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 via 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

Zope 4 no longer ships with a builtin web server, so when you want to interact with Zope via browser you have to setup a WSGI server.

Note

For usage on a production server you will probably want to setup a reverse proxy in front of the WSGI server.

The WSGI server receives the request and hands it over to Zope, where 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 the WSGI server. Finally, the WSGI server passes the response back to your web browser.

The publishing process is summarized in [2-1]

../_images/2-1.png

2.1 Object publishing

Typically the published object is a persistent object that the published module loads from the ZODB. See Chapter 6 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. A request is sent 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 the results and passes them back.

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

4.2.1. 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.

../_images/2-2.png

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 closer look at traversal.

4.2.2. 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 methods, too.

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

Finally, published objects cannot be Python modules.

4.2.3. 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):
    """Return subobjects depending on cookie contents."""
    if request.cookies.has_key('special'):
        return self.special_subobjects.get(key, None)
    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 subobjects 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"]

4.2.4. 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.

4.2.5. Character Encodings for Responses

If the published method returns an object of type binary, 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, which is also the default encoding.

If the Content-Type header does not include a charset or is not set at all, the default encoding is set.

If you want to manually set a Content-Type header you have to set a value like text/html; charset=UTF-8.

4.2.6. HTTP Responses

Usually, 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.

Note

When the return value is empty, e.g. an empty list, instead of returning an empty page, Zope issues a header with a 204 status code.

Depending on the used client, it looks like nothing happens.

Optionally, the published method can return a tuple with the title and the body of the response. In this case, the publisher returns a generated HTML page, with the first item of the tuple used for the value of the HTML title tag of the page, and the second item as the content of the HTML body tag.

For example a response of:

("my_title", "my_text")

is turned into this HTML page:

<html>
<head><title>my_title</title></head>
<body>my_text</body>
</html>

4.2.7. 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 class"""

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

    def two(self):
        """render page 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 index_html without specifying the method name in the URL, relative links returned by index_html won’t work right.

For example:

class Example:
    """example class""""

     def index_html(self):
         """render default view"""
        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 between the head tags in the HTML output of index_html 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, 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.

4.2.8. 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 and also how exceptions are used to set the HTTP response code.

4.2.9. 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.

4.2.10. Traversal and Acquisition

Note

Simply put, acquisition means that a Zope object can acquire any attribute of its parents.

For detailed information about acquisition please refer to chapter 7.

Acquisition affects traversal in several ways. The most obvious way 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 the following object hierarchy:

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. 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 number_of_items(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/number_of_items uses acquisition along containment lines to publish the number_of_items method (assuming that apple doesn’t have a number_of_items 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.

4.2.11. 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 8 “Security”.

4.2.12. 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 historically did not support creating attributes. 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 8 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.

If validation succeeds the publisher assigns the user object to the request variable AUTHENTICATED_USER. The publisher places no restrictions on user objects.

4.2.13. Zope Security

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.

Note

For more information on Acquisition, visit one of the following resources:

  • chapter 7 “Acquisition” of this Zope Developer’s Guide
  • chapter 8 “Acquisition” of The Zope Book
  • the excellent readme of the “Acquisition” package

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 8, “Security”.

4.2.14. Calling the Published Object

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 by name."""
    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.

4.2.15. Marshalling Arguments from the Request

Zope responds to requests, specified via URL, request headers and an optional request body. A URL consists of various parts, among them a path and a query, see RFC 2396 for details.

Zope uses the path to locate an object, method or view for producing the response (this process is called traversal) and query - if present - as a specification for request parameters. Additionally, request parameters can come from the optional request body.

Zope preprocesses the incoming request information and makes the result available in the so called request object. This way, the response generation code can access all relevant request information in an easy and natural (pythonic) way. Preprocessing transforms the request parameters into request (or form) variables. They are made available via the request object’s form attribute (a dict) or directly via the request object itself, as long as they are not hidden by other request information.

The request parameters coming from the query have the form name=value and are separated by &; request parameters from a request body can have different forms and can be separated in different ways dependent on the request Content-Type, but they, too, have a name and a value.

All request parameter names and values are strings. A parameter value, however, often designates a value of a specific type, e.g. an integer or a datetime. The response generating code can be simplified significantly when it does not need to make the type conversion itself. In addition, in some cases the request parameters are not independent from one another but related. In those cases it can help if the related parameters are aggregated into a single object. Zope supports both cases but it needs directives to guide the process. It uses name suffixes of the form :directive to specify such directives. For example, the parameter i:int=1 tells Zope to convert the value '1' to an integer and use it as value for request variable i; the parameter sequence x.name:record=Peter&x.age:int:record=10 tells Zope to construct a record x with attributes name and age and respective values 'Peter' and 10.

The publisher also marshals arguments from CGI environment variables and cookies. When locating arguments, the publisher first looks in other (i.e. explicitly set or special) request variables, then CGI environment variables, then form variables, 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 marshalled from the CGI environment first, before the form data.

The publisher provides a number of additional special variables such as URL, URLn, BASEn and others, which are derived from the request.

Unfortunately, there is no current documentation for those variables.

4.2.15.1. Argument Conversion

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

def one_third(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 one_third?number:int=66.

Some converters employ special logic for the conversion. For example, both tokens as well as lines convert to a list of strings but tokens splits the input at whitespace, lines at newlines.

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. Also converts a list/tuple of variables to a list/tuple of integers.
  • long – Converts a variable to a Python integer. Strips the trailing “L” symbol at the end of the value. Also converts a list/tuple of variables to a list/tuple of integers.
  • float – Converts a variable to a Python floating point number. Also converts a list/tuple of variables to a list/tuple of floats.
  • string – Converts a variable to a native string. So the result is str, no matter which Python version you are on.
  • ustring – Converts a variable to a Python unicode string.
  • bytes – Converts a variable to a Python bytes object/string.
  • required – Raises an exception if the variable is not present or 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.
  • date_international – Converts a string to a DateTime object, but especially treats ambiguous dates as “days before month before year”. This useful if you need to parse non-US dates.
  • lines – Converts a variable to a Python list of native strings by splitting the string on line breaks. Also converts list/tuple of variables to list/tuple of native strings.
  • tokens – Converts a variable to a Python list of native strings by splitting the variable on whitespace.
  • text – Converts a variable to a native 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 always converts into unicode strings.

The full list of supported converters can be found in ZPublisher.Converters.type_converters.

If the publisher cannot coerce a request parameter 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.

Note

Client-side validation with HTML 5 and/or JavaScript may improve the usability of the application, but it is never a replacement for server side validation.

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">

4.2.15.2. Aggregators

An aggregator directive tells Zope how to process parameters with the same or a similar name.

Zope supports the following aggregators:

  • list – collect all values with this name into a list. If there are two or more parameters with the same name they are collected into a list by default. The list aggregator is mainly used to ensure that the parameter leads to a list value even in the case that there is only one of them.
  • tuple – collect all values with this name into a tuple.
  • default – use the value of this parameter as a default value; it can be overridden by a parameter of the same name without the default directive.
  • record – this directive assumes that the parameter name starts with var.attr. It tells Zope to create a request variable var of type record (more precisely, a ZPublisher.HTTPRequest.record instance) and set its attribute attr to the parameter value. If such a request variable already exists, then only its attribute attr is updated.
  • records – this directive is similar to record. However, var gets as value not a single record but a list of records. Zope starts a new record (and appends it to the list) when the current request parameter would override an attribute in the last record of the list constructed so far (or this list is empty).
  • ignore_empty – this directive causes Zope to ignore the parameter if its value is empty.
4.2.15.2.1. An aggregator in detail: the record argument

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 the 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 an empty 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 marshalled 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.

Note

Records do not work with input fields of type radio as you might expect, as all radio fields with the same name are considered as one group - even if they are in different records. That means, activating one radio button will also deactivate all other radio buttons from the other records.

Attention

When using records please note that there is a known issue when you use a form, where checkboxes are used in the first “column”.

As browsers leave out empty checkboxes when sending a request, the object publisher may not be able to match checked checkboxes with the correct record.

This behaviour cannot not be fixed.

If you want a checkbox as the first form field, you can work around the problem by using a hidden input field.

Code example with applied workaround:

<form action="records_parse">
    <p>
    <input type="hidden" name="index.dummy:records" value="dummy" />
    <input type="checkbox" name="index.enabled:records" value="1" checked="checked" />
    <input type="text" name="index.name:records" value="index 1" />
    <p>
    <input type="hidden" name="index.dummy:records" value="dummy" />
    <input type="checkbox" name="index.enabled:records" value="2" />
    <input type="text" name="index.name:records" value="index 2" />
    <p>
    <input type="submit" name="submit" value="send" />
</form>

4.2.15.3. Specifying argument character encodings

An encoding directive tells the converting process the encoding of the parameter value. Typical encodings are e.g. “utf8” or “latin1”.

An encoding directive is ignored if the parameter does not have a converter directive as well. If there is no encoding directive, the converter uses the default encoding as specified by the Zope configuration option zpublisher-default-encoding. The default value for this configuration option in Zope 4 is utf-8.

In principle, Zope supports any encoding known by the codecs module. However, the converter may impose restrictions.

Special cases

If your pages use a different encoding, such as Windows-1252 or ISO-8859-1, which was the default encoding for HTML 4, you have to add the encoding, eg :cp1252, for all argument type converts, such as follows:

<input type="text" name="name:cp1252:ustring">
<input type="checkbox" name="numbers:list:int:cp1252" value="1">
<input type="checkbox" name="numbers:list:int:cp1252" value="1">

Note

For a full list of supported encodings, please have a look at:

https://docs.python.org/3.7/library/codecs.html#standard-encodings

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 character encoding for boolean, int, long, float and date types.

Note

The form submission encoding can be overridden by the accept-charset attribute of the form tag:

https://www.w3.org/TR/html5/sec-forms.html#selecting-a-form-submission-encoding

4.2.15.4. Method Arguments

Normally, a request parameter is transformed into a request variable and made available via the form attribute of the request object. The method directive tells Zope to extend the path used for traversal.

You can use a method directive 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 the use of the method argument type. The method type allows the request variable PATH_INFO to be augmented using information from a form item’s 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.

Zope supports the following method directives: method (synonym action), and default_method (synonym default_action). A path extension specified by a default_method directive is overridden by a method directive.

4.2.15.5. Processing model for request data marshaling

Zope processes the request parameters in ZPublisher.HTTPRequest.HTTPRequest.processInputs.

This section describes the complex processing model in some detail as its various steps and peculiar logic may lead to surprises. If you are developing with Zope as opposed to developing Zope itelf, you may skip over these details.

In a preliminary step the request parameters are collected from the potential sources, i.e. the “query” and request body (if present), and normalized. The result is a sequence of name/value pairs, each describing a single request parameter.

Zope then sets up some variables:

form
as target for the collected form variables
defaults
as target for the collected form variable defaults
tuple_items
to remember which form variable should be tuples
method
as target for the path extension from method directives.

It then loops over the request parameter sequence.

For each request parameter, the processing consists of the following steps:

  1. Some variables are set up:

    isFileUpload

    does the parameter represent an uploaded file?

    converter_type

    the most recently seen converter from a converter directive

    character_encoding

    the most recently seen encoding from an encoding directive

    flags

    to indicate which processing types are requested via directives

    Processing types are “ignore”, “aggregate as sequence”, “aggregate as record”, “aggregate as records”, “use as default”, “convert” (using converter_type and character_encoding).

  2. The parameter value is checked to see if it is a file upload. In this case, it is wrapped into a FileUpload, and isFileUpload is updated.

  3. All directives in the paramter name are examined from right to left and the variables set up in step 1 are updated accordingly. :tuple directives update flags and tuple_items, and method directives update flags and method.

  4. The actions stored in flags during step 3 are executed.

    If flags indicate the use as default, the step operates on defaults, otherwise on form.

After all request parameters have been processed request variables from defaults are put into form as long as it does not contain that variable already. If a method directive has been encountered the traversal path is extended accordingly.

As a security measure, mainly for DTML use, request variables are not only made available in the request attribute form. A (somewhat) secured version of them is also stored in the attribute taintedform. In the tainted request variable variant, strings potentially containing HTML fragments use TaintedString as data type rather than the normal str. DTML will automatically quote those values to give some protection against cross site scripting attacks via HTML injection. With the more modern page templates, all values (not only tainted ones) are quoted by default. They typically do not use the tainted form of the request variables.

4.2.15.6. Known issues and caveats

  1. There is almost no error handling:

    • unrecognized directives are silently ignored
    • if a request paramater contains several converter directives, the leftmost wins
    • if a request paramter contains several encoding directives, the leftmost wins
    • if a request parameter contains an encoding but no converter directive, the encoding directive is silently ignored
    • some directive combinations do not make sense (e.g. :record:records); for them, some of the directives are silently ignored
  2. Usually, the order of aggregator directives in a request parameter does not matter. However, this is not the case for the :tuple directive. To really produce a tuple request variable, it must be the left most directive; otherwise, it is equivalent to :list.

    In addition, :tuple is always equivalent to :list for request variables aggregated as record or sequence of records.

  3. The main use case for the :default directive is to provide a default value for form controls (e.g. checkboxes) for which the browser may or may not pass on a value when the form is submitted. Unfortunately, this only works at the top level. It does not work for subcomponents, e.g. an attribute of a “record”. As a consequence, if a request parameter combines :default with another aggregator directive, the result may be unexpected.

  4. The request preprocessing happens at a very early stage, before traversal has taken place. As a consequence, important configuration for application specific error handling may not yet have taken effect. Exceptions raised during this stage are reported and tracked only via “root level” error handling. For the reason it is typically better to use a form framework such as z3c.form or zope.formlib for form processing rather than the built-in features described in this document.

4.2.16. Exceptions

When the object publisher catches an unhandled exception, it tries to match it with a set of predifined exceptions coming from the zExceptions package, such as HTTPNoContent, HTTPNotFound, HTTPUnauthorized.

If there is a match, the exception gets upgraded to the matching zException, which then results in a proper response returned to the browser, including an appropriate HTTP status code.

Note

For a full list of exceptions please directly refer to the implemented exception classes within the zExceptions package.

Attention

When you create a custom exception, please make sure not to inherit from BaseException, but from Exception or one of its child classes, otherwise you’ll run into an exception in waitress.

Note

Beginning with Zope 4, a standard installation no longer comes with a standard_error_message.

There are two ways to catch and render an exception:

  • create a standard_error_message, which can be a DTML Method, DTML Document, Script (Python) or Page Template
  • create an exception view, see blog post Catching and rendering exceptions

If the exception is not handled, it travels up the WSGI stack.

What happens then depends entirely on the possibly installed WSGI middleware or the WSGI server. By default Zope uses waitress and by default waitress returns an error message as follows:

Internal Server Error

The server encountered an unexpected internal server error

(generated by waitress)

Note

Further information:

Debugging Zope applications under WSGI

4.2.16.1. 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.

If an unhandled exception is raised during the publishing process, Zope aborts the transaction. When a ConflictError occurs, Zope retries the request up to three times by default. You can change that number in the zope.conf by adding a max_conflict_retries directive.

Note

For further information on transactions please refer to chapter 6 ZODB Persistent Components.

4.2.17. Manual Access to Request and 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 request and response can be looked up in the source code.

We’ll look at a few common uses of the request and response. If you need access to the complete API, please directly refer to the source code.

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 calculate(data, REQUEST=None):
    ...
    result = ...
    if REQUEST is not None:
        return "<html><p>Result: %s </p></html>" % result
    return result

The calculate 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.

4.3. Other Network Protocols

4.3.1. XML-RPC

XML-RPC is a light-weight remote procedure call (RPC) protocol that uses XML to encode its calls and HTTP as a transport mechanism.

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:

from xmlrpc.client import ServerProxy as proxy
folder = proxy("http://example.com/myfolder")
folder_id = folder.getId()

You can also do traversal via a dot notation.

For example:

from xmlrpc.client import ServerProxy as proxy

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

# walking directly up to the published object
folderB = proxy("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.

Note

XML-RPC does not support keyword arguments.

4.4. 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.