IPEP 20: Informal structure of cell metadata

Berk Birand edited this page Jul 3, 2014 · 4 revisions
Clone this wiki locally
key value
Status Active
Author Matthias Bussonnier <bussonniermatthias@gmail.com>
Created August 7, 2013
Updated -
Discussion
Implementation

This IPEP proposes to add informal documentation for expected metadata fields and their structure.

Metadata are optional

Unlike the rest of the fields of the .ipynb format, which should be strictly defined, metadata can support arbitrary JSON structures. Hence, it is difficult for third-party plugins to know what they can write/read and what the expected structure of data is that they are supposed to handle.

The 3 types of metadata available are:

  • notebook-level metadata;
  • cell-level metadata;
  • mimetype-level metadata.

[Note that there is no worksheet-level metadata, since the worksheet structure will be removed in 2.0.]

This proposal describes the use of cell-level metadata, but we should bear in mind that such a description should also be done for the 2 other types of metadata.

General advice on metadata

IPython notebook file are meant to be cross-language, hence the file format should help with this as much as possible.

metadata type

For cross-language compatibility and efficiency, it is advised to avoid having the same field in the metadata structure being able to store data of different type.

metadata value

For the same reason, it is advised in general to avoid when possible to rely on undefined or None values, as well as value that could be considered as such. In particular some languages might not make the distinction between the absence of the field and its value being set to undefined or fact that empty string '', the number 0 (zero) and the boolean false might be indistinguishable.

metadata presence/conservation

Implementation should not make any assumption in the metadata presence or the value contained in it. The metadata filed itself should always be present in the relevant places, but should be set to at least empty dictionary when no metadata is present. Application/plugin should try as much as possible to keep the structure of metadata they don't know how to handle unless if requested by user interaction. This encompass both unknown field, as well as known field with unknown structure/type.

avoid name conflict

To avoid name collision, it is advised to avoid generic name like value , key, ..., and recommended to store plugin/application specific value under a subkey with an explicit name.

Cell metadata fields: Name and tags

For IPython Notebook 2.0, it has been decided to include two optional fields in metadata that could be expected by the Notebook and NbViewer, name and tags. The current proposal will explain the goal and usage of the name and tags fields, as well as a few good practices when reading/writing them.

These two fields will be added because of the need when converting a notebook to be able to identify an individual cell, as well as groups of cells. This is not needed for the correct use of the notebook, so such data belong in the metadata field of a given cell.

Name

The name field is designed to contain a value of type String which is a priori unique in a notebook, and which should allow to reference a cell in a unique way. This field (if present) will mainly be used in the user interface, so it is recommended to be meaningful to the user, easy to type and easy to remember.

The naming of the key as name was chosen in favor of id as it is more meaningful for the user; id is reserved for future usage, as a mandatory field, not in metadata that will not be user-modifiable. The concept of the name field is similar to that of id for HTML tags.

Reading/writing values

When reading these values, an implementation should be able to make the assumption that the name field is unique across a notebook. When writing this value to a cell metadata field, implementations should do their best to avoid having duplicate names across cells, warning the user about duplication and proposing an alternative.

Allowed value

As well as for other metadta, and in addition, the value of the name field should be a non-empty String. If the value is an empty string or undefined, an implementation is able to assume that the field is non-existent or not set. If the value is something other than a string, implementations can also act as if the field was not set, but are advised to do their best not to modify the current value unless explicitly requested by the user.

Allowed characters.

Do we stick to ASCII? Do we recommend not to use spaces? Try to avoid quotes? -- single? double? Do we want case sensitivity?

tags

Tags are:

  • a list of strings

with the following properties:

  • each string avoids commas, since these are often used as separators when a user types.
  • the order doesn't count
  • each string must be unique
  • basically the same recommendation as for name, except for uniqueness?