{"id":2890,"date":"2017-10-24T14:43:55","date_gmt":"2017-10-24T12:43:55","guid":{"rendered":"http:\/\/blog.gocept.com\/?p=2890"},"modified":"2018-01-17T10:23:10","modified_gmt":"2018-01-17T09:23:10","slug":"zope4-errorhandling","status":"publish","type":"post","link":"https:\/\/blog.gocept.com\/2017\/10\/24\/zope4-errorhandling\/","title":{"rendered":"Catching and rendering exceptions"},"content":{"rendered":"

TL;DR:<\/strong> You have to write an exception view in file system code which is rendered when an exception occurs.<\/p>\n

History<\/h3>\n

If an exception occurred in Zope 2 the standard_error_message<\/code>\u00a0(an object in the ZODB) was rendered. This way the error page could be customised through the web.<\/p>\n

When using a WSGI server on Zope 2 the standard_error_message<\/code>\u00a0is no longer used. The exceptions have to be handled in a WSGI middleware. (This is a sub-optimal solution as the middleware is not run in the same execution context where the exception occurred.)<\/p>\n

Thats why error handling changed again in Zope 4: \u00a0Like Zope 3 (aka BlueBream)\u00a0Zope 4\u00a0tries to lookup an exception view if an exception occurs. If the lookup succeeds (aka there is an exception view registered for the current exception) this view is rendered as response. This approach allows different views for different exceptions. The\u00a0standard_error_message<\/code> is even gone when installing Zope 4 from scratch.<\/p>\n

<\/p>\n

Base solution<\/h3>\n

The exception view has to be created in the file system and registered via ZCML. If you do not have a file system package yet \u2013 where this code can be placed \u2013 you can create a new package e. g. by using paster<\/code>:<\/p>\n

$ bin\/pip install PasteScript\r\n$ bin\/paster create -t basic_package errorviews\r\n$ bin\/pip install -e errorviews<\/pre>\n
Where errorviews<\/code>\u00a0is the name of my example package.<\/p>\n
In the existing\u00a0errorviews\/__init__.py<\/code>enter the following code:<\/p>\n
class SiteErrorView(object):\r\n    \"\"\"View rendered on SiteError.\"\"\"\r\n\r\n    def __call__(self):\r\n        return \"SiteError!\"<\/pre>\n
This view returns the string SiteError!<\/code>\u00a0instead of the standard error message. It has to be registered via ZCML. Write a file named configure.zcml<\/code>\u00a0right besides __init__.py<\/code>:<\/p>\n
<configure\r\n  xmlns=\"http:\/\/namespaces.zope.org\/zope\"\r\n  xmlns:browser=\"http:\/\/namespaces.zope.org\/browser\">\r\n\r\n  <browser:page\r\n    for=\"Exception\"\r\n    name=\"index.html\"\r\n    class=\".SiteErrorView\"\r\n    permission=\"zope.Public\"\r\n    \/>\r\n\r\n<\/configure><\/pre>\n
The view gets registered for\u00a0Exception<\/code>\u00a0and all classes inheriting from it. (This might be a bit too general for use in actual code, I know.) Zope 4 expects that the name is\u00a0index.html<\/code>. class<\/code>\u00a0 is a relative dotted path to the view class.<\/p>\n
If you put these files into a Zope Product they could be picked automatically. If you created an\u00a0errorviews<\/code>\u00a0package like me you have to register it in etc\/site.zcml<\/code>\u00a0. Put the following line near the end before <\/configure><\/code>:<\/p>\n
\u00a0<include package=\"errorviews\" \/><\/pre>\n
After re-starting Zope each exception renders as SiteError!<\/code><\/p>\n
Using a PageTemplate<\/h3>\n
Writing HTML in a Python class is not very convenient. It is easier to use a PageTemplate to store the templating code. Create a file error.pt<\/code>\u00a0right beside\u00a0__init__.py<\/code>:<\/p>\n
<html>\r\n <body>\r\n   <h1>SiteError occurred<\/h1>\r\n <\/body>\r\n<\/html>\r\n<\/pre>\n
In\u00a0configure.zcml<\/code>\u00a0replace class=\".SiteErrorView\"<\/code> by template=\"error.pt\"<\/code>. (The view class is not used in this example.) After re-starting Zope each exception renders the HTML page.<\/p>\n
Back to standard_error_message<\/code><\/h3>\n
If you have an existing Data.fs<\/code>\u00a0and want to re-use standard_error_message<\/code>\u00a0you might try the following hack: Change\u00a0configure.zcml<\/code>\u00a0(back) to the XML code shown in the section “Base solution”. \u00a0Change __init__.py<\/code>\u00a0to the following content:<\/p>\n
class SiteErrorView(object):\r\n    \"\"\"View rendered on SiteError.\"\"\"\r\n\r\n    def __call__(self):\r\n        root = self.request['PARENTS'][-1]\r\n        return root.standard_error_message(\r\n            error_type=self.context.__class__.__name__,\r\n            error_value=str(self.context))<\/pre>\n
The code in the example expects that\u00a0standard_error_message<\/code>\u00a0 is a DTMLMethod<\/code>\u00a0(as provided by Zope 2.13). The arguments error_type<\/code>\u00a0 and error_value<\/code>\u00a0 allow using\u00a0&dtml-error_type;<\/code> and &dtml-error_value;<\/code>\u00a0in the DTMLMethod<\/code>\u00a0as before.<\/p>\n
Conclusion<\/h3>\n
Zope 4 has a nice and flexible concept to render error pages. But it requires at least some Python code in the file system. Even this can be tricked back to standard_error_message<\/code>\u00a0if needed. But I think this should only be used for old applications or as an interim solution.<\/p>\n","protected":false},"excerpt":{"rendered":"
Error handling in Zope 4<\/p>\n","protected":false},"author":15344399,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_coblocks_attr":"","_coblocks_dimensions":"","_coblocks_responsive_height":"","_coblocks_accordion_ie_support":"","advanced_seo_description":"","jetpack_seo_html_title":"","jetpack_seo_noindex":false,"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_newsletter_tier_id":0,"footnotes":"","jetpack_publicize_message":"","jetpack_is_tweetstorm":false,"jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","enabled":false}}},"categories":[10221],"tags":[196,832,6,72208,594099838],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_likes_enabled":true,"jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/pFP3y-KC","jetpack-related-posts":[{"id":2789,"url":"https:\/\/blog.gocept.com\/2017\/10\/25\/testing-zope-4beta2\/","url_meta":{"origin":2890,"position":0},"title":"Migrating from Zope 2.13 to 4.0b2","author":"Michael Howitz","date":"October 25, 2017","format":false,"excerpt":"Beta-Testing Zope 4 together with PerFact Innovation","rel":"","context":"In "en"","block_context":{"text":"en","link":"https:\/\/blog.gocept.com\/category\/en\/"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":1714,"url":"https:\/\/blog.gocept.com\/2016\/09\/29\/zope-resurrection-part-1-reanimation\/","url_meta":{"origin":2890,"position":1},"title":"Zope Resurrection Part 1 \u2013 Reanimation","author":"Michael Howitz","date":"September 29, 2016","format":false,"excerpt":"Now we\u00a0are helping\u00a0Zope in the Python 3 wonderland:\u00a0Almost 20 people started with the reanimation of Zope. 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