Diagnosing errors in Pylons 1.0 to Pyramid 1.0a1+ Transition

My first attempt at migrating a Pylons project to Pyramid was accomplished without too much difficulty. That project was relatively small, however, this project hasn’t been put into production.

A brief history of this application:

We have a legacy PHP application which consists of 78k lines of code. Originally we started rewriting the application in Turbogears 2.0 and later moved over to Pylons with ToscaWidgets. We used ToscaWidgets because all of our forms had been written for TG2. The application never made it into production, but, was substantial enough that we felt it would be a good test to convert it to Pyramid since we’re focusing our development efforts on Pyramid.

Initially I started to write a script to do the migration from Pylons to Pyramid and from ToscaWidgets to Deform but abandoned that. Changing the controllers from Pylons to Pyramid was fairly easy.

Most of the pylons imports were commented out and replaced with:

import deform
import colander

from pyramid.response import Response
from pyramid.view import action
from pyramid.security import authenticated_userid
from pyramid.threadlocal import get_current_request
import webhelpers.paginate as paginate

References to tmpl_context. were altered, our __before__ action for authkit was removed and replaced with an __init__ since we were using handlers. Routes were modified and given unique names, templates were modified to remove references to ${h. and ${tmpl_context., and we began the process of stepping through the application. Initially I had written a script to convert the ToscaWidgets form models over to Deform schemas, but, after two hours, it became obvious it would take more time to do that than to manually recreate the forms. After working through much of the process, I’m debating whether this project should have been migrated over to FormAlchemy as almost every form is a duplicate of the SQL schema. Since it isn’t in production, we still have some time to make that decision.

What follows is a summary of the errors received and what caused the errors.

TypeError: ‘NoneType’ object is not iterable

I ran into this while importing some routes from an existing project. Since routes must be uniquely named in Pyramid, a route with the same name will replace a prior route. Some of the route names have gotten quite unwieldy.

TypeError: object.__new__() takes no parameters

Solution, add the __init__ block to your class in your handler.

class YourClass(object):
    def __init__(self, request):
        self.request = request

ValueError: Non-response object returned from view named (and no renderer): {‘template’: ‘billing_index’}

Turbogears 2.0 code:

    def index(self, **kw):
        return dict(template='billing_index')

modify to:

    @action(renderer='billing_index.mako', permission='client')
    def index(self, **kw):
        return {}

AttributeError: ‘Undefined’ object has no attribute ‘form’

Pyramid doesn’t pass the tmpl_context., c. or h. through to the template, so, the return values of each of the actions needs to pass the fields required. As a result, any template code that replies on these globals needs to be modified, and the corresponding action needs to return the values in the dictionary. You can use one of the pylons templates which will instantiate a subscriber method to replicate the Pylons globals if you want.

AttributeError: ‘Undefined’ object has no attribute ‘literal’

Existing forms are relying on the pylons h. global.


UnboundLocalError: local variable ‘clients’ referenced before assignment

This occurs when you remove tmpl_context. and are left with a variable assigned that matches the class. For example:

tmpl_context.clients = meta.Session.query(clients).filter_by(client_id==1).all()

When tmpl_context. is removed, the class clients is cast incorrectly.

AttributeError: ‘Undefined’ object has no attribute ‘pager’

Paginate itemset isn’t being passed to the template.

TypeError: ‘NoneType’ object is not iterable

Paginate is getting the result set, rather than the paginate set passed in the return dictionary.

NameError: global name ‘request’ is not defined

request.matchdict/request.params -> self.request.matchdict/self.request.params

In a handler, request. is referred to as self.request. If you’ve converted things over to a handler rather than writing the individual routes for each action, you’ll need to preface any request. with self.

NotImplementedError: no URL generator available

Webhelpers pagination doesn’t know how to generate URLs in Pyramid, but, you can use a callable to generate the URLs. This requires access to pyramid.threadlocal which is generally not recommended, but, does allow you to use paginate until a pyramid compatible paginate is written.

The generator you need looks like this:

from webhelpers.util import update_params
from pyramid.threadlocal import get_current_request

def get_page_url(**kw):
    return update_params(get_current_request().path_info, **kw)

In your paginate block, you need to add the following:

        paginator = paginate.Page(
            page=int(self.request.params.get('page', 1)),
            items_per_page = 40,

RuntimeError: Caught exception rendering template. TypeError: ‘int’ object is not iterable

deform select widget requires tuple for the dropdown creation. Toscawidgets would build the right hand side if it was passed a list of IDs.

TypeError: ‘Undefined’ object is unsubscriptable

Returning a dict, removing tmpl_context., a ${value[‘asdf’]} is missing ‘value’:value being passed in the return dict.

TypeError: sequence item 10: expected string or Unicode, Undefined found

When using deform, return {‘form’:form.render()} rather than return {‘form’:form}.


This is the second application I’ve converted from Pylons 1.0 to Pyramid and most of the issues have been syntax issues. Hopefully the error summary above will save someone some time.

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