Goal: Learn about variable types in CDAT.

A variable is essentially a multidimensional data array, augmented with a domain, a set of attributes, and optionally a spatial and/or temporal coordinate system. It can be obtained either from a file, a collection of files, or as the result of computation. Correspondingly there are three types of variables in CDMS:

• A file variable is a variable associated with a single data file. Setting or referencing a file variable generates I/O operations
  
• A transient variable is an ‘in-memory’ object not associated with a file or dataset. Transient variables result from a computation or I/O operation. 

• A dataset variable is a variable associated with a collection of files. Reference to a dataset variable reads data, possibly from multiple files. Dataset variables are read-only.

File variable:

# import necessary module
import cdms

# open the file to read, return "f" - the file object
f = cdms.open(’sample.nc’,’r+’)

# get a file variable u from the opened file
u = f.getVariable('u')
# or equivalent, u2 is also a file variable
u2 = f['u']
u
<Variable: u, file: sample.nc, shape: (1, 2, 80, 97)>
 

Transient variable:

# to get a transient variable use the following syntax:
u = f('u')  # notice the patentesis are "( )" here
u
u
array(
array (1,2,80,97) , type = f, has 15520 elements)

Dataset variable:

# open many files described in the 'sample.xml' file
filepath = os.path.join(sys.prefix, 'sample_data/cdtest10.xml')
f2 = cdms.open(filepath)

f2.listall
<bound method Dataset.listall of <Dataset: 'cdtest10', 
  URI: '.../sample_data/cdtest10.xml', 
  mode: 'r', status: open>>

# check what variables exist
f2.listvariables()
['bounds_latitude', 'bounds_time', 'bounds_longitude', 
  'u', 't', 'v']

u3 = f2('u') # u3 is a dataset variable

# get all info about this variable
u.info()
*** Description of Slab u ***
id: u
shape: (48, 16, 32)
filename: cdtest10
missing_value: 0
comments:
grid_name: grid_16x32
grid_type: None
time_statistic:
long_name:
units: m/s
datatype: Int
name_in_file: u
Grid has Python id -0x491b9214.
Gridtype: generic
Grid shape: (16, 32)
Order: yx
** Dimension 1 **
   id: time_48
   Designated a time axis.
   units:  months since 2000-1
   Length: 48
   First:  0.0
   Last:   59.0
   Other axis attributes:
      name_in_file: time
      datatype: Double
      partition: [[ 0,12,]
 [12,24,]
 [24,36,]
 [36,48,] ]
      isvar: true
   Python id:  -0x491b9394
** Dimension 2 **
   id: latitude
   Designated a latitude axis.
   units:
   Length: 16
   First:  -90.0
   Last:   90.0
   Other axis attributes:
      datatype: Double
      isvar: true
      axis: Y
   Python id:  -0x491b9294
** Dimension 3 **
   id: longitude
   Designated a longitude axis.
   units:
   Length: 32
   First:  0.0
   Last:   348.75
   Other axis attributes:
      modulo: 360.0
      datatype: Double
      isvar: true
      axis: X
      topology: circular
   Python id:  -0x491b9374
*** End of description for u ***