Constructors and Object Initialization¶

Mapping imposes no restrictions or requirements on the constructor (__init__) method for the class. You are free to require any arguments for the function that you wish, assign attributes to the instance that are unknown to the ORM, and generally do anything else you would normally do when writing a constructor for a Python class.

The SQLAlchemy ORM does not call __init__ when recreating objects from database rows. The ORM’s process is somewhat akin to the Python standard library’s pickle module, invoking the low level __new__ method and then quietly restoring attributes directly on the instance rather than calling __init__.

If you need to do some setup on database-loaded instances before they’re ready to use, you can use the @reconstructor decorator to tag a method as the ORM counterpart to __init__. SQLAlchemy will call this method with no arguments every time it loads or reconstructs one of your instances. This is useful for recreating transient properties that are normally assigned in your __init__:

from sqlalchemy import orm

class MyMappedClass(object):
    def __init__(self, data):
        self.data = data
        # we need stuff on all instances, but not in the database.
        self.stuff = []

    @orm.reconstructor
    def init_on_load(self):
        self.stuff = []

When obj = MyMappedClass() is executed, Python calls the __init__ method as normal and the data argument is required. When instances are loaded during a Query operation as in query(MyMappedClass).one(), init_on_load is called.

Any method may be tagged as the reconstructor(), even the __init__ method. SQLAlchemy will call the reconstructor method with no arguments. Scalar (non-collection) database-mapped attributes of the instance will be available for use within the function. Eagerly-loaded collections are generally not yet available and will usually only contain the first element. ORM state changes made to objects at this stage will not be recorded for the next flush() operation, so the activity within a reconstructor should be conservative.

reconstructor() is a shortcut into a larger system of “instance level” events, which can be subscribed to using the event API - see InstanceEvents for the full API description of these events.

sqlalchemy.orm.reconstructor(fn)¶

Decorate a method as the ‘reconstructor’ hook.

Designates a method as the “reconstructor”, an __init__-like method that will be called by the ORM after the instance has been loaded from the database or otherwise reconstituted.

The reconstructor will be invoked with no arguments. Scalar (non-collection) database-mapped attributes of the instance will be available for use within the function. Eagerly-loaded collections are generally not yet available and will usually only contain the first element. ORM state changes made to objects at this stage will not be recorded for the next flush() operation, so the activity within a reconstructor should be conservative.