Motivation¶
SDSS has a long history of using flags to indicate the quality of data.
These flags have usually been stored as 64-bit integers.
When more than 64 flag definitions have been needed, multiple flag columns have been used.
For example, APOGEE_TARGET1 and APOGEE_TARGET2 are two 64-bit flag columns used by APOGEE to indicate targeting information.
In APOGEE-2, another three columns were added: APOGEE2_TARGET1, APOGEE2_TARGET2, and APOGEE2_TARGET3.
This is manageable if there are only a few columns, but becomes unweildly when there are many. If the targeting information in SDSS-V were stored in this way, it would require dozens of flag columns. To check whether a flag is set, you’d need to know which column to look up for that flag, and the bit position for that flag.
The semaphore package provides a way to store flags in a single column.
The underlying flags are still stored as integers in a 2D array, but this package provides a way to create, store, and query an unlimited number of flags in a single column.
Installation¶
The current version is 0.2.3. You can install the package by doing
$ pip install sdss-semaphore