How to Overload Python Assignment

Is it possible to override the assignment ('=') operator in Python?

It is not possible to override the implementation of x = y. See Facts and Myths about Python Names and Values for details of what assignment means.

You can override x.a = y, with __setattr__, it is (roughly) x.__setattr__('a', y).

You can override x[k] = y with __setitem__, it is (roughly) x.__setitem__(k, y).

But you can't override x = y.

How to Emulate Assignment Operator Overloading in Python?

I ended up creating a Model metaclass called ModelMeta that registers the typed attributes.

See http://github.com/espeed/bulbs/blob/master/bulbs/model.py

In this case, the typed attributes are graph-database "properties", which are all subclasses of the Property class.

See https://github.com/espeed/bulbs/blob/master/bulbs/property.py

Here's an example Model declaration:

# people.py

from bulbs.model import Node, Relationship
from bulbs.property import String, Integer, DateTime
from bulbs.utils import current_datetime

class Person(Node):

    element_type = "person"

    name = String(nullable=False)
    age = Integer()

class Knows(Relationship):

    label = "knows"

    created = DateTime(default=current_datetime, nullable=False)

Example usage:

>>> from people import Person
>>> from bulbs.neo4jserver import Graph
>>> g = Graph()

# Add a "people" proxy to the Graph object for the Person model:
>>> g.add_proxy("people", Person)

# Use it to create a Person node, which also saves it in the database:
>>> james = g.people.create(name="James")
>>> james.eid
3
>>> james.name
'James'

# Get the node (again) from the database by its element ID:
>>> james = g.people.get(james.eid)

# Update the node and save it in the database:
>>> james.age = 34
>>> james.save()

# Lookup people using the Person model's primary index:
>>> nodes = g.people.index.lookup(name="James")

See...

• Bulbs Model API: http://bulbflow.com/docs/api/bulbs/model/
• Bulbs Model Quickstart: http://bulbflow.com/quickstart/#models

Assignment operator overloading in python Abstract Syntax Trees

You do not need to use astunparse, the ast module includes an unparse method:

import ast
class AssignOverload(ast.NodeTransformer):
   def visit_Assign(self, node):
      return ast.Call(func=ast.Name(id='copy_variable_value'), 
         args=[ast.Constant(value=ast.unparse(i)) for i in [*node.targets, node.value]], 
         keywords=[])

code_chunk = "town_slot=cxt.my_town"
a = AssignOverload()
result = a.visit(ast.parse(code_chunk))
print(ast.unparse(result))

Output:

copy_variable_value('town_slot', 'cxt.my_town')

overloading augmented arithmetic assignments in python

You need to add return self to your method. Explanation:

The semantics of a += b, when type(a) has a special method __iadd__, are defined to be:

  a = a.__iadd__(b)

so if __iadd__ returns something different than self, that's what will be bound to name a after the operation. By missing a return statement, the method you posted is equivalent to one with return None.

Python: overloading tuple multi-assignment capabilities?

Yes. Implement __iter__().

class unpackable_dict(dict):
    def __iter__(self):
        return (self[key] for key in sorted(self.keys()))

d = unpackable_dict(a=1, b=2)
a, b = d

The reason you normally can't unpack values from a dict like you can a tuple is that dicts don't have a defined order. I've used a generator expression to get the items out in the sorted order of their keys. You might consider using an OrderedDict instead, though.

How can I overload the assignment of a class member?

Can't you use properties and override setter for the field?

Citing from django documentation:

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)

    def _get_full_name(self):
        return "%s %s" % (self.first_name, self.last_name)

    def _set_full_name(self, combined_name):
        self.first_name, self.last_name = combined_name.split(' ', 1)

    full_name = property(_get_full_name)

    full_name_2 = property(_get_full_name, _set_full_name)

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