6. Interview Questions- Events and Internals

Source: https://docs.sqlalchemy.org/en/20/orm/extending.html

1. What is the purpose of extending the SQLAlchemy ORM?

Answer: To customize behavior such as object instantiation, attribute access, change tracking, or mapping configuration beyond default behavior.

2. What are ORM events in SQLAlchemy?

Answer: Hooks that let you intercept and respond to changes in ORM objects or sessions—e.g., object load, insert, update, delete, etc.

3. How do you register an event for when a class is loaded from the database?

Answer:

from sqlalchemy import event

@event.listens_for(MyClass, "load")
def receive_load(instance, context):
    print(f"{instance} was loaded")

4. What does the `@event.listens_for(..., 'init')` decorator do?

Answer: It intercepts when a new instance of a class is created (before it's added to a session or persisted).

5. What’s the difference between `load` and `refresh` events?

Answer:

load: Triggered when the object is loaded from the DB the first time.
refresh: Triggered when an existing instance is reloaded from the DB (e.g., via refresh()).

6. How can you track changes to a particular attribute?

Answer:

@event.listens_for(MyClass.attr_name, "set")
def attr_set_listener(target, value, oldvalue, initiator):
    print(f"Changed from {oldvalue} to {value}")

7. What event is used to intercept before insert or update in ORM?

Answer:

@event.listens_for(MyClass, "before_insert")
@event.listens_for(MyClass, "before_update")

8. Can you intercept attribute-level access (e.g., `get`, `set`) using ORM tools?

Answer: Yes, using attribute instrumentation events like "set" and "init_scalar".

9. What does the `"append"` event on a relationship do?

Answer: It triggers when a new item is added to a collection-based relationship.

10. How can you perform actions when an object is added to a relationship?

Answer:

@event.listens_for(MyClass.related_items, "append")
def on_append(target, value, initiator):
    print(f"{value} added to {target}")

11. What is the use of `init()` vs. `@event.listens_for(..., 'init')`?

Answer: __init__() is Python-native object creation. The init event allows SQLAlchemy to instrument or wrap creation dynamically.

12. How do you extend a mapped class with mixins in SQLAlchemy?

Answer: Define shared logic (columns, methods, relationships) in a base class without __tablename__, and inherit it in ORM classes.

13. What is the difference between a mixin and an abstract base in SQLAlchemy ORM?

Answer:

Mixin: Reusable logic; not mapped by itself.
Abstract base: May be mapped, but marked with __abstract__ = True.

14. Can attribute-level events be registered after class definition?

Answer: Yes, via the event.listen() API or using @event.listens_for() decorator.

15. How do you track when a relationship collection is cleared?

Answer:

@event.listens_for(MyClass.related_items, "remove")
def on_remove(target, value, initiator):
    print(f"{value} removed from {target}")

16. How do you prevent a certain value from being assigned to a field?

Answer: Use the "set" attribute event and raise an exception based on the value.

17. What is a class manager in SQLAlchemy ORM internals?

Answer: It’s an internal object that manages instrumentation, lifecycle events, and tracking for ORM-mapped classes.

18. How does SQLAlchemy know which attributes are mapped and instrumented?

Answer: Through the class’s mapper and the instrumentation system via DeclarativeBase or registry().

19. Can you define global listeners that affect all ORM classes?

Answer: Yes, by targeting the base Mapper or InstanceEvents on all mapped classes.

20. What’s the difference between instance-level and class-level event listeners?

Answer:

Instance-level: React to changes on specific instances (e.g., attribute set).
Class-level: Affect all instances of the class during lifecycle phases like insert, load, etc.

21. How do you intercept an object being marked as deleted?

Answer:

@event.listens_for(MyClass, "after_delete")
def receive_after_delete(mapper, connection, target):
    print(f"Deleted: {target}")

22. What event can you use to automatically populate a timestamp on insert?

Answer:

@event.listens_for(MyClass, "before_insert")
def set_created_at(mapper, connection, target):
    target.created_at = datetime.utcnow()

23. How do you instrument a class without using declarative?

Answer: You can use registry().map_imperatively() and still attach class- or attribute-level listeners.

24. What’s the use of `@event.listens_for(Mapper, 'after_configured')`?

Answer: It lets you execute logic after all ORM mappings have been configured (e.g., apply validations or constraints globally).

25. What does the `"append"` event emit in terms of arguments?

Answer: It passes the target (the parent), value (the new child), and initiator (instrumentation metadata).

26. Can you listen to events on unmapped classes?

Answer: No, ORM-specific events only apply to mapped classes or instrumented attributes.

27. What’s the difference between `"set"` and `"modified"` events on an attribute?

Answer:

"set": Triggers when a value is assigned.
"modified": Triggers when a value changes meaningfully (often used for dirty tracking).

28. How do you dynamically add a listener at runtime?

Answer:

from sqlalchemy import event
event.listen(SomeClass, "load", some_handler)

29. What is the `init_scalar` event used for?

Answer: It fires when a new instance is constructed and scalar attributes are initialized (typically None or default values).

30. How can you track whether a particular column was changed during a session?

Answer: Use attribute-level "set" event or use sqlalchemy.inspect(obj).attrs.column_name.history.

31. How do you mark a mixin as declarative-safe?

Answer: Use the @declarative_mixin decorator:

from sqlalchemy.orm import declarative_mixin

@declarative_mixin
class TimestampMixin:
    created_at = mapped_column(...)

32. What does `@declarative_mixin` offer over a plain class?

Answer: It signals SQLAlchemy that the class is intended as a mixin for mapping and avoids accidental mapping of the mixin itself.

33. Can `@event.listens_for` be used on hybrid properties?

Answer: Yes, but it should be attached to the underlying attribute, not the property method.

34. How do you register an event on all mappers globally?

Answer:

@event.listens_for(Mapper, "before_insert")
def global_before_insert(mapper, connection, target):
    ...

35. What is the initiator argument in attribute events?

Answer: It carries metadata about the operation, including the key name, parent object, and whether the event is user-triggered.

36. Can listeners be removed once added?

Answer: Yes, using event.remove() with the same signature used to register.

37. What’s the purpose of `dispatch` on ORM classes or attributes?

Answer: It exposes the event registry for that target, allowing inspection or programmatic listener attachment.

38. How can you use ORM events to implement soft deletes?

Answer: Intercept the before_delete event, cancel the actual delete, and instead set a deleted flag:

@event.listens_for(MyClass, "before_delete")
def soft_delete(mapper, connection, target):
    raise orm_exc.ObjectDeletedError()  # Or set flag and skip deletion

39. How can you override attribute access behavior in an ORM-mapped class?

Answer: Use descriptors or attribute-level instrumentation with events like "set" and "get".

40. What is the role of the `MapperEvents` class in SQLAlchemy?

Answer: It defines the set of ORM-level event names like before_insert, after_update, before_flush, etc., for mapped classes.

41. What is the purpose of `__declare_last__()` in a mapped class?

Answer: It is a class method that SQLAlchemy calls after all mappings are configured. It’s often used to register class-specific events or constraints.

class MyModel(Base):
    @classmethod
    def __declare_last__(cls):
        event.listen(cls, "before_insert", custom_handler)

42. Can you override default behavior of an attribute (e.g., validation) using events?

Answer: Yes. Use the "set" event to intercept assignment and apply validation logic.

@event.listens_for(MyModel.my_column, "set")
def validate_column(instance, value, oldvalue, initiator):
    if value < 0:
        raise ValueError("Negative values not allowed")

43. What’s the role of `MapperExtension` in SQLAlchemy?

Answer: MapperExtension was a legacy system used to hook into ORM behavior. It is now replaced by the modern event system (e.g., before_insert, after_update).

44. Can `@event.listens_for` be applied to both attributes and relationships?

Answer: Yes. It can listen to scalar fields ("set", "modified") and collection-based relationships ("append", "remove", "bulk_replace").

45. What event should you use to modify values coming from the database during object load?

Answer: The "load" event on the mapped class:

@event.listens_for(MyModel, "load")
def customize_load(instance, context):
    instance.custom_attr = "transformed"

46. How do you make a declarative mixin class that contributes indexes or constraints?

Answer: Use __table_args__ in the mixin:

@declarative_mixin
class IndexMixin:
    __table_args__ = (Index("ix_name", "name"), )

47. How can you use events to inject behavior across multiple ORM classes?

Answer: Attach listeners to the base Mapper class or apply the same mixin with __declare_last__() in child classes.

48. What’s the purpose of the `bulk_replace` event on relationships?

Answer: It fires when a full list of items is replaced on a collection-based relationship, allowing inspection or intervention.

49. Can you attach listeners to hybrid or computed properties?

Answer: Direct event listening is not available on @hybrid_property, but you can hook into the underlying columns or override the property logic.

50. How does SQLAlchemy handle multiple mixins with overlapping attributes?

Answer: The last mixin in the MRO (method resolution order) overrides earlier ones. Conflicts should be resolved manually to avoid ambiguous behavior.

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Last updated 8 months ago

hashtag1. What is the purpose of extending the SQLAlchemy ORM?

hashtag2. What are ORM events in SQLAlchemy?

hashtag3. How do you register an event for when a class is loaded from the database?

hashtag4. What does the @event.listens_for(..., 'init') decorator do?

hashtag5. What’s the difference between load and refresh events?

hashtag6. How can you track changes to a particular attribute?

hashtag7. What event is used to intercept before insert or update in ORM?

hashtag8. Can you intercept attribute-level access (e.g., __get__, __set__) using ORM tools?

hashtag9. What does the "append" event on a relationship do?

hashtag10. How can you perform actions when an object is added to a relationship?

hashtag11. What is the use of __init__() vs. @event.listens_for(..., 'init')?

hashtag12. How do you extend a mapped class with mixins in SQLAlchemy?

hashtag13. What is the difference between a mixin and an abstract base in SQLAlchemy ORM?

hashtag14. Can attribute-level events be registered after class definition?

hashtag15. How do you track when a relationship collection is cleared?

hashtag16. How do you prevent a certain value from being assigned to a field?

hashtag17. What is a class manager in SQLAlchemy ORM internals?

hashtag18. How does SQLAlchemy know which attributes are mapped and instrumented?

hashtag19. Can you define global listeners that affect all ORM classes?

hashtag20. What’s the difference between instance-level and class-level event listeners?

hashtag21. How do you intercept an object being marked as deleted?

hashtag22. What event can you use to automatically populate a timestamp on insert?

hashtag23. How do you instrument a class without using declarative?

hashtag24. What’s the use of @event.listens_for(Mapper, 'after_configured')?

hashtag25. What does the "append" event emit in terms of arguments?

hashtag26. Can you listen to events on unmapped classes?

hashtag27. What’s the difference between "set" and "modified" events on an attribute?

hashtag28. How do you dynamically add a listener at runtime?

hashtag29. What is the init_scalar event used for?

hashtag30. How can you track whether a particular column was changed during a session?

hashtag31. How do you mark a mixin as declarative-safe?

hashtag32. What does @declarative_mixin offer over a plain class?

hashtag33. Can @event.listens_for be used on hybrid properties?

hashtag34. How do you register an event on all mappers globally?

hashtag35. What is the initiator argument in attribute events?

hashtag36. Can listeners be removed once added?

hashtag37. What’s the purpose of dispatch on ORM classes or attributes?

hashtag38. How can you use ORM events to implement soft deletes?

hashtag39. How can you override attribute access behavior in an ORM-mapped class?

hashtag40. What is the role of the MapperEvents class in SQLAlchemy?

hashtag41. What is the purpose of __declare_last__() in a mapped class?

hashtag42. Can you override default behavior of an attribute (e.g., validation) using events?

hashtag43. What’s the role of MapperExtension in SQLAlchemy?

hashtag44. Can @event.listens_for be applied to both attributes and relationships?

hashtag45. What event should you use to modify values coming from the database during object load?

hashtag46. How do you make a declarative mixin class that contributes indexes or constraints?

hashtag47. How can you use events to inject behavior across multiple ORM classes?

hashtag48. What’s the purpose of the bulk_replace event on relationships?

hashtag49. Can you attach listeners to hybrid or computed properties?

hashtag50. How does SQLAlchemy handle multiple mixins with overlapping attributes?

1. What is the purpose of extending the SQLAlchemy ORM?

2. What are ORM events in SQLAlchemy?

3. How do you register an event for when a class is loaded from the database?

4. What does the `@event.listens_for(..., 'init')` decorator do?

5. What’s the difference between `load` and `refresh` events?

6. How can you track changes to a particular attribute?

7. What event is used to intercept before insert or update in ORM?

8. Can you intercept attribute-level access (e.g., `get`, `set`) using ORM tools?

9. What does the `"append"` event on a relationship do?

10. How can you perform actions when an object is added to a relationship?

11. What is the use of `init()` vs. `@event.listens_for(..., 'init')`?

12. How do you extend a mapped class with mixins in SQLAlchemy?

13. What is the difference between a mixin and an abstract base in SQLAlchemy ORM?

14. Can attribute-level events be registered after class definition?

15. How do you track when a relationship collection is cleared?

16. How do you prevent a certain value from being assigned to a field?

17. What is a class manager in SQLAlchemy ORM internals?

18. How does SQLAlchemy know which attributes are mapped and instrumented?

19. Can you define global listeners that affect all ORM classes?

20. What’s the difference between instance-level and class-level event listeners?

21. How do you intercept an object being marked as deleted?

22. What event can you use to automatically populate a timestamp on insert?

23. How do you instrument a class without using declarative?

24. What’s the use of `@event.listens_for(Mapper, 'after_configured')`?

25. What does the `"append"` event emit in terms of arguments?

26. Can you listen to events on unmapped classes?

27. What’s the difference between `"set"` and `"modified"` events on an attribute?

28. How do you dynamically add a listener at runtime?

29. What is the `init_scalar` event used for?

30. How can you track whether a particular column was changed during a session?

31. How do you mark a mixin as declarative-safe?

32. What does `@declarative_mixin` offer over a plain class?

33. Can `@event.listens_for` be used on hybrid properties?

34. How do you register an event on all mappers globally?

35. What is the initiator argument in attribute events?

36. Can listeners be removed once added?

37. What’s the purpose of `dispatch` on ORM classes or attributes?

38. How can you use ORM events to implement soft deletes?

39. How can you override attribute access behavior in an ORM-mapped class?

40. What is the role of the `MapperEvents` class in SQLAlchemy?

41. What is the purpose of `__declare_last__()` in a mapped class?

42. Can you override default behavior of an attribute (e.g., validation) using events?

43. What’s the role of `MapperExtension` in SQLAlchemy?

44. Can `@event.listens_for` be applied to both attributes and relationships?

45. What event should you use to modify values coming from the database during object load?

46. How do you make a declarative mixin class that contributes indexes or constraints?

47. How can you use events to inject behavior across multiple ORM classes?

48. What’s the purpose of the `bulk_replace` event on relationships?

49. Can you attach listeners to hybrid or computed properties?

50. How does SQLAlchemy handle multiple mixins with overlapping attributes?