to apply some character encoding. The published method can choose which character encoding it uses by setting a ‘Content-Type’ response header which includes a ‘charset’ property (setting response headers is explained later in this chapter). A common choice of character encoding is UTF-8. To cause the publisher to send unicode results as UTF-8 you need to set a ‘Content-Type’ header with the value ‘text/html; charset=UTF-8’

If the ‘Content-Type’ header does not include a charser property (or if this header has not been set by the published method) then the publisher will choose a default character encoding. Today this default is ISO-8859-1 (also known as Latin-1) for compatability with old versions of Zope which did not include Unicode support. At some time in the future this default is likely to change to UTF-8.

4.9. HTTP Responses

Normally the published method returns a string which is considered the body of the HTTP response. The response headers can be controlled by calling methods on the response object, which is described later in the chapter. Optionally, the published method can return a tuple with the title, and body of the response. In this case, the publisher returns an generated HTML page, with the first item of the tuple used for the HTML ‘title’ of the page, and the second item as the contents of the HTML ‘body’ tag. For example a response of:

('response', 'the response')

is turned into this HTML page:

<html>
<head><title>response</title></head>
<body>the response</body>
</html>

4.10. Controlling Base HREF

When you publish an object that returns HTML relative links should allow you to navigate between methods. Consider this example:

class Example:
    "example"

    def one(self):
        "method one"
        return """<html>
                  <head>
                  <title>one</title>
                  </head>
                  <body>
                  <a href="two">two</a>
                  </body>
                  </html>"""

    def two(self):
        "method two"
        return """<html>
                  <head>
                  <title>two</title>
                  </head>
                  <body>
                  <a href="one">one</a>
                  </body>
                  </html>"""

However, the default method, ‘index_html’ presents a problem. Since you can access the ‘index_html’ method without specifying the method name in the URL, relative links returned by the ‘index_html’ method won’t work right. For example:

class Example:
    "example"

     def index_html(self):
        return """<html>
                  <head>
                  <title>one</title>
                  </head>
                  <body>
                  <a href="one">one</a><br>
                  <a href="two">two</a>
                  </body>
                  </html>"""
     ...

If you publish an instance of the ‘Example’ class with the URL ‘http://zope/example‘, then the relative link to method ‘one’ will be ‘http://zope/one‘, instead of the correct link, ‘http://zope/example/one‘.

Zope solves this problem for you by inserting a ‘base’ tag inside the ‘head’ tag in the HTML output of ‘index_html’ method when it is accessed as the default method. You will probably never notice this, but if you see a mysterious ‘base’ tag in your HTML output, know you know where it came from. You can avoid this behavior by manually setting your own base with a ‘base’ tag in your ‘index_html’ method output.

4.10.1. Response Headers

The publisher and the web server take care of setting response headers such as ‘Content-Length’ and ‘Content-Type’. Later in the chapter you’ll find out how to control these headers. Later you’ll also find out how exceptions are used to set the HTTP response code.

4.10.2. Pre-Traversal Hook

The pre-traversal hook allows your objects to take special action before they are traversed. This is useful for doing things like changing the request. Applications of this include special authentication controls, and virtual hosting support.

If your object has a method named ‘__before_publishing_traverse__’, the publisher will call it with the current object and the request, before traversing your object. Most often your method will change the request. The publisher ignores anything you return from the pre-traversal hook method.

The ‘ZPublisher.BeforeTraverse’ module contains some functions that help you register pre-traversal callbacks. This allows you to perform fairly complex callbacks to multiple objects when a given object is about to be traversed.

4.10.3. Traversal and Acquisition

Acquisition affects traversal in several ways. See Chapter 5, “Acquisition” for more information on acquisition. The most obvious way in which acquisition affects traversal is in locating the next object in a path. As we discussed earlier, the next object during traversal is often found using ‘getattr’. Since acquisition affects ‘getattr’, it will affect traversal. The upshot is that when you are traversing objects that support implicit acquisition, you can use traversal to walk over acquired objects. Consider the object hierarchy rooted in ‘fruit’:

from Acquisition import Implicit

class Node(Implicit):
    ...

fruit=Node()
fruit.apple=Node()
fruit.orange=Node()
fruit.apple.strawberry=Node()
fruit.orange.banana=Node()

When publishing these objects, acquisition can come into play. For example, consider the URL /fruit/apple/orange. The publisher would traverse from ‘fruit’, to ‘apple’, and then using acquisition, it would traverse to ‘orange’.

Mixing acquisition and traversal can get complex. Consider the URL /fruit/apple/orange/strawberry/banana. This URL is functional but confusing. Here’s an even more perverse but legal URL /fruit/apple/orange/orange/apple/apple/banana.

In general you should limit yourself to constructing URLs which use acquisition to acquire along containment, rather than context lines. It’s reasonable to publish an object or method that you acquire from your container, but it’s probably a bad idea to publish an object or method that your acquire from outside your container. For example:

from Acquisition import Implicit

class Basket(Implicit):
    ...
    def numberOfItems(self):
        "Returns the number of contained items"
        ...

class Vegetable(Implicit):
    ...
    def texture(self):
        "Returns the texture of the vegetable."

class Fruit(Implicit):
    ...
    def color(self):
        "Returns the color of the fruit."

 basket=Basket()
 basket.apple=Fruit()
 basket.carrot=Vegetable()

The URL /basket/apple/numberOfItems uses acquisition along containment lines to publish the ‘numberOfItems’ method (assuming that ‘apple’ doesn’t have a ‘numberOfItems’ attribute). However, the URL /basket/carrot/apple/texture uses acquisition to locate the ‘texture’ method from the ‘apple’ object’s context, rather than from its container. While this distinction may be obscure, the guiding idea is to keep URLs as simple as possible. By keeping acquisition simple and along containment lines your application increases in clarity, and decreases in fragility.

A second usage of acquisition in traversal concerns the request. The publisher tries to make the request available to the published object via acquisition. It does this by wrapping the first object in an acquisition wrapper that allows it to acquire the request with the name ‘REQUEST’. This means that you can normally acquire the request in the published object like so:

request=self.REQUEST # for implicit acquirers

or like so:

request=self.aq_acquire('REQUEST') # for explicit acquirers

Of course, this will not work if your objects do not support acquisition, or if any traversed objects have an attribute named ‘REQUEST’.

Finally, acquisition has a totally different role in object publishing related to security which we’ll examine next.

4.10.4. Traversal and Security

As the publisher moves from object to object during traversal it makes security checks. The current user must be authorized to access each object along the traversal path. The publisher controls access in a number of ways. For more information about Zope security, see Chapter 6, “Security”.

4.10.5. Basic Publisher Security

The publisher imposes a few basic restrictions on traversable objects. These restrictions are the same of those for publishable objects. As previously stated, publishable objects must have doc strings and must not have names beginning with underscore.

The following details are not important if you are using the Zope framework. However, if your are publishing your own modules, the rest of this section will be helpful.

The publisher checks authorization by examining the ‘__roles__’ attribute of each object as it performs traversal. If present, the ‘__roles__’ attribute should be ‘None’ or a list of role names. If it is None, the object is considered public. Otherwise the access to the object requires validation.

Some objects such as functions and methods do not support creating attributes (at least they didn’t before Python 2). Consequently, if the object has no ‘__roles__’ attribute, the publisher will look for an attribute on the object’s parent with the name of the object followed by ‘__roles__’. For example, a function named ‘getInfo’ would store its roles in its parent’s ‘getInfo__roles__’ attribute.

If an object has a ‘__roles__’ attribute that is not empty and not ‘None’, the publisher tries to find a user database to authenticate the user. It searches for user databases by looking for an ‘__allow_groups__’ attribute, first in the published object, then in the previously traversed object, and so on until a user database is found.

When a user database is found, the publisher attempts to validate the user against the user database. If validation fails, then the publisher will continue searching for user databases until the user can be validated or until no more user databases can be found.

The user database may be an object that provides a validate method:

validate(request, http_authorization, roles)

where ‘request’ is a mapping object that contains request information, ‘http_authorization’ is the value of the HTTP ‘Authorization’ header or ‘None’ if no authorization header was provided, and ‘roles’ is a list of user role names.

The validate method returns a user object if succeeds, and ‘None’ if it cannot validate the user. See Chapter 6 for more information on user objects. Normally, if the validate method returns ‘None’, the publisher will try to use other user databases, however, a user database can prevent this by raising an exception.

If validation fails, Zope will return an HTTP header that causes your browser to display a user name and password dialog. You can control the realm name used for basic authentication by providing a module variable named ‘__bobo_realm__’. Most web browsers display the realm name in the user name and password dialog box.

If validation succeeds the publisher assigns the user object to the request variable, ‘AUTHENTICATED_USER’. The publisher places no restriction on user objects.

Zope Security

When using Zope rather than publishing your own modules, the publisher uses acquisition to locate user folders and perform security checks. The upshot of this is that your published objects must inherit from ‘Acquisition.Implicit’ or ‘Acquisition.Explicit’. See Chapter 5, “Acquisition”, for more information about these classes. Also when traversing each object must be returned in an acquisition context. This is done automatically when traversing via ‘getattr’, but you must wrap traversed objects manually when using ‘__getitem__’ and ‘__bobo_traverse__’. For example:

class Example(Acquisition.Explicit):
    ...

    def __bobo_traverse__(self, name, request):
        ...
        next_object=self._get_next_object(name)
        return  next_object.__of__(self)

Finally, traversal security can be circumvented with the ‘__allow_access_to_unprotected_subobjects__’ attribute as described in Chapter 6, “Security”.

4.11. Environment Variables

You can control some facets of the publisher’s operation by setting environment variables.

• ‘Z_DEBUG_MODE’ – Sets debug mode. In debug mode tracebacks are not hidden in error pages. Also debug mode causes ‘DTMLFile’ objects, External Methods and help topics to reload their contents from disk when changed. You can also set debug mode with the ‘-D’ switch when starting Zope.
• ‘Z_REALM’ – Sets the basic authorization realm. This controls the realm name as it appears in the web browser’s username and password dialog. You can also set the realm with the ‘__bobo_realm__’ module variable, as mentioned previously.
• ‘PROFILE_PUBLISHER’ – Turns on profiling and sets the name of the profile file. See the Python documentation for more information about the Python profiler.

Many more options can be set using switches on the startup script. See the Zope Administrator’s Guide for more information.

4.11.1. Testing

ZPublisher comes with built-in support for testing and working with the Python debugger. This topic is covered in more detail in Chapter 7, “Testing and Debugging”.

4.11.2. Publishable Module

If you are using the Zope framework, this section will be irrelevant to you. However, if you are publishing your own modules with ‘ZPublisher’ read on.

The publisher begins the traversal process by locating an object in the module’s global namespace that corresponds to the first element of the path. Alternately the first object can be located by one of two hooks.

If the module defines a ‘web_objects’ or ‘bobo_application’ object, the first object is searched for in those objects. The search happens according to the normal rules of traversal, using ‘__bobo_traverse__’, ‘getattr’, and ‘__getitem__’.

The module can receive callbacks before and after traversal. If the module defines a ‘__bobo_before__’ object, it will be called with no arguments before traversal. Its return value is ignored. Likewise, if the module defines a ‘__bobo_after__’ object, it will be called after traversal with no arguments. These callbacks can be used for things like acquiring and releasing locks.

4.11.3. Calling the Published Object

Now that we’ve covered how the publisher located the published object and what it does with the results of calling it, let’s take a closer look at how the published object is called.

The publisher marshals arguments from the request and automatically makes them available to the published object. This allows you to accept parameters from web forms without having to parse the forms. Your objects usually don’t have to do anything special to be called from the web. Consider this function:

def greet(name):
    "greet someone"
    return "Hello, %s" % name

You can provide the ‘name’ argument to this function by calling it with a URL like greet?name=World. You can also call it with a HTTP ‘POST’ request which includes ‘name’ as a form variable.

In the next sections we’ll take a closer look at how the publisher marshals arguments.

4.11.4. Marshalling Arguments from the Request

The publisher marshals form data from GET and POST requests. Simple form fields are made available as Python strings. Multiple fields such as form check boxes and multiple selection lists become sequences of strings. File upload fields are represented with ‘FileUpload’ objects. File upload objects behave like normal Python file objects and additionally have a ‘filename’ attribute which is the name of the file and a ‘headers’ attribute which is a dictionary of file upload headers.

The publisher also marshals arguments from CGI environment variables and cookies. When locating arguments, the publisher first looks in CGI environment variables, then other request variables, then form data, and finally cookies. Once a variable is found, no further searching is done. So for example, if your published object expects to be called with a form variable named ‘SERVER_URL’, it will fail, since this argument will be marshaled from the CGI environment first, before the form data.

The publisher provides a number of additional special variables such as ‘URL0’ which are derived from the request. These are covered in the ‘HTTPRequest’ API documentation.

4.11.5. Argument Conversion

The publisher supports argument conversion. For example consider this function:

def onethird(number):
    "returns the number divided by three"
    return number / 3.0

This function cannot be called from the web because by default the publisher marshals arguments into strings, not numbers. This is why the publisher provides a number of converters. To signal an argument conversion you name your form variables with a colon followed by a type conversion code. For example, to call the above function with 66 as the argument you can use this URL onethird?number:int=66 The publisher supports many converters:

• boolean – Converts a variable to true or false. Variables that are 0, None, an empty string, or an empty sequence are false, all others are true.
• int – Converts a variable to a Python integer.
• long – Converts a variable to a Python long integer.
• float – Converts a variable to a Python floating point number.
• string – Converts a variable to a Python string.
• ustring – Converts a variable to a Python unicode string.
• required – Raises an exception if the variable is not present or is an empty string.
• ignore_empty – Excludes a variable from the request if the variable is an empty string.
• date – Converts a string to a DateTime object. The formats accepted are fairly flexible, for example ‘10/16/2000’, ‘12:01:13 pm’.
• list – Converts a variable to a Python list of values, even if there is only one value.
• tuple – Converts a variable to a Python tuple of values, even if there is only one value.
• lines – Converts a string to a Python list of values by splitting the string on line breaks.
• tokens – Converts a string to a Python list of values by splitting the string on spaces.
• text – Converts a variable to a string with normalized line breaks. Different browsers on various platforms encode line endings differently, so this converter makes sure the line endings are consistent, regardless of how they were encoded by the browser.
• ulines, utokens, utext – like lines, tokens, text, but using unicode strings instead of plain strings.

If the publisher cannot coerce a request variable into the type required by the type converter it will raise an error. This is useful for simple applications, but restricts your ability to tailor error messages. If you wish to provide your own error messages, you should convert arguments manually in your published objects rather than relying on the publisher for coercion. Another possibility is to use JavaScript to validate input on the client-side before it is submitted to the server.

You can combine type converters to a limited extent. For example you could create a list of integers like so:

<input type="checkbox" name="numbers:list:int" value="1">
<input type="checkbox" name="numbers:list:int" value="2">
<input type="checkbox" name="numbers:list:int" value="3">

In addition to these type converters, the publisher also supports method and record arguments.

4.11.6. Character Encodings for Arguments

The publisher needs to know what character encoding was used by the browser to encode form fields into the request. That depends on whether the form was submitted using GET or POST (which the publisher can work out for itself) and on the character encoding used by the page which contained the form (for which the publisher needs your help).

In some cases you need to add a specification of the character encoding to each fields type converter. The full details of how this works are explained below, however most users do not need to deal with the full details:

1. If your pages all use the UTF-8 character encoding (or at least all the pages that contain forms) the browsers will always use UTF-8 for arguments. You need to add ‘:utf8’ into all argument type converts. For example:

   <input type=”text” name=”name:utf8:ustring”> <input type=”checkbox” name=”numbers:list:int:utf8” value=”1”> <input type=”checkbox” name=”numbers:list:int:utf8” value=”1”>

   % Anonymous User - Apr. 6, 2004 5:56 pm:

   121

2. If your pages all use a character encoding which has ASCII as a subset (such as Latin-1, UTF-8, etc) then you do not need to specify any chatacter encoding for boolean, int, long, float, and date types. You can also omit the character encoding type converter from string, tokens, lines, and text types if you only need to handle ASCII characters in that form field.

4.11.6.1. Character Encodings for Arguments; The Full Story

If you are not in one of those two easy categories, you first need to determine which character encoding will be used by the browser to encode the arguments in submitted forms.

1. Forms submitted using GET, or using POST with “application/x-www-form-urlencoded” (the default)

   1. Page uses an encoding of unicode: Forms are submitted using UTF8, as required by RFC 2718 2.2.5
   2. Page uses another regional 8 bit encoding: Forms are often submitted using the same encoding as the page. If you choose to use such an encoding then you should also verify how browsers behave.

2. Forms submitted using “multipart/form-data”:

   According to HTML 4.01 (section 17.13.4) browsers should state which character encoding they are using for each field in a Content-Type header, however this is poorly supported. The current crop of browsers appear to use the same encoding as the page containing the form.

   Every field needs that character encoding name appended to is converter. The tag parser insists that tags must only use alphanumberic characters or an underscore, so you might need to use a short form of the encoding name from the Python ‘encodings’ library package (such as utf8 rather than UTF-8).

4.11.7. Method Arguments

Sometimes you may wish to control which object is published based on form data. For example, you might want to have a form with a select list that calls different methods depending on the item chosen. Similarly, you might want to have multiple submit buttons which invoke a different method for each button.

The publisher provides a way to select methods using form variables through use of the method argument type. The method type allows the request ‘PATH_INFO’ to be augmented using information from a form item name or value.

If the name of a form field is ‘:method’, then the value of the field is added to ‘PATH_INFO’. For example, if the original ‘PATH_INFO’ is ‘foo/bar’ and the value of a ‘:method’ field is ‘x/y’, then ‘PATH_INFO’ is transformed to ‘foo/bar/x/y’. This is useful when presenting a select list. Method names can be placed in the select option values.

If the name of a form field ends in ‘:method’ then the part of the name before ‘:method’ is added to ‘PATH_INFO’. For example, if the original ‘PATH_INFO’ is ‘foo/bar’ and there is a ‘x/y:method’ field, then ‘PATH_INFO’ is transformed to ‘foo/bar/x/y’. In this case, the form value is ignored. This is useful for mapping submit buttons to methods, since submit button values are displayed and should, therefore, not contain method names.

Only one method field should be provided. If more than one method field is included in the request, the behavior is undefined.

4.11.8. Record Arguments

Sometimes you may wish to consolidate form data into a structure rather than pass arguments individually. Record arguments allow you to do this.

The ‘record’ type converter allows you to combine multiple form variables into a single input variable. For example:

<input name="date.year:record:int">
<input name="date.month:record:int">
<input name="date.day:record:int">

This form will result in a single variable, ‘date’, with attributes ‘year’, ‘month’, and ‘day’.

You can skip empty record elements with the ‘ignore_empty’ converter. For example:

<input type="text" name="person.email:record:ignore_empty">

When the email form field is left blank the publisher skips over the variable rather than returning a null string as its value. When the record ‘person’ is returned it will not have an ‘email’ attribute if the user did not enter one.

You can also provide default values for record elements with the ‘default’ converter. For example:

<input type="hidden"
       name="pizza.toppings:record:list:default"
       value="All">
<select multiple name="pizza.toppings:record:list:ignore_empty">
<option>Cheese</option>
<option>Onions</option>
<option>Anchovies</option>
<option>Olives</option>
<option>Garlic<option>
</select>

The ‘default’ type allows a specified value to be inserted when the form field is left blank. In the above example, if the user does not select values from the list of toppings, the default value will be used. The record ‘pizza’ will have the attribute ‘toppings’ and its value will be the list containing the word “All” (if the field is empty) or a list containing the selected toppings.

You can even marshal large amounts of form data into multiple records with the ‘records’ type converter. Here’s an example:

<h2>Member One</h2>
Name:
<input type="text" name="members.name:records"><BR>
Email:
<input type="text" name="members.email:records"><BR>
Age:
<input type="text" name="members.age:int:records"><BR>

<H2>Member Two</H2>
Name:
<input type="text" name="members.name:records"><BR>
Email:
<input type="text" name="members.email:records"><BR>
Age:
<input type="text" name="members.age:int:records"><BR>

This form data will be marshaled into a list of records named ‘members’. Each record will have a ‘name’, ‘email’, and ‘age’ attribute.

Record marshalling provides you with the ability to create complex forms. However, it is a good idea to keep your web interfaces as simple as possible.

4.11.9. Exceptions

Unhandled exceptions are caught by the object publisher and are translated automatically to nicely formatted HTTP output.

When an exception is raised, the exception type is mapped to an HTTP code by matching the value of the exception type with a list of standard HTTP status names. Any exception types that do not match standard HTTP status names are mapped to “Internal Error” (500). The standard HTTP status names are: “OK”, “Created”, “Accepted”, “No Content”, “Multiple Choices”, “Redirect”, “Moved Permanently”, “Moved Temporarily”, “Not Modified”, “Bad Request”, “Unauthorized”, “Forbidden”, “Not Found”, “Internal Error”, “Not Implemented”, “Bad Gateway”, and “Service Unavailable”. Variations on these names with different cases and without spaces are also valid.

An attempt is made to use the exception value as the body of the returned response. The object publisher will examine the exception value. If the value is a string that contains some white space, then it will be used as the body of the return error message. If it appears to be HTML, the error content type will be set to ‘text/html’, otherwise, it will be set to ‘text/plain’. If the exception value is not a string containing white space, then the object publisher will generate its own error message.

There are two exceptions to the above rule:

1. If the exception type is: “Redirect”, “Multiple Choices” “Moved Permanently”, “Moved Temporarily”, or “Not Modified”, and the exception value is an absolute URI, then no body will be provided and a ‘Location’ header will be included in the output with the given URI.
2. If the exception type is “No Content”, then no body will be returned.

When a body is returned, traceback information will be included in a comment in the output. As mentioned earlier, the environment variable ‘Z_DEBUG_MODE’ can be used to control how tracebacks are included. If this variable is set then tracebacks are included in ‘PRE’ tags, rather than in comments. This is very handy during debugging.

4.11.10. Exceptions and Transactions

When Zope receives a request it begins a transaction. Then it begins the process of traversal. Zope automatically commits the transaction after the published object is found and called. So normally each web request constitutes one transaction which Zope takes care of for you. See Chapter 4. for more information on transactions.

If an unhandled exception is raised during the publishing process, Zope aborts the transaction. As detailed in Chapter 4. Zope handles ‘ConflictErrors’ by re-trying the request up to three times. This is done with the ‘zpublisher_exception_hook’.

In addition, the error hook is used to return an error message to the user. In Zope the error hook creates error messages by calling the ‘raise_standardErrorMessage’ method. This method is implemented by ‘SimpleItem.Item’. It acquires the ‘standard_error_message’ DTML object, and calls it with information about the exception.

You will almost never need to override the ‘raise_standardErrorMessage’ method in your own classes, since it is only needed to handle errors that are raised by other components. For most errors, you can simply catch the exceptions normally in your code and log error messages as needed. If you need to, you should be able to customize application error reporting by overriding the ‘standard_error_message’ DTML object in your application.

4.11.11. Manual Access to Request and Response

You do not need to access the request and response directly most of the time. In fact, it is a major design goal of the publisher that most of the time your objects need not even be aware that they are being published on the web. However, you have the ability to exert more precise control over reading the request and returning the response.

Normally published objects access the request and response by listing them in the signature of the published method. If this is not possible you can usually use acquisition to get a reference to the request. Once you have the request, you can always get the response from the request like so:

response=REQUEST.RESPONSE

The APIs of the request and response are covered in the API documentation. Here we’ll look at a few common uses of the request and response.

One reason to access the request is to get more precise information about form data. As we mentioned earlier, argument marshalling comes from a number of places including cookies, form data, and the CGI environment. For example, you can use the request to differentiate between form and cookie data:

cookies = REQUEST.cookies # a dictionary of cookie data
form = REQUEST.form # a dictionary of form data

One common use of the response object is to set response headers. Normally the publisher in concert with the web server will take care of response headers for you. However, sometimes you may wish manually control headers:

RESPONSE.setHeader('Pragma', 'No-Cache')

Another reason to access the response is to stream response data. You can do this with the ‘write’ method:

while 1:
    data=getMoreData() #this call may block for a while
    if not data:
        break
    RESPONSE.write(data)

Here’s a final example that shows how to detect if your method is being called from the web. Consider this function:

def feedParrot(parrot_id, REQUEST=None):
    ...

    if REQUEST is not None:
        return "<html><p>Parrot %s fed</p></html>" % parrot_id

The ‘feedParrot’ function can be called from Python, and also from the web. By including ‘REQUEST=None’ in the signature you can differentiate between being called from Python and being called form the web. When the function is called from Python nothing is returned, but when it is called from the web the function returns an HTML confirmation message.

4.12. Other Network Protocols

4.12.1. FTP

Zope comes with an FTP server which allows users to treat the Zope object hierarchy like a file server. As covered in Chapter 3, Zope comes with base classes (‘SimpleItem’ and ‘ObjectManager’) which provide simple FTP support for all Zope objects. The FTP API is covered in the API reference.

To support FTP in your objects you’ll need to find a way to represent your object’s state as a file. This is not possible or reasonable for all types of objects. You should also consider what users will do with your objects once they access them via FTP. You should find out which tools users are likely to edit your object files. For example, XML may provide a good way to represent your object’s state, but it may not be easily editable by your users. Here’s an example class that represents itself as a file using RFC 822 format:

from rfc822 import Message
from cStringIO import StringIO

class Person(...):

    def __init__(self, name, email, age):
        self.name=name
        self.email=email
        self.age=age

    def writeState(self):
        "Returns object state as a string"
        return "Name: %s\nEmail: %s\nAge: %s" % (self.name,
                                                 self.email,
                                                 self.age)
    def readState(self, data):
        "Sets object state given a string"
        m=Message(StringIO(data))
        self.name=m['name']
        self.email=m['email']
        self.age=int(m['age'])

The ‘writeState’ and ‘readState’ methods serialize and unserialize the ‘name’, ‘age’, and ‘email’ attributes to and from a string. There are more efficient ways besides RFC 822 to store instance attributes in a file, however RFC 822 is a simple format for users to edit with text editors.

To support FTP all you need to do at this point is implement the ‘manage_FTPget’ and ‘PUT’ methods. For example:

def manage_FTPget(self):
    "Returns state for FTP"
    return self.writeState()

def PUT(self, REQUEST):
    "Sets state from FTP"
     self.readState(REQUEST['BODY'])

You may also choose to implement a ‘get_size’ method which returns the size of the string returned by ‘manage_FTPget’. This is only necessary if calling ‘manage_FTPget’ is expensive, and there is a more efficient way to get the size of the file. In the case of this example, there is no reason to implement a ‘get_size’ method.

One side effect of implementing ‘PUT’ is that your object now supports HTTP PUT publishing. See the next section on WebDAV for more information on HTTP PUT.

That’s all there is to making your object work with FTP. As you’ll see next WebDAV support is similar.

4.12.2. WebDAV

WebDAV is a protocol for collaboratively edit and manage files on remote servers. It provides much the same functionality as FTP, but it works over HTTP.

It is not difficult to implement WebDAV support for your objects. Like FTP, the most difficult part is to figure out how to represent your objects as files.

Your class must inherit from ‘webdav.Resource’ to get basic DAV support. However, since ‘SimpleItem’ inherits from ‘Resource’, your class probably already inherits from ‘Resource’. For container classes you must inherit from ‘webdav.Collection’. However, since ‘ObjectManager’ inherits from ‘Collection’ you are already set so long as you inherit from ‘ObjectManager’.

In addition to inheriting from basic DAV classes, your classes must implement ‘PUT’ and ‘manage_FTPget’. These two methods are also required for FTP support. So by implementing WebDAV support, you also implement FTP support.

The permissions that you assign to these two methods will control the ability to read and write to your class through WebDAV, but the ability to see your objects is controlled through the “WebDAV access” permission.

4.12.3. Supporting Write Locking

Write locking is a feature of WebDAV that allows users to put lock on objects they are working on. Support write locking s easy. To implement write locking you must assert that your lass implements the ‘WriteLockInterface’. For example:

from webdav.WriteLockInterface import WriteLockInterface

class MyContentClass(OFS.SimpleItem.Item, Persistent):
    __implements__ = (WriteLockInterface,)

It’s sufficient to inherit from ‘SimpleItem.Item’, since it inherits from ‘webdav.Resource’, which provides write locking long with other DAV support.

In addition, your ‘PUT’ method should begin with calls to dav__init’ and ‘dav_simpleifhandler’. For example:

def PUT(self, REQUEST, RESPONSE):
    """
    Implement WebDAV/HTTP PUT/FTP put method for this object.
    """
    self.dav__init(REQUEST, RESPONSE)
    self.dav__simpleifhandler(REQUEST, RESPONSE)
    ...

Finally your class’s edit methods should check to determine whether your object is locked using the ‘ws_isLocked’ method. If someone attempts to change your object when it is locked you should raise the ‘ResourceLockedError’. For example:

from webdav import ResourceLockedError

class MyContentClass(...):
    ...

    def edit(self, ...):
        if self.ws_isLocked():
            raise ResourceLockedError
        ...

WebDAV support is not difficult to implement, and as more WebDAV editors become available, it will become more valuable. If you choose to add FTP support to your class you should probably go ahead and support WebDAV too since it is so easy once you’ve added FTP support.

4.12.4. XML-RPC

XML-RPC is a light-weight Remote Procedure Call protocol that uses XML for encoding and HTTP for transport. Fredrick Lund maintains a Python <XML-RPC module <http://www.pythonware.com/products/xmlrpc>`_ .

All objects in Zope support XML-RPC publishing. Generally you will select a published object as the end-point and select one of its methods as the method. For example you can call the ‘getId’ method on a Zope folder at ‘http://example.com/myfolder‘ like so:

import xmlrpclib
folder = xmlrpclib.Server('http://example.com/myfolder')
ids = folder.getId()

You can also do traversal via a dotted method name. For example:

import xmlrpclib

# traversal via dotted method name
app = xmlrpclib.Server('http://example.com/app')
id1 = app.folderA.folderB.getId()

# walking directly up to the published object
folderB = xmlrpclib.Server('http://example.com/app/folderA/folderB')
id2 = folderB.getId()

print id1 == id2

This example shows different routes to the same object publishing call.

XML-RPC supports marshalling of basic Python types for both publishing requests and responses. The upshot of this arrangement is that when you are designing methods for use via XML-RPC you should limit your arguments and return values to simple values such as Python strings, lists, numbers and dictionaries. You should not accept or return Zope objects from methods that will be called via XML-RPC.

XML-RPC does not support keyword arguments. This is a problem if your method expect keyword arguments. This problem is noticeable when calling DTMLMethods and DTMLDocuments with XML-RPC. Normally a DTML object should be called with the request as the first argument, and additional variables as keyword arguments. You can get around this problem by passing a dictionary as the first argument. This will allow your DTML methods and documents to reference your variables with the ‘var’ tag. However, you cannot do the following:

<dtml-var expr="REQUEST['argument']">

Although the following will work:

<dtml-var expr="_['argument']">

This is because in this case arguments are in the DTML namespace, but they are not coming from the web request.

In general it is not a good idea to call DTML from XML-RPC since DTML usually expects to be called from normal HTTP requests.

One thing to be aware of is that Zope returns ‘false’ for published objects which return None since XML-RPC has no concept of null.

Another issue you may run into is that ‘xmlrpclib’ does not yet support HTTP basic authentication. This makes it difficult to call protected web resources. One solution is to patch ‘xmlrpclib’. Another solution is to accept authentication credentials in the signature of your published method.

4.13. Summary

Object publishing is a simple and powerful way to bring objects to the web. Two of Zope’s most appealing qualities is how it maps objects to URLs, and you don’t need to concern yourself with web plumbing. If you wish, there are quite a few details that you can use to customize how your objects are located and published.