Requests and traversal¶
How does Zope handle requests and translate paths to published objects?
What happens when a request is received?¶
A request is received either via a WSGI pipeline or the
Medusa web server. Using Medusa, it first hits
zhttp_server, which consumes the request until it has enough to act
on. At this point
is called. This constructs a
from the Medusa
environment and prepares a
ZServerHTTPResponse, a subclass of
The actual request is delegated to a threadpool. In a
non-WSGI setup, this is managed by
(note the typo in the module name!). This keeps track of
the requests and (skeletal) responses to be processed, and
passes them to an instance of a
also deals with thread locking.
will call either
ZPublisher.WSGIPublisher.publish_module, depending on the deployment mode, with the request and
the response. The non-WSGI version also takes a module
name to publish, which is
Zope2. This is a relic of the Bobo publisher, which could
publish other modules with a
variable set (recall that this variable was set in the
startup phase described above).
The remainder of this section will describe the non-WSGI publisher. The WSGI publisher performs the same actions, but deals in WSGI environs and response body iterators.
There are two versions of
publish_module, one with profiling and one without.
(without profiling) performs the following actions:
Set the default ZTK skin on the request, by adapting the
publish(), which does the real publication.
- Handle errors.
Write the response body to
stdout, which is wired up to be the HTTP response stream.
The more interesting function is
publish(). This starts by calling
to get the information about the published module (which,
recall, is almost always going to be
Zope2). The results are cached, so this will only do its work
(bobo_before, bobo_after, object, realm, debug_mode, err_hook, validated_hook, transactions_manager) = get_module_info(module_name)
The returned variables are:
bobo_before, set via a module level variable
__bobo_before__. This is a callable that will be invoked immediately before publication.
bobo_after, set via a module level variable
__bobo_after__. This is a callable that will be invoked immediately after publication.
objectto publish, which defaults to the module itself, but can be set via the module-level variable
realm, set via the module level variable
__bobo_realm__, or a global default which can be set the
debug_mode, a boolean set using the module level variable
err_hook, set via the module level variable
zpublisher_exception_hook. This is used to handle error responses (more below).
validated_hook, set via the module level variable
zpublisher_validated_hook. This is used to initialize a security manager once authentication and authorization have taken place (more below).
transactions_manager, set via the module level variable
zpublisher_transactions_manager, but defaulting to the
DefaultTransactionsManagerwhich uses the
transactionAPI to manage transactions.
The publisher then performs the following steps:
Create a new
processInputs()on the request to process request parameters and the request body so that the Zope request object works as advertised.
If the request contains a key
SUBMITwith the value
canceland a key
cancel_actionwith a path, a
Redirectexception is raised, which will cause an HTTP 302 redirect to be raised.
realmon the response, as returned by
bobo_before()is set, it is called with no arguments.
Set the initial value for
request['PARENTS']to be the published object. This will be the
ZApplicationWrapperset during the startup phase.
Begin a transaction using the
Traverse to the actual object being published (e.g. a view) by calling
object=request.traverse(path, validated_hook=validated_hook), where
request['PATH_INFO']. More on traversal below.
Note the path and authenticated user in the transaction.
Call the object being pusblished using
result=mapply(object, request.args, request, call_object,1, missing_name, dont_publish_class, request, bind=1)
ZPublisher.mapply.mapply()method is somewhat complicated, but in essence all it does is to call either a published method, or a published instance with a
request.argscan contain positional arguments supplied in an XML-RPC call, but is usually empty. The
requestis passed to act as a dictionary of keyword arguments, which allows request parameters to be turned into method parameters to a published method.
The other parameters are about policy — we call any object (e.g. a method or object with a
__call__method) to resolve it, but we don't publish class objects (which would in effect instantiate them). We do allow binding of
selffor methods on objects, and we pass the
requestas context for debugging.
Set the result of the
mapply()call as the response body. As a marker, the response object itself can be returned from the callable that
mapply()invokes to bypass this behavior, i.e. if the published object set the response body itself.
Commit the transaction using the
Return the response object, which is then used by the ZServer to write to stdout.
If an exception happens during this process, the
is called. This is allowed to raise a
exception. Regardless, the event
is notified before the transaction is aborted, and then
is raised after the
interaction is ended.
If the request supports retry, it will be retried by
cloning it and calling
recursively. All HTTP requests support retry, but only up
to a limit of
retry_max_count, which by default is 3. Retry is mainly used to retry in
the case of write-conflict errors.
If there is no error hook installed, a simple abort is encountered, with no retry.
The default error hook is an instance of
Zope2.startup.ZPublisherExceptionHook. This handles exceptions by performing the following
Redirectexceptions are re-raised.
ConflictError, which indicates a write-conflict in the ZODB, is turned into a
Retryexception so that request can be retried.
Other exception are stored in the
__error_log__acquired from the published object, if possible.
If a view named
index.htmlis registered with the exception type as its context, this is resolved and returned as the response.
If the published object or any of its acquisition
parents have a method
raise_standardErrorMessage(), this will be called to create an error message instead of using the view approach. This is called with a first argument of whichever object in the acquisition chain has an attribute
standard_error_message, as well as the request and traceback information.
When handling an exception by returning an error message,
with the exception type (class) as an argument. The
name of the exception class is then used to look
up the status code in the
ZPublisher.HTTPResponse. Hence, raising an exception called
will automatically set the response code to 404.
How does publication traversal work?¶
Traversal is the process during which the path elements of a URL are resolved to an actual object to publish (there is also path traversal, used in TAL expressions in page templates, which is similar, but implemented differently — see below).
Traversal is invoked during object publication, which
with the path from the request (the
CGI variable). This method is inordinately complicated,
mostly because it caters for a lot of edge cases. The
basic idea is pretty simple, though: each path element
represents an item to traverse to, from the preceding
object (its parent). Traversal can mean dictionary-like
__getitem__), attribute-like access (
__getattr__), or one of a number of different hooks for overriding
or extending traversal. Once the final element on the path
is found, the user's access to it is validated, before it
is returned to be passed to
Here are the gory details:
Clean up the path up by stripping leading and trailing slashes, explicitly disallowing access to things like
aq_self, and resolving
..elements as in filesystem paths.
Check if the top-level object (the application root) has a
__bobo_traverse__method (it almost certainly will — as shown above, there is a wrapper around the application root that implements this method to open and close the ZODB connection upon traversal). If so, call it to obtain a new top level object (which will be the real Zope application root in the ZODB).
Aquisition-wrap the top-level object in a
RequestContainer. This is the fake root object that makes it possible to acquire the attribute
REQUESTfrom any traversed-to context.
Record the request variable
ACTUAL_URL, which is the inbound URL plus the original path. Hence, this variable provides access to the URL as the user saw it.
Set up (and later, pop from) the request variable
TraversalRequestNameStack. This is a stack of path elements still to be processed. Traversal hooks sometimes use this to look ahead at the path elements that have not been traversed to and, in some cases, modify the stack to trick traversal into going somewhere other than what the inbound path specified.
In a loop, process the traversal name stack:
Check if the current object (initially the application root) has a method
__before_publishing_traverse__. If so, call it with the request as an argument. This hook is used by many parts of Zope, CMF and Plone to support things like content object method aliases, setting the CMF skin from the request, or making the
portal_factorytool work. This method cannot easily change the traversal path, except by modifying
If there are more elements in the path, pop the next element.
Append this to the variable
request['URL'], which contains the traversal URL. Various traversal tricks may mean this is not quite the same as what the user sees in their address bar, but it should be a valid, traversable URL.
Attempt to traverse to the next object using the name popped from the path stack. This takes place in the
traverseName()method of the request:
If the name starts with a
@, parse it as a traversal namespace. (A name starting with an
@is taken as a shorthand for
++view++<name>, i.e. an entry in the
++view++traversal namespace. Other namespaces include
++etc++.) If a traversal namespace is found, attempt to look up an adapter from the current traversal object and the request to
zope.traversing.interfaces.ITraversablewith a name matching the traversal namespace (e.g.
view). Then call its
traverse()method with the name of the next entry on the traversal stack as an argument. This is expected to return an object to traverse to next. If this succeeds, acquisition-wrap the returned object in the parent object.
Note: As this implies, objects returned from the
traverse()method of an
ITraversableadapter are not expected to be acquisition-wrapped. This is in contrast to objects returned by
__getattr__(), or a custom
IPublishTraverseadapter (see below), which are expected to be wrapped.
If there is no namespace traversal adapter, find an
IPublishTraverseobject in one of three places:
- If the current traversal object implements it directly, use that;
if there is an adapter from the current
object and the request to
IPublishTraverse, use that; or,
fall back to the
DefaultPublishTraverseimplementation found in
Then call the
publishTraverse()method to find an object to traverse to and return that (without acquisition-wrapping it).
IPublishTraverseis a common way to allow further traversal from a view, with paths like
.../@@foo/some/path, where the
@@fooview either implements or is adaptable to
DefaultPublishTraverseis used in most cases, either directly or as a fallback from custom implementations. It works like this:
If the name starts with an underscore, raise a
If the object has a
__bobo_traverse__method, call it with the request and the name of the next entry on the traversal stack as arguments. It may return either an object, or a tuple of objects. In the latter case, amend the request parents list as if traversal had happened over all the elements in the tuple except the last one, and treat that as the next object.
__bobo_traverse__call fails by raising an
NotFoundexception, attempt to look up a view with the traversal name (which would have been given without the explicit
@@prefix). If this succeeds, set the status code to 200 (the preceding failure may have set it to 404), acquisition-wrap the view if applicable, and return it.
If there was no
__bobo_traverse__, or if it raised the special exception
ZPublisher.interfaces.UseTraversalDefault, try the following:
Attempt to look up the name as an attribute
of the current object, using
aq_base(i.e. explicitly not acquiring from parents of the current object). If this succeeds, return the attribute, which is expected to be acquisition-wrapped if applicable (i.e. the parent object extends
- Next, try to look up a view using the same semantics as above
aq_basecheck, i.e. allowing acquired attributes.
If that fails, raise a
KeyErrorto indicate the object could not be found (this is later turned into a 404 response).
- Attempt to look up the name as an attribute of the current object, using
If we now have a sub-object, check that it has a docstring. If it does not, raise a
The requirement for a docstring is an ancient and primitive security restriction, since Zope can be used to publish all kinds of Python objects. It is mostly a nuisance these days, but note that views and custom
IPublishTraversetraversal do not have this restriction.
Next, raise a
Forbiddenexception if traversal resolved a primitive or built-in list, tuple, set or dict — these are not directly traversable.
Finally, return the object.
NotFoundexception is raised during name resolution, return a 404 response by raising an exception. Similarly, if a
Forbiddenexception is raised, set and return a 403 response.
Once the end of the path is reached, we have the most specific item mentioned in the (possibly mutated) path. However, this may choose to delegate to another object (usually a subobject) through a mechanism known as "browser default", which is similar to the way web servers often serve an
index.htmlfile by default when traversing to a folder.
A browser publisher is described by the interface
IBrowserPublisher, which is a sub-interface of
IPublishTraverseand is implemented by the
DefaultPublishTraverseclass. Again, the
IBrowserPublisherfor the traversed-to object is found in one of three ways: * the object may implement it itself; or * it may be adaptable, with the request, to this interface; or * the fallback
DefaultPublishTraversemay be used. The
browserDefault()method on the
IBrowserPublisheris then called with the request as an argument.
The return value from
browserDefault()is a tuple of a parent object (usually the most recently traversed-to object, i.e.
self.contextin the adapter) and a tuple of further names to traverse to from this parent.
The default implementation in
If the object has a method
__browser_default__(), delegate to this.
IDefaultViewNamehas been registered for the context in ZCML, look up and use this. This is deprecated, however.
self.context, (), i.e. no further traversal required.
- If the object has a method
If a further path is returned and it has more than one element, add its elements to the
TraversalRequestNameStackand continue traversal as if these elements had been part of the original path all along.
If there is only one element in the further path returned by
browserDefault(), use this as the next entry name and continue traversal to this.
If no further path is used, fall back on the default method name
index_html()(applicable for HTTP
POSTrequests — there is special handling of other HTTP verbs for WebDAV that we won't go into here) and continue traversal to this.
If there is no
index_html()method, use the traversed-to object itself as the final entry, so break out of the traversal loop. We always end up here eventually: if the browser default element or
index_html()method is the last item we traverse to, eventually we reach something publishable.
This object will most likely be called (through
mapply()), so we ensure the roles used in security checks are obtained from the
__call__()method of the traversed-to object (note: function and method objects also have a
Once we have reached the end of the traversal stack (phew!), we make sure the
parentslist is in the right order (it is built in reverse order), even if there was a failure. Hence,
request['PARENTS']is always a useful indicator of what objects have been traversed over, with the last item being the special request container and the penultimate item being the application root.
We then set
request['PUBLISHED']to be the published callable. Note that this is usually a view or page template, though for content types like
Imageit is the
index_html()method of the content object itself.
Next, we validate that the current user has sufficient permissions to call the published object. If not, a 403 response is returned by calling
The authentication works as follows:
The roles required to access the traversed-to object
are fetched by calling
getRoles(), first on the application root, and, if applicable, on the
__call__()method of the traversed-to object.
A user folder (i.e.
acl_users) is obtained by looking for the special attribute
__allow_groups__on the published object or one of its parents. This attribute is set by user folders on their parent container when they are added.
validate()method of the user folder is called (there is a fallback called
old_validate(), used if there is no user folder, but that should never happen in a modern Zope installation). This either returns a user object or
None, if the user is not found in this user folder, or there is a user, but the user cannot be authorized by this user folder.
Noneis returned, the search continues up the list of traversal parents until a suitable user folder is found. If no such user folder is found, an
Unauthorizedexception is raised, unless there are no security declarations on the context.
If a user with permissions is found, and the
validated_hookis set (found via
get_module_info()as described above), it is called with the request and user as arguments. The standard
newSecurityManager()with the user, which sets the security context for the remainder of the request.
The user is then saved in the request variable
AUTHENTICATED_USER. The true traversal path is saved in the request variable
- The roles required to access the traversed-to object are fetched by calling
Finally, if any post-traverse functions have been registered (by using the
post_traverse()method of the request to register functions and optional static arguments), they are called in the order they were registered. If any post-traverse function returns a value other than
None, no further post-traverse functions are called, and the return value is used as the return value of the
traverse()function, discarding the actual object that was traversed to and security checked.
How does path traversal work?¶
Path traversal is invoked when using path expressions in
page templates or action expressions (e.g.
context/Title). It may be invoked explicitly in code using the methods
(which performs security checks) or
(which does not), defined in
and mixed into most persistent items in Zope. This is
semantically similar to publication (URL) traversal as
described above, but is not identical — see below.
All the logic is in the
method, which takes an optional argument
that is set to
when called via
restrictedTraverse(). It takes a
string or element list as an argument, and optionally a
default to return if traversal fails. If no default is
specified, an exception will be raised if traversal fails.
This may either be an
exception, depending on what type of traversal failed.
will perform a security check using
for every step of traversal. This is different to URL
traversal, which only validates at the end of traversal.
The implementation does the following:
Strip any trailing slash from the
If the path starts with a slash, begin traversal from
the physical application root. Otherwise, start from
self. If performing restricted traversal from the application root, validate access to it.
For each slash-separated name element of the path:
If the name starts with an underscore, raise a
zExceptions.NotFoundexception — traversal to names starting with an underscore is never allowed.
If the name is
.., get the acquisition parent of the current traversal object and continue traversal from here after validating access if applicable.
Otherwise, if the name starts with a
@, perform traversal namespace lookup as described for publication traversal above. If this throws a
LocationError, fail with an
AttributeError. If it succeeds, acquisition-wrap the result if possible and validate access to it if applicable before continuing traversal from this object.
Otherwise, if the object has a
__bobo_traverse__()hook, invoke it to get the next object to traverse to. If this succeeds, validate access to the result if applicable, taking into account that it could be a method or non-security aware object, and that it may or may not be acquisition-wrapped. Then continue traversal from this object.
If there was no
__bobo_traverse__(), or if it returned or raised the sentinel
ZPublisher.interfaces.UseTraversalDefault, attempt to obtain a non-acquired attribute of the current object with the applicable name. If one is found, continue traversal from this. If security checking is being performed, use
AccessControl.ZopeGuardsto get the attribute, which may raise
Unauthorized. (This is the special
getattr()that is also used for all attribute access by untrusted Python code.) Otherwise, use standard
Otherwise, attempt dictionary-like (
__getitem__) access and validate the result if applicable before continuing traversal from this object.
If any of the above failed with an
KeyError, attempt to look up a view on the current traversal object with the given name. If one is found, acquisition-wrap it if possible and validate access if applicable, before continuing traversal from the view instance.
If there is no view, but there was a
__bobo_traverse__, fail by re- raising the original exception. The logic behind this is that if there is a
__bobo_traverse__(), we should not attempt to acquire attributes.
Assuming we still don't have a value and there was
__bobo_traverse__(), attempt to acquire an attribute, using either
guarded_getattr()depending on whether security checks are being made and continue traversal from the result if this succeeds.
- If the name starts with an underscore, raise a
- If we reach the end of the path, return the most recently traversed-to object.
If an exception of any kind (other than a
ConflictError) is thrown and a
defaultwas passed in, return this rather than letting the exception bubble up to the caller.
Note: This logic does not check for the
hooks, although they do allow traversal to a view