# Properties

Properties are custom string key/value pairs that can be attached to a copy of any object. For example, the key/value pair "project"/"cancer" can be attached to an object.

The properties associated with an object are returned by the `describe` method, and modified by the `setProperties` method.

## API method: `/class-xxxx/setProperties`

### Specification

Sets properties on an object in the specified project. To remove a property altogether, its value needs to be set to the JSON `null` (instead of a string). This call updates the properties of the object by merging any old (previously existing) ones with what is provided in the input, the newer ones taking precedence when the same key appears in the old.

Best practices: to fully reset properties (remove all existing key/value pairs and replace them with new ones), first issue a describe call to get the names of all properties, then issue a `setProperties` request to set the values of those properties to `null`.

### Inputs

* `project` **string** (required) ID of the project or container containing the object to be modified.
* `properties` **mapping** (required) Properties to modify.
  * **key** — the name of the property to modify.
  * **value** **string or null** — a new string value for the property, or `null` to unset the property.

### Outputs

* `id` **string** ID of the manipulated object.

### Errors

* InvalidInput
  * There exists at least one value in `properties` which is neither a string nor the JSON `null`
  * For each property key-value pair, the size, encoded in UTF-8, of the property key may not exceed 100 bytes and the property value may not exceed 700 bytes
* ResourceNotFound (the specified object does not exist, the specified project does not exist, or the specified project does not contain the specified object)
* PermissionDenied (If the object is open, UPLOAD access is required, otherwise CONTRIBUTE access is required for the specified project)

### Examples

| Input                 | Action                                          |
| --------------------- | ----------------------------------------------- |
| `{}`                  | No action taken.                                |
| `{"tissue": "liver"}` | The property "tissue" is set to value "liver".  |
| `{"tissue": null}`    | The property "tissue" is removed, if it exists. |


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