# Objective
There is a lot of `world.entities().len()`, especially in tests. In
tests, usually, the assumption is made that empty worlds do not contain
any entities. This is about to change (#19711), and as such all of these
tests are failing for that PR.
## Solution
`num_entities` is a convenience method that returns the number of
entities inside a world. It can later be adapted to exclude 'unexpected'
entities, associated with internal data structures such as Resources,
Queries, Systems. In general I argue for a separation of concepts where
`World` ignores internal entities in methods such as `iter_entities()`
and `clear_entities()`, that discussion is, however, separate from this
PR.
## Testing
I replaced most occurrences of `world.entities().len()` with
`world.num_entities()` and the tests passed.
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Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
6dbe3600ed
