Navigating a Cube
After loading any cube, you will want to investigate precisely what it contains. This section is all about accessing and manipulating the metadata contained within a cube.
Cube String Representations
We have already seen a basic string representation of a cube when printing:
>>> import iris >>> filename = iris.sample_data_path('rotated_pole.nc') >>> cube = iris.load_cube(filename) >>> print(cube) air_pressure_at_sea_level / (Pa) (grid_latitude: 22; grid_longitude: 36) Dimension coordinates: grid_latitude x - grid_longitude - x Scalar coordinates: forecast_period 0.0 hours forecast_reference_time 2006-06-15 00:00:00 time 2006-06-15 00:00:00 Attributes: Conventions CF-1.5 STASH m01s16i222 source Data from Met Office Unified Model 6.01
This representation is equivalent to passing the cube to the
str() function. This function can be used on
any Python variable to get a string representation of that variable.
Similarly there exist other standard functions for interrogating your variable:
type() for example:
print(str(cube)) print(repr(cube)) print(type(cube))
Other, more verbose, functions also exist which give information on what you can do with any given
variable. In most cases it is reasonable to ignore anything starting with a “
_” (underscore) or a “
__” (double underscore):
Working With Cubes
print(cube.standard_name) print(cube.long_name) print(cube.units)
Interrogating these with the standard
type() function will tell you that
are either a string or
units is an instance of
iris.unit.Unit. A more in depth discussion on
the cube units and their functional effects can be found at the end of Cube Maths.
You can access a string representing the “name” of a cube with the
The result of which is always a string.
When loading from most file formats in Iris, the data itself is not loaded until the first time that the data is requested. Hence you may have noticed that running the previous command for the first time takes a little longer than it does for subsequent calls.
For this reason, when you have a large cube it is strongly recommended that you do not access the cube’s data unless
you need to.
ndim attributes exists on a cube, which
can tell you the shape of the cube’s data without loading it:
For more on the benefits, handling and uses of lazy data, see Real and Lazy Data
You can change the units of a cube using the
convert_units() method. For example:
Some cubes represent a processed phenomenon which are represented with cell methods, these can be accessed on a
cube with the
Accessing Coordinates on the Cube
for coord in cube.coords(): print(coord.name())
Alternatively, we can use list comprehension to store the names in a list:
coord_names = [coord.name() for coord in cube.coords()]
The result is a basic Python list which could be sorted alphabetically and joined together:
>>> print(', '.join(sorted(coord_names))) forecast_period, forecast_reference_time, grid_latitude, grid_longitude, time
To get an individual coordinate given its name, the
Cube.coord method can be used:
coord = cube.coord('grid_latitude') print(type(coord))
print(coord.standard_name) print(coord.long_name) print(coord.units)
Adding Metadata to a Cube
>>> import iris.coords >>> new_coord = iris.coords.AuxCoord(1, long_name='my_custom_coordinate', units='no_unit') >>> cube.add_aux_coord(new_coord) >>> print(cube) air_pressure_at_sea_level / (Pa) (grid_latitude: 22; grid_longitude: 36) Dimension coordinates: grid_latitude x - grid_longitude - x Scalar coordinates: forecast_period 0.0 hours forecast_reference_time 2006-06-15 00:00:00 my_custom_coordinate 1 time 2006-06-15 00:00:00 Attributes: Conventions CF-1.5 STASH m01s16i222 source Data from Met Office Unified Model 6.01
my_custom_coordinate now exists on the cube and is listed under the non-dimensioned single valued scalar coordinates.
Adding and Removing Metadata to the Cube at Load Time
Sometimes when loading a cube problems occur when the amount of metadata is more or less than expected. This is often caused by one of the following:
The file does not contain enough metadata, and therefore the cube cannot know everything about the file.
Some of the metadata of the file is contained in the filename, but is not part of the actual file.
There is not enough metadata loaded from the original file as Iris has not handled the format fully. (in which case, please let us know about it)
The callback is a user defined function which must have the calling sequence
function(cube, field, filename)
which can make any modifications to the cube in-place, or alternatively return a completely new cube instance.
Suppose we wish to load a lagged ensemble dataset from the Met Office’s GloSea4 model. The data for this example represents 13 ensemble members of 6 one month timesteps; the logistics of the model mean that the run is spread over several days.
If we try to load the data directly for
>>> filename = iris.sample_data_path('GloSea4', '*.pp') >>> print(iris.load(filename, 'surface_temperature')) 0: surface_temperature / (K) (time: 6; forecast_reference_time: 2; latitude: 145; longitude: 192) 1: surface_temperature / (K) (time: 6; forecast_reference_time: 2; latitude: 145; longitude: 192) 2: surface_temperature / (K) (realization: 9; time: 6; latitude: 145; longitude: 192)
We get multiple cubes some with more dimensions than expected, some without a
realization (i.e. ensemble member) dimension.
In this case, two of the PP files have been encoded without the appropriate
realization number attribute, which means that
the appropriate coordinate cannot be added to the resultant cube. Fortunately, the missing attribute has been encoded in the filename
which, given the filename, we could extract:
filename = iris.sample_data_path('GloSea4', 'ensemble_001.pp') realization = int(filename[-6:-3]) print(realization)
We can solve this problem by adding the appropriate metadata, on load, by using a callback function, which runs on a field by field basis before they are automatically merged together:
import numpy as np import iris import iris.coords as icoords def lagged_ensemble_callback(cube, field, filename): # Add our own realization coordinate if it doesn't already exist. if not cube.coords('realization'): realization = np.int32(filename[-6:-3]) ensemble_coord = icoords.AuxCoord(realization, standard_name='realization', units="1") cube.add_aux_coord(ensemble_coord) filename = iris.sample_data_path('GloSea4', '*.pp') print(iris.load(filename, 'surface_temperature', callback=lagged_ensemble_callback))
The result is a single cube which represents the data in a form that was expected:
0: surface_temperature / (K) (realization: 13; time: 6; latitude: 145; longitude: 192)