Introduction to the cf.FieldList object

A cf.FieldList object is an ordered sequence of cf.Field objects.

It supports nearly all of the python list-like operations, the exceptions being the arithmetic and comparison operators for which it has its own definitions. For example:

>>> fl
[<CF Field: x_wind(grid_latitude(110), grid_longitude(106)) m s-1>,
 <CF Field: air_temperature(time(12), latitude(73), longitude(96)) K>]
>>> fl[0]
<CF Field: air_temperature(time(12), latitude(73), longitude(96)) K>
>>> fl[::-1]
[<CF Field: air_temperature(time(12), latitude(73), longitude(96)) K>,
 <CF Field: x_wind(grid_latitude(110), grid_longitude(106)) m s-1>]
>>> fl[slice(1, -1, 2)]
[<CF Field: x_wind(grid_latitude(110), grid_longitude(106)) m s-1>]

Note that an indexing by an integer returns an individual field, but other types of index always return a field list.

>>> len(fl)
2
>>> f = fl.pop()
>>> f
<CF Field: air_temperature(time(12), latitude(73), longitude(96)) K>
>>> len(fl)
1
>>> fl.append(f)
>>> len(fl)
2
>>> f in fl
True
>>> from operator import attrgetter
>>> fl
[<CF Field: x_wind(grid_latitude(110), grid_longitude(106)) m s-1>,
 <CF Field: air_temperature(time(12), latitude(73), longitude(96)) K>]
>>> fl.sort(key=attrgetter('standard_name'))
[<CF Field: air_temperature(time(12), latitude(73), longitude(96)) K>,
 <CF Field: x_wind(grid_latitude(110), grid_longitude(106)) m s-1>]

Selecting fields

One or more fields from a field list may be selected with the select method. This returns a single field or a new field list, depending on how many fields are selected. For example.

>>> fl
[<CF Field: x_wind(grid_latitude(110), grid_longitude(106)) m s-1>,
 <CF Field: air_temperature(time(12), latitude(73), longitude(96)) K>]
>>> fl.select('air_temperature')
<CF Field: air_temperature(time(12), latitude(73), longitude(96)) K>]
>>> fl.select('[air_temperature|x_wind]')
[<CF Field: x_wind(grid_latitude(110), grid_longitude(106)) m s-1>,
 <CF Field: air_temperature(time(12), latitude(73), longitude(96)) K>]

Using field methods

A subset of a field’s callable methods are also available to a field list object (see the list of field list methods). In general, these are methods which, on a field, return a field or None. These methods are applied to each constituent field independently, so

>>> gl = fl.max()
>>> fl.max(i=True)

is exactly equivalent to

>>> gl = cf.FieldList([f.max() for f in fl])
>>> for f in fl:
...     f.max(i=True)

Arithmetic and comparison

All of the operators defined for a field are also allowed for field list, the operation applying to each field independently. For example the commands:

>>> gl = fl + 2
>>> gl = 2 // fl
>>> gl = fl == 0
>>> fl += 2

are exactly equivalent to:

>>> gl = cf.FieldList(f + 2 for f in fl)
>>> gl = cf.FieldList(2 // f for f in fl)
>>> gl = cf.FieldList(f == 0 for f in fl)
>>> for f in fl:
...     f += 2