Semantic of None in Configs

[innerlee]

Background

In a layered config system, a later layer will override config values of previous layers. A typical layered configuration system is https://docs.microsoft.com/en-us/aspnet/core/fundamentals/configuration/?view=aspnetcore-5.0, where a rough order of config files.json -> ENV VARIABLES -> cmd line args is presented.

Motivation

In such config systems, we need a way to represent the semantic of "not-set". For example, suppose we have an argument normalization type in both config file and in cmd args.

• Case 1. Use None to represent not set
  • Then we can make the normalization type in cmd args defaults to None, and values in config files would be kept.
• Case 2. None is a valid normalization type.
  • Then there is no way to specify normalization type in config file, because it will be overridden anyway.

Proposal

• For all our configurations, reserve None as not set. If you want to represent none, use str 'none'.
• In the configuration merging logic, ignore an entry if the value is None.

Status Quo

The current merging logic in https://github.com/open-mmlab/mmcv/blob/master/mmcv/utils/config.py#L193-L244 does not treat None in any special way, and causing problems in #463.