Execution phases#

Part objects have this life cycle:

  1. Definition

  2. Construction

  3. Bind

  4. Traversal (e.g. render to html, respond to ajax, custom report creation)

At definition time we can have just a bunch of dicts. This is really a stacking and merging of namespaces.

At construction time we take the definition namespaces and materialize them into proper Table, Column, Form etc objects.

At bind time we:

  • register parents

  • evaluate callables into real values

  • invoke any user defined on_bind handlers

At traversal time we are good to go and can now invoke the final methods of all objects. We can now render html, respond to ajax, etc.


“Bind” is when we take an abstract declaration of what we want and convert it into the “bound” concrete expression of that. It consists of these parts:

  1. Copy of the part. (We set a member _declared to point to the original definition if you need to refer to it for debugging purposes.)

  2. Set the parent and set _is_bound to True

  3. Style application

  4. Call the parts on_bind method

The parts are responsible for calling bind(parent=self) on all their children in on_bind.

The root object of the graph is initialized with bind(request=request). Only one object can be the root.

Namespace dispatching#

I’ve already hinted at this above in the example where we do columns__foo__include=False. This is an example of the powerful namespace dispatch mechanism from iommi.declarative. It’s inspired by the query syntax of Django where you use __ to jump namespace. (If you’re not familiar with Django, here’s the gist of it: you can do Table.objects.filter(foreign_key__column='foo') to filter.) We really like this style and have expanded on it. It enables functions to expose the full API of functions it calls while still keeping the code simple. Here’s a contrived example:

from iommi.declarative.dispatch import dispatch
from iommi.declarative.namespace import EMPTY

    b__x=1,  # these are default values. "b" here is implicitly
             # defining a namespace with a member "x" set to 1
def a(foo, b, c):
    print('foo:', foo)

    d=EMPTY,  # explicit namespace
def some_function(x, d):
    print('x:', x)

def another_function(y=None, z=None):
    if y:
        print('y:', y)
    if z:
        print('z:', z)

# now to call a()!
# output:
# foo: q
# x: 1
# y: 2

a('q', b__x=5)
# foo: q
# x: 5
# y: 2

a('q', b__d__z=5)
# foo: q
# x: 1
# z: 5
# y: 2

This is really useful for the Table class as it means we can expose the full feature set of the underling Query and Form classes by just dispatching keyword arguments downstream. It also enables us to bundle commonly used features in what we call “shortcuts”, which are pre-packaged sets of defaults.


To customize iommi you can pass functions/lambdas in many places. This makes it super easy and fast to customize things, but how does this all work? Let’s start with a concrete example:

    columns__name__cell__format=lambda value, **_: f'{value} !!!',

This will change the rendering of Dios name from Dio to Dio !!!. The obvious question here is: what other keyword arguments besides value do I get? In this case you get:

request        WSGIRequest
table          Table
column         Column
traversable    Column
value          str
row            Artist

The general idea here that you should get all useful objects up the tree and as they are named it becomes easy to understand what is happening when reading these functions. If you have an iommi object you can call the method iommi_evaluate_parameters() on it to retrieve this dict.

traversable is exactly the same object as column. It’s the general name of the closest object (or the leaf) for that callback. You can think of it as similar to self. This is useful for creating functions that you can use for Field, Column, and Filter; as the keyword argument traversable is the same, but they will get field, column, and filter as the specific keyword arguments. Prefer the specific name if possible since it makes the code more readable.


It is a good idea to always give your callbacks **_ even if you match all keyword arguments. We don’t consider adding keyword arguments a breaking change so additional keyword arguments can be added at any time.

Evaluate - under the hood#

There are three functions that handle the evaluation of callables into values when needed. All of these pass values straight through, which is why you can write e.g. display_name='Artist' instead of having to write lambdas for simple values.

  • evaluate: evaluates non-strict, which means it will allow functions that don’t match the given signature to pass through

  • evaluate_strict: evaluates strictly, which means functions that don’t match the given signature will be an error

  • evaluate_strict_container: used for evaluating all the values of a dict

Each object in the tree declares what it adds to the evaluate parameters with a method own_evaluate_parameters. For example Table adds just one argument table which is itself. The method iommi_evaluate_parameters gives you all the evaluate parameters up the tree from where you are.

There are two special cases: traversable which is the leaf node, and request which is the http request object.