# \[Breaking change\] New changes to select, annotate, key\_by interface for 0.2

**URL:** <https://discuss.hail.is/t/breaking-change-new-changes-to-select-annotate-key-by-interface-for-0-2/491>\
**Category:** Updates\
**Created:** [May 9, 2018, 5:24pm UTC](https://discuss.hail.is/t/breaking-change-new-changes-to-select-annotate-key-by-interface-for-0-2/491 "2018-05-09T17:24:57Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![wang](https://yyz2.discourse-cdn.com/flex036/user_avatar/discuss.hail.is/wang/32/146_2.png) [@wang](https://discuss.hail.is/u/wang)\
**Post date:** [May 9, 2018, 5:24pm UTC](https://discuss.hail.is/t/breaking-change-new-changes-to-select-annotate-key-by-interface-for-0-2/491/1 "2018-05-09T17:24:57Z")

</div>

[https://github.com/hail-is/hail/pull/3297](https://github.com/hail-is/hail/pull/3297)

This PR will change the way keys on Tables and MatrixTables are dealt with in certain functions. (The doc links will be updated once this pull request lands.)

#### [`key_by(*expr, **named_exprs)`](https://hail.is/docs/devel/hail.Table.html#hail.Table.key_by) (and `key_rows_by` and `key_cols_by`)

is the only method that can modify key fields. The interface is identical to the current `select` interface, where non-named exprs must be field references (not necessarily top-level), but non-field-reference expressions can still be used if a name is provided. All unused former key expressions are retained as value fields.

###### [`partition_rows_by(partition_key, *exprs, **named_exprs)`](https://hail.is/docs/devel/hail.MatrixTable.html#hail.MatrixTable.partition_rows_by)

lets you specify a partition key from the key fields in the new matrix table. It’s identical to `key_rows_by`, but takes a list of key fields as its first arguement as a partition key. This interface is still subject to change.

#### [`annotate(**named_exprs)`](https://hail.is/docs/devel/hail.Table.html#hail.Table.annotate) (and `annotate_rows` and `annotate_cols`)

An attempt to annotate over a key field will cause an error. If you actually want to annotate over a key field, use `key_by` directly (if preserving the field as a key), or specify a new key and then call annotate.

#### [`select(*exprs, **named_exprs)`](https://hail.is/docs/devel/hail.Table.html#hail.Table.select) (and `select_rows` and `select_cols`)

likewise deals only with value fields. All key fields are automatically preserved; if `locus` and `alleles` are already row keys in a `MatrixTable`, you can do `mt.select_rows()` to drop all the other fields. If you want to

#### [`drop(*fields)`](https://hail.is/docs/devel/hail.Table.html#hail.Table.drop)

In order to drop a field, remove it from the key first with key\_by.

#### [`transmute(**named_exprs)`](https://hail.is/docs/devel/hail.Table.html#hail.Table.transmute)

Restrictions on what can be overwritten with `transmute` are identical to the restrictions on `annotate`. The main change is that referencing a key field is allowed, but will not cause the field to be dropped. Value fields will continue to be dropped.