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Nion utility classes.

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Nion Utilities

The Nion Utils library (used in Nion Swift)

Nion utility classes.


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A utility library of useful Python objects.

  • Events

  • Observable

  • Bindings, Converters, Models

  • Geometry

  • Persistence

  • Process, Threads

  • Publish and Subscribe

  • Reference Counting

  • Stream

  • Structured Model

These objects are primarily used within the Nion Python libraries, but may be useful in general usage too.

This project is funded by Nion Co. as part of the Nion Swift imaging and analysis platform. The code is available under the Apache License, Version 2.0.


Requires Python 3.9 or later.

Getting Help and Contributing

If you find a bug, please file an issue on GitHub. You can also contact us directly at

This is primarily a library focused on providing support to higher level Nion Python libraries. If you are using this in your own project, we will accept bug fixes and minor feature improvements via pull requests. For larger features or additions, please contact us to discuss.

This library includes some direct tests, but is also tested via other Nion Python projects. Any contribution will need to pass the entire suite of tests. New contributions should be submitted with new tests.

Summary of Features


Events can be used by objects to notify other objects when something of interest occurs. The source object “fires” the event, optionally passing parameters, and the “listener” receives a function call. The source object determines when to fire an event. The event can have multiple listeners. The listeners are called synchronously in the order in which they are added, and the source can fire unconditionally, or until a listener returns True or False.

from nion.utils import Event

class Telescope:
  def __init__(self):
    self.new_planet_detected_event = Event.Event()

  def on_external_event(self, coordinates):

def handle_new_planet(coordinates):
  print("New planet coordinates at " + str(coordinates))

telescope = Telescope()
listener = telescope.new_planet_detected_event.listen(handle_new_planet)

listener.close()  # when finished


The Observable based class defines five basic events for watching for direct changes to an object such as a property changing, an object being set or cleared, or an item being inserted or removed from a list. The observable is used along with events to implement bindings.

from nion.utils import Observable

class MyClass(Observable.Observable):
  def __init__(self):
    self.__weight = 1.0

  def weight(self):
    return self.__weight

  def weight(self, new_weight):
    self.__weight = new_weight

myc = MyClass()

def property_changed(key):
  if key == "weight":
    print("The weight changed " + str(myc.weight))

listener = myc.property_changed_event.listen(property_changed)

listener.close()  # when finished

Bindings, Converters, Models

Bindings connect a value in a source object to a value in a target object. Bindings can be one way bindings from source to target, or two way bindings from source to target and from target to source. Bindings can bind property values, lists, or an item in a tuple between the source and target. Bindings work using the Observable events and subsequently are implemented via Events.

Bindings can optionally make value conversions between the source and target. For instance, a binding between a float property and a user interface text field will need to convert from float to string and back. Converters between value and strings are included for integer, float, percentage, check state, and UUID to strings.


Classes for integer and float based points, sizes, and rectangles are included. Additional geometry routines are also included, for instance: line midpoint.


The PersistentObject based class defines a basic structure for storing objects and their relationship to each other into a persistent storage context. PersistentObjects can store basic properties, single objects (to-one relationship) and lists of objects (to-many relationship). Subclasses must strictly notify the PersistentObject of changes to their persistent data and follow certain guidelines. Doing so allows the object to be stored persistently and restored from persistent storage.

Properties in the PersistentObject can have validators, converters, change notifications, and more. Items and relationships have change notifications and more.

The PersistentStorageContext defines an interfaces which manages a collection of PersistentObjects. It must be able to store a simple dict structure for properties, items, and lists.

Process, Threads

Process defines classes to facilitate a threaded queue, which executes its items serially, and thread set which executes the most recent item in the set.

ThreadPool defines a threaded dispatcher with the ability to limit dispatch frequency and a thread pool with the ability to execute explicitly without threads for testing.

Publish and Subscribe

Publish and subscribe implements a basic publish and subscribe mechanism. It is should be considered experimental and is not recommended for use.

Reference Counting

The ReferenceCounted base class provides an explicitly reference counted object that is unique from regular Python reference counting in that it provides precise control of when the reference is acquired and released. The about_to_delete method is called when reference count reaches zero.


The Stream classes provide a async-based stream of values that can be controlled using standard reactive operators such as sample, debounce, and combine. The stream source is an Event named value_stream and the source object must provide both the value_stream and a value property.

Structured Model

The Structured Model classes provide a way to describe a data structure which can produce a modifiable and observable object to be used as a model for other utility classes such as binding and events.

Changelog (nionutils)

0.4.8 (2023-08-10)

  • Add support for custom ValueStream compare operator.

0.4.7 (2023-06-19)

  • Add explicit support for Python 3.11; drop support for Python 3.8.

  • Add an auditing capability for performance auditing.

  • Add ability to add/remove streams from CombineLatestStream.

  • Add ability to display datetime in local time and with format in DateTime converter.

  • Add ValuesToIndexConverter.

0.4.6 (2022-11-04)

  • Minor maintenance.

0.4.5 (2022-09-13)

  • Add minor Color method to get RGB values.

  • Add DateTime utilities (utcnow for Windows, strictly increasing).

0.4.4 (2022-07-25)

  • Add a Color utility class.

  • Fix issues with FloatToScaledIntegerConverter when min/max are invalid.

  • Fix possible race condition in thread pool.

0.4.3 (2022-05-28)

  • Improve convert_back method of IntegerToStringConverter for fuzzy conversion.

0.4.2 (2022-02-18)

  • Ensure component prioritization works.

  • Fix top level namespace.

0.4.1 (2021-12-13)

  • Minor typing and return type issues.

0.4.0 (2021-11-10)

  • Eliminate need to call close methods on models, streams, listeners, etc.

  • Drop support for Python 3.7, add support for Python 3.10.

  • Enable strict typing.

  • Remove unused ConcatStream.

  • Make ReferenceCounted work with Python references, not explicit ref count.

  • Add useful Geometry functions: empty_rect, int_rect, as_tuple.

  • Remove unused/deprecated PersistentObject.

  • Add property changed property model for monitoring observables.

  • Change Recorder to allow customer logger.

  • Change ThreadPool to be no-close.

  • Fix issue in single SingleItemDispatcher so it actually delays.

  • Change SingleItemDispatcher to be no-close.

  • Extend sync processes function to cancel outstanding async functions.

  • Add experimental stream, value change, and reactor functions.

  • Add method to sync but not close event loop.

  • Add a single item dispatcher to thread pool.

  • Deprecate ListBinding class.

0.3.26 (2021-03-12)

  • Add select forward/backward methods to IndexedSelection.

0.3.25 (2021-02-02)

  • Add None and fuzzy options to int converter.

0.3.24 (2020-12-07)

  • Fix issue updating selection on master instead of filtered items.

  • Add ListPropertyModel to treat a list like a single property.

0.3.23 (2020-11-06)

  • Change list model to more efficiently send change events.

  • Change selection to (optionally) fire changed messages when adjusting indexes.

0.3.22 (2020-10-06)

  • Fix property binding inconsistency.

0.3.21 (2020-08-31)

  • Fix issue with stream calling stale function.

  • Filtered lists no longer access their container’s master items when closing.

  • Add rotate methods to FloatPoint and FloatSize.

  • Improve LogTicker. Add support for major and minor ticks.

  • Fix case of extending selection with no anchor.

  • Add separate LogTicker class. Renamed old Ticker to LinearTicker. Add base Ticker class.

  • Add date time to string converter.

  • Extend PropertyChangedEventStream to optionally take an input stream rather than direct object.

  • Add added/discarded notifications to Observable for set-like behavior.

  • Add a pathlib.Path converter.

  • Improve performance of filtered list models.

  • Add Registry function to send registry events to existing components. Useful for initialization.

  • Add geometry rectangle functions for intersect and union.

  • Add geometry functions to convert from int to float versions.

0.3.20 (2020-01-28)

  • Add various geometry functions; facilitate geometry objects conversions to tuples.

  • Add Process.close_event_loop for standardized way of closing event loops.

  • Improve geometry comparisons so handle other being None.

0.3.19 (2019-06-27)

  • Add method to clear TaskQueue.

  • Make event listeners context manager aware.

  • Improve stack traceback during events (fire, listen, handler).

  • Add auto-close (based on weak refs) and tracing (debugging) to Event objects.

0.3.18 (2019-03-11)

  • Ensure FuncStreamValueModel handles threading properly.

0.3.17 (2019-02-27)

  • Add ConcatStream and PropertyChangedEventStream.

  • Add standardized [notify] item_content_changed event to Observable.

  • Make item_changed_event optional for items within FilteredListModel.

  • Add floordiv operator to IntSize.

0.3.16 (2018-12-11)

  • Change list model text filter to use straight text rather than regular expressions.

0.3.15 (2018-11-13)

  • Allow recorder object to be closed.

  • Improve release of objects when closing MappedListModel.

  • Add close method to ListModel for consistency.

  • Allow persistent objects to delay writes and handle external data.

  • Allow persistent relationships to define storage key.

  • Extend Registry to allow registering same component with additional component types.

0.3.14 (2018-09-13)

  • Allow default values in persistent factory callback.

0.3.13 (2018-09-11)

  • Allow persistent items to be hidden (like properties).

  • Allow persistent interface to use get_properties instead of properties attribute when saving.

  • Allow FilteredListModel to have separate master/item property names.

0.3.12 (2018-07-23)

  • Fix bug where unregistered objects were not reported correctly.

  • Add model changed event to structured model to monitor deep changes.

0.3.11 (2018-06-25)

  • Improve str conversion in Geometry classes (include x/y).

  • Add a get_component method to Registry for easier lookup.

  • Treat ‘.’ in float numbers as decimal point independent of locale when parsing, leave locale decimal point valid too.

0.3.10 (2018-05-10)

  • Initial version online.

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