Tagging datasets and bands

GDAL’s data model includes collections of key, value pairs for major classes. In that model, these are “metadata”, but since they don’t have to be just for metadata, these key, value pairs are called “tags” in rasterio.

Reading tags

I’m going to use the rasterio interactive inspector in these examples below.

$ rio insp tests/data/RGB.byte.tif
Rasterio 1.2.0 Interactive Inspector (Python 3.7.8)
Type "src.name", "src.read(1)", or "help(src)" for more information.

Tags belong to namespaces. To get a copy of a dataset’s tags from the default namespace, call tags() with no arguments.

>>> import rasterio
>>> src = rasterio.open("tests/data/RGB.byte.tif")
>>> src.tags()
{'AREA_OR_POINT': 'Area'}

A dataset’s bands may have tags, too. Here are the tags from the default namespace for the first band, accessed using the positional band index argument of tags().

>>> src.tags(1)['STATISTICS_MEAN']

These are the tags that came with the sample data I’m using to test rasterio. In practice, maintaining stats in the tags can be unreliable as there is no automatic update of the tags when the band’s image data changes.

The 3 standard, non-default GDAL tag namespaces are ‘SUBDATASETS’, ‘IMAGE_STRUCTURE’, and ‘RPC’. You can get the tags from these namespaces using the ns keyword of tags().

>>> src.tags(ns='IMAGE_STRUCTURE')
>>> src.tags(ns='SUBDATASETS')
>>> src.tags(ns='RPC')

A special case for GDAL tag namespaces are those prefixed with ‘xml’ e.g. ‘xml:TRE’ or ‘xml:VRT’. GDAL will treat these namespaces as a single xml string.

Writing tags

You can add new tags to a dataset or band, in the default or another namespace, using the update_tags() method. Unicode tag values, too, at least for TIFF files.

import rasterio

with rasterio.open(
        height=10) as dst:

    dst.update_tags(a='1', b='2')
    dst.update_tags(1, c=3)
    with pytest.raises(ValueError):
        dst.update_tags(4, d=4)

    # True
    assert dst.tags() == {'a': '1', 'b': '2'}
    # True
    assert dst.tags(1) == {'c': '3' }

    dst.update_tags(ns='rasterio_testing', rus=u'другая строка')
    # True
    assert dst.tags(ns='rasterio_testing') == {'rus': u'другая строка'}

As with image data, tags aren’t written to the file on disk until the dataset is closed.