GroupView

class lsst.afw.table.GroupView(schema, ids, groups)

Bases: collections.abc.Mapping

A mapping (i.e. dict-like object) that provides convenient operations on the concatenated catalogs returned by a MultiMatch object.

A GroupView provides access to a catalog of grouped objects, in which the grouping is indicated by a field for which all records in a group have the same value. Once constructed, it allows operations similar to those supported by SQL “GROUP BY”, such as filtering and aggregate calculation.

Parameters:

schema : lsst.afw.table.Schema

Catalog schema to use for the grouped object catalog.

ids : List

List of identifying keys for the groups in the catalog.

groups : List

List of catalog subsets associated with each key in ids.

Methods Summary

aggregate(function[, field, dtype])	Run an aggregate function on each group, returning an array with one element for each group.
apply(function[, field, dtype])	Run a non-aggregate function on each group, returning an array with one element for each record.
build(catalog[, groupField])	Construct a GroupView from a concatenated catalog.
get(k[,d])
items()
keys()
values()
where(predicate)	Return a new GroupView that contains only groups for which the given predicate function returns True.

Methods Documentation

aggregate(function, field=None, dtype=<class 'float'>)

Run an aggregate function on each group, returning an array with one element for each group.

Parameters:

function :

Callable object that computes the aggregate value. If field is None, called with the entire subset catalog as an argument. If field is not None, called with an array view into that field.

field : str, optional

A string name or Key object that indicates a single field the aggregate is computed over.

dtype :

Data type of the output array.

Returns:

result : Array of dtype

Aggregated values for each group.

apply(function, field=None, dtype=<class 'float'>)

Run a non-aggregate function on each group, returning an array with one element for each record.

Parameters:

function :

Callable object that computes the aggregate value. If field is None, called with the entire subset catalog as an argument. If field is not None, called with an array view into that field.

field : str

A string name or Key object that indicates a single field the aggregate is computed over.

dtype :

Data type for the output array.

Returns:

result : numpy.array of dtype

Result of the function calculated on an element-by-element basis.

classmethod build(catalog, groupField='object')

Construct a GroupView from a concatenated catalog.

Parameters:

catalog : lsst.afw.table.base.Catalog

Input catalog, containing records grouped by a field in which all records in the same group have the same value. Must be sorted by the group field.

groupField : str, optional

Name or Key for the field that indicates groups. Defaults to “object”.

Returns:

groupCatalog : lsst.afw.table.GroupView

Constructed GroupView from the input concatenated catalog.

get(k[, d]) → D[k] if k in D, else d. d defaults to None.

items() → a set-like object providing a view on D's items

keys() → a set-like object providing a view on D's keys

values() → an object providing a view on D's values

where(predicate)

Return a new GroupView that contains only groups for which the given predicate function returns True.

The predicate function is called once for each group, and passed a single argument: the subset catalog for that group.

Parameters:

predicate :

Function to identify which groups should be included in the output.

Returns:

outGroupView : lsst.afw.table.GroupView

Subset GroupView containing only groups that match the predicate.