.. _loading_iris_cubes: =================== Loading Iris Cubes =================== To load a single file into a **list** of Iris cubes the :py:func:`iris.load` function is used:: import iris filename = '/path/to/file' cubes = iris.load(filename) Iris will attempt to return **as few cubes as possible** by collecting together multiple fields with a shared standard name into a single multidimensional cube. The :py:func:`iris.load` function automatically recognises the format of the given files and attempts to produce Iris Cubes from their contents. .. note:: Currently there is support for CF NetCDF, GRIB 1 & 2, PP and FieldsFiles file formats with a framework for this to be extended to custom formats. In order to find out what has been loaded, the result can be printed: >>> import iris >>> filename = iris.sample_data_path('uk_hires.pp') >>> cubes = iris.load(filename) >>> print(cubes) 0: air_potential_temperature / (K) (time: 3; model_level_number: 7; grid_latitude: 204; grid_longitude: 187) 1: surface_altitude / (m) (grid_latitude: 204; grid_longitude: 187) This shows that there were 2 cubes as a result of loading the file, they were: ``air_potential_temperature`` and ``surface_altitude``. The ``surface_altitude`` cube was 2 dimensional with: * the two dimensions have extents of 204 and 187 respectively and are represented by the ``grid_latitude`` and ``grid_longitude`` coordinates. The ``air_potential_temperature`` cubes were 4 dimensional with: * the same length ``grid_latitude`` and ``grid_longitude`` dimensions as ``surface_altitide`` * a ``time`` dimension of length 3 * a ``model_level_number`` dimension of length 7 .. note:: The result of :func:`iris.load` is **always** a :class:`iris.cube.CubeList` (even if it only contains one :class:`iris.cube.Cube` - see :ref:`strict-loading`). Anything that can be done with a Python :class:`list` can be done with an :class:`iris.cube.CubeList`. The order of this list should not be relied upon. Ways of loading a specific cube or cubes are covered in :ref:`constrained-loading` and :ref:`strict-loading`. .. hint:: Throughout this user guide you will see the function ``iris.sample_data_path`` being used to get the filename for the resources used in the examples. The result of this function is just a string. Using this function allows us to provide examples which will work across platforms and with data installed in different locations, however in practice you will want to use your own strings:: filename = '/path/to/file' cubes = iris.load(filename) To get the air potential temperature cube from the list of cubes returned by :py:func:`iris.load` in the previous example, list indexing can be used: >>> import iris >>> filename = iris.sample_data_path('uk_hires.pp') >>> cubes = iris.load(filename) >>> # get the first cube (list indexing is 0 based) >>> air_potential_temperature = cubes[0] >>> print(air_potential_temperature) air_potential_temperature / (K) (time: 3; model_level_number: 7; grid_latitude: 204; grid_longitude: 187) Dimension coordinates: time x - - - model_level_number - x - - grid_latitude - - x - grid_longitude - - - x Auxiliary coordinates: forecast_period x - - - level_height - x - - sigma - x - - surface_altitude - - x x Derived coordinates: altitude - x x x Scalar coordinates: forecast_reference_time 2009-11-19 04:00:00 Attributes: STASH m01s00i004 source 'Data from Met Office Unified Model' um_version '7.3' Notice that the result of printing a **cube** is a little more verbose than it was when printing a **list of cubes**. In addition to the very short summary which is provided when printing a list of cubes, information is provided on the coordinates which constitute the cube in question. This was the output discussed at the end of the :doc:`iris_cubes` section. .. note:: Dimensioned coordinates will have a dimension marker ``x`` in the appropriate column for each cube data dimension that they describe. Loading Multiple Files ----------------------- To load more than one file into a list of cubes, a list of filenames can be provided to :py:func:`iris.load`:: filenames = [iris.sample_data_path('uk_hires.pp'), iris.sample_data_path('air_temp.pp')] cubes = iris.load(filenames) It is also possible to load one or more files with wildcard substitution using the expansion rules defined :py:mod:`fnmatch`. For example, to match **zero or more characters** in the filename, star wildcards can be used:: filename = iris.sample_data_path('GloSea4', '*.pp') cubes = iris.load(filename) .. note:: The cubes returned will not necessarily be in the same order as the order of the filenames. Lazy Loading ------------ In fact when Iris loads data from most file types, it normally only reads the essential descriptive information or metadata : the bulk of the actual data content will only be loaded later, as it is needed. This is referred to as 'lazy' data. It allows loading to be much quicker, and to occupy less memory. For more on the benefits, handling and uses of lazy data, see :doc:`Real and Lazy Data `. .. _constrained-loading: Constrained Loading ----------------------- Given a large dataset, it is possible to restrict or constrain the load to match specific Iris cube metadata. Constrained loading provides the ability to generate a cube from a specific subset of data that is of particular interest. As we have seen, loading the following file creates several Cubes:: filename = iris.sample_data_path('uk_hires.pp') cubes = iris.load(filename) Specifying a name as a constraint argument to :py:func:`iris.load` will mean only cubes with matching :meth:`name ` will be returned:: filename = iris.sample_data_path('uk_hires.pp') cubes = iris.load(filename, 'surface_altitude') Note that, the provided name will match against either the standard name, long name, NetCDF variable name or STASH metadata of a cube. Therefore, the previous example using the ``surface_altitude`` standard name constraint can also be achieved using the STASH value of ``m01s00i033``:: filename = iris.sample_data_path('uk_hires.pp') cubes = iris.load(filename, 'm01s00i033') If further specific name constraint control is required i.e., to constrain against a combination of standard name, long name, NetCDF variable name and/or STASH metadata, consider using the :class:`iris.NameConstraint`. For example, to constrain against both a standard name of ``surface_altitude`` **and** a STASH of ``m01s00i033``:: filename = iris.sample_data_path('uk_hires.pp') constraint = iris.NameConstraint(standard_name='surface_altitude', STASH='m01s00i033') cubes = iris.load(filename, constraint) To constrain the load to multiple distinct constraints, a list of constraints can be provided. This is equivalent to running load once for each constraint but is likely to be more efficient:: filename = iris.sample_data_path('uk_hires.pp') cubes = iris.load(filename, ['air_potential_temperature', 'surface_altitude']) The :class:`iris.Constraint` class can be used to restrict coordinate values on load. For example, to constrain the load to match a specific ``model_level_number``:: filename = iris.sample_data_path('uk_hires.pp') level_10 = iris.Constraint(model_level_number=10) cubes = iris.load(filename, level_10) Further details on using :class:`iris.Constraint` are discussed later in :ref:`cube_extraction`. .. _strict-loading: Strict Loading -------------- The :py:func:`iris.load_cube` and :py:func:`iris.load_cubes` functions are similar to :py:func:`iris.load` except they can only return *one cube per constraint*. The :func:`iris.load_cube` function accepts a single constraint and returns a single cube. The :func:`iris.load_cubes` function accepts any number of constraints and returns a list of cubes (as an `iris.cube.CubeList`). Providing no constraints to :func:`iris.load_cube` or :func:`iris.load_cubes` is equivalent to requesting exactly one cube of any type. A single cube is loaded in the following example:: >>> filename = iris.sample_data_path('air_temp.pp') >>> cube = iris.load_cube(filename) >>> print(cube) air_temperature / (K) (latitude: 73; longitude: 96) Dimension coordinates: latitude x - longitude - x ... Cell methods: 0 time: mean However, when attempting to load data which would result in anything other than one cube, an exception is raised:: >>> filename = iris.sample_data_path('uk_hires.pp') >>> cube = iris.load_cube(filename) Traceback (most recent call last): ... iris.exceptions.ConstraintMismatchError: Expected exactly one cube, found 2. .. note:: All the load functions share many of the same features, hence multiple files could be loaded with wildcard filenames or by providing a list of filenames. The strict nature of :func:`iris.load_cube` and :func:`iris.load_cubes` means that, when combined with constrained loading, it is possible to ensure that precisely what was asked for on load is given - otherwise an exception is raised. This fact can be utilised to make code only run successfully if the data provided has the expected criteria. For example, suppose that code needed ``air_potential_temperature`` in order to run:: import iris filename = iris.sample_data_path('uk_hires.pp') air_pot_temp = iris.load_cube(filename, 'air_potential_temperature') print(air_pot_temp) Should the file not produce exactly one cube with a standard name of 'air_potential_temperature', an exception will be raised. Similarly, supposing a routine needed both 'surface_altitude' and 'air_potential_temperature' to be able to run:: import iris filename = iris.sample_data_path('uk_hires.pp') altitude_cube, pot_temp_cube = iris.load_cubes(filename, ['surface_altitude', 'air_potential_temperature']) The result of :func:`iris.load_cubes` in this case will be a list of 2 cubes ordered by the constraints provided. Multiple assignment has been used to put these two cubes into separate variables. .. note:: In Python, lists of a pre-known length and order can be exploited using *multiple assignment*: >>> number_one, number_two = [1, 2] >>> print(number_one) 1 >>> print(number_two) 2