Python class method chaining

Question

To avoid getting lost in architectural decisions, I'll ask this with an analogous example:

lets say I wanted a Python class pattern like this:

queue = TaskQueue(broker_conn)
queue.region("DFW").task(fn, "some arg")

The question here is how do I get a design a class such that certain methods can be "chained" in this fashion.

task() would require access to the queue class instance attributes and the operations of task depends on the output of region().

I see SQLalchemy does this (see below) but am having difficulty digging through their code and isolating this pattern.

query = db.query(Task).filter(Task.objectid==10100)

Martijn Pieters · Accepted Answer · 2016-03-27 18:51:29Z

17

SQLAlchemy produces a clone on such calls, see Generative._generate() method which simply returns a clone of the current object.

On each generative method call (such as .filter(), .orderby(), etc.) a new clone is returned, with a specific aspect altered (such as the query tree expanded, etc.).

SQLAlchemy uses a @_generative decorator to mark methods that must operate and return a clone here, swapping out self for the produced clone.

Using this pattern in your own code is quite simple:

from functools import wraps

class GenerativeBase(object):
    def _generate(self):
        s = self.__class__.__new__(self.__class__)
        s.__dict__ = self.__dict__.copy()
        return s

def _generative(func):
    @wraps(func)
    def decorator(self, *args, **kw):
        new_self = self._generate()
        func(new_self, *args, **kw)
        return new_self
    return decorator


class TaskQueue(GenerativeBase):
    @_generative
    def region(self, reg_id):
        self.reg_id = reg_id

    @_generative
    def task(self, callable, *args, **kw):
        self.tasks.append((callable, args, kw))

Each call to .region() or .task() will now produce a clone, which the decorated method updates by altering the state. The clone is then returned, leaving the original instance object unchanged.

edited Mar 27, 2016 at 18:51

answered Feb 14, 2014 at 17:35

Martijn Pieters

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11 Comments

NFicano Over a year ago

Actually I think the solution is what I'm looking for, but having difficulty implementing it: pastebin.com/HpsgK0T5 - I been trying to debug it, but have been unsuccessful, 'TaskQueue' object is not callable

Martijn Pieters Over a year ago

@NFicano: you changed the signature of the decorator; you replaced self with fn.

Martijn Pieters Over a year ago

@NFicano: I see now that I did make a typo in my answer; corrected fn(...) to func(...).

NFicano Over a year ago

AH no I didn't see your edit, I knew fn wasn't right, but couldn't figure out what it was suppose to refer to, thanks again!

Martijn Pieters Over a year ago

@HalcyonAbrahamRamirez: no, it is the actual class object. The same object that type(instance) returns.

|

Community · Accepted Answer · 2017-05-23 10:29:57Z

6

Just return the current object from region method, like this

def region(self, my_string):
    ...
    ...
    return self

Since region returns the current object which has the task function, the chaining is possible now.

Note:

As @chepner mentioned in the comments section, make sure that region makes changes to the object self.

edited May 23, 2017 at 10:29

CommunityBot

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answered Feb 14, 2014 at 17:25

thefourtheye

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5 Comments

chepner Over a year ago

Also, region needs to modify self in some way, since presumably queue.region("DFW").task(fn, "some arg") should be different from queue.task(fn, "some arg"). Or, region() should return not self, but some other TaskQueue object (or, whatever object has a task method).

Martijn Pieters Over a year ago

You'll need to return a clone here; one that is a copy of self but with the changes applied.

thefourtheye Over a year ago

@MartijnPieters Why so? Could you please explain?

Martijn Pieters Over a year ago

@thefourtheye: That's what SQLAlchemy does: return you a new instance object that then allows further filtering. The original query object is not affected.

Halcyon Abraham Ramirez Over a year ago

@thefourtheye how exactly do we modify self?

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