# fiftyone.core.singletons FiftyOne singleton implementations. Copyright 2017-2026, Voxel51, Inc.
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**Classes:** | [`DatasetSingleton`](#fiftyone.core.singletons.DatasetSingleton)(\*args, \*\*kwargs) | Singleton metaclass for [`fiftyone.core.dataset.Dataset`](fiftyone.core.dataset.md#fiftyone.core.dataset.Dataset). | |----------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------| | [`DocumentSingleton`](#fiftyone.core.singletons.DocumentSingleton) | Singleton metaclass interface for [`fiftyone.core.document.Document`](fiftyone.core.document.md#fiftyone.core.document.Document) subclasses. | | [`SampleSingleton`](#fiftyone.core.singletons.SampleSingleton)(\*args, \*\*kwargs) | Singleton metaclass for [`fiftyone.core.sample.Sample`](fiftyone.core.sample.md#fiftyone.core.sample.Sample). | | [`FrameSingleton`](#fiftyone.core.singletons.FrameSingleton)(\*args, \*\*kwargs) | Singleton metaclass for [`fiftyone.core.frame.Frame`](fiftyone.core.frame.md#fiftyone.core.frame.Frame). | ### *class* fiftyone.core.singletons.DatasetSingleton(\*args, \*\*kwargs) Bases: `type` Singleton metaclass for [`fiftyone.core.dataset.Dataset`](fiftyone.core.dataset.md#fiftyone.core.dataset.Dataset). Datasets are singletons keyed by the dataset’s `name`. Note that new [`fiftyone.core.dataset.Dataset`](fiftyone.core.dataset.md#fiftyone.core.dataset.Dataset) instances are always created if the `_create == True`. **Methods:** | [`mro`](#fiftyone.core.singletons.DatasetSingleton.mro)() | Return a type's method resolution order. | |-------------------------------------------------------------|--------------------------------------------| #### mro() Return a type’s method resolution order. ### *class* fiftyone.core.singletons.DocumentSingleton Bases: `type` Singleton metaclass interface for [`fiftyone.core.document.Document`](fiftyone.core.document.md#fiftyone.core.document.Document) subclasses. The methods declared by this interface are used by the [`fiftyone.core.document.Document`](fiftyone.core.document.md#fiftyone.core.document.Document) class to manage all instances of a class that implements this type. **Methods:** | [`mro`](#fiftyone.core.singletons.DocumentSingleton.mro)() | Return a type's method resolution order. | |--------------------------------------------------------------|--------------------------------------------| #### mro() Return a type’s method resolution order. ### *class* fiftyone.core.singletons.SampleSingleton(\*args, \*\*kwargs) Bases: [`DocumentSingleton`](#fiftyone.core.singletons.DocumentSingleton) Singleton metaclass for [`fiftyone.core.sample.Sample`](fiftyone.core.sample.md#fiftyone.core.sample.Sample). This metaclass maintains a weakref dictionary of all in-memory [`fiftyone.core.sample.Sample`](fiftyone.core.sample.md#fiftyone.core.sample.Sample) instances keyed by `[collection name][sample ID]`. **Methods:** | [`mro`](#fiftyone.core.singletons.SampleSingleton.mro)() | Return a type's method resolution order. | |------------------------------------------------------------|--------------------------------------------| #### mro() Return a type’s method resolution order. ### *class* fiftyone.core.singletons.FrameSingleton(\*args, \*\*kwargs) Bases: [`DocumentSingleton`](#fiftyone.core.singletons.DocumentSingleton) Singleton metaclass for [`fiftyone.core.frame.Frame`](fiftyone.core.frame.md#fiftyone.core.frame.Frame). This metaclass maintains a weakref dictionary of all in-memory [`fiftyone.core.frame.Frame`](fiftyone.core.frame.md#fiftyone.core.frame.Frame) instances keyed by `[collection name][sample ID][frame number]`. **Methods:** | [`mro`](#fiftyone.core.singletons.FrameSingleton.mro)() | Return a type's method resolution order. | |-----------------------------------------------------------|--------------------------------------------| #### mro() Return a type’s method resolution order.