, external=None, extra=None, verbose=None, warnings=False, warn_valid=False, mask=True, _implementation=<CFDMImplementation: >)[source]

Read field constructs from a dataset.

The dataset may be a netCDF file on disk or on an OPeNDAP server, or a CDL file on disk (see below).

The returned field constructs are sorted by the netCDF variable names of their corresponding data variables.

CDL files

A file is considered to be a CDL representation of a netCDF dataset if it is a text file whose first non-comment line starts with the seven characters “netcdf ” (six letters followed by a space). A comment line is identified as one which starts with any amount white space (including none) followed by “//” (two slashes). It is converted to a temporary netCDF4 file using the external ncgen command, and the temporary file persists until the end of the Python session, at which time it is automatically deleted. The CDL file may omit data array values (as would be the case, for example, if the file was created with the -h or -c option to ncdump), in which case the the relevant constructs in memory will be created with data with all missing values.

NetCDF unlimited dimensions

Domain axis constructs that correspond to NetCDF unlimited dimensions may be accessed with the nc_is_unlimited and nc_set_unlimited methods of a domain axis construct.

NetCDF hierarchical groups

Hierarchical groups in CF provide a mechanism to structure variables within netCDF4 datasets. Field constructs are constructed from grouped datasets by applying the well defined rules in the CF conventions for resolving references to out-of-group netCDF variables and dimensions. The group structure is preserved in the field construct’s netCDF interface. Groups were incorporated into C-1.8. For files with groups that state compliance to earlier versions of the CF conventions, the groups will be interpreted as per the latest release of CF.


If the dataset is partially CF-compliant to the extent that it is not possible to unambiguously map an element of the netCDF dataset to an element of the CF data model, then a field construct is still returned, but may be incomplete. This is so that datasets which are partially conformant may nonetheless be modified in memory and written to new datasets.

Such “structural” non-compliance would occur, for example, if the “coordinates” attribute of a CF-netCDF data variable refers to another variable that does not exist, or refers to a variable that spans a netCDF dimension that does not apply to the data variable. Other types of non-compliance are not checked, such whether or not controlled vocabularies have been adhered to. The structural compliance of the dataset may be checked with the dataset_compliance method of the field construct, as well as optionally displayed when the dataset is read by setting the warnings parameter.


Descriptive properties are always read into memory, but lazy loading is employed for all data arrays, which means that no data is read into memory until the data is required for inspection or to modify the array contents. This maximises the number of field constructs that may be read within a session, and makes the read operation fast.

New in version (cfdm): 1.7.0

filename: str

The file name or OPenDAP URL of the dataset.

Relative paths are allowed, and standard tilde and shell parameter expansions are applied to the string.

Parameter example:

The file in the user’s home directory could be described by any of the following: '$HOME/', '${HOME}/', '~/', '~/tmp/../'.

external: (sequence of) str, optional

Read external variables (i.e. variables which are named by attributes, but are not present, in the parent file given by the filename parameter) from the given external files. Ignored if the parent file does not contain a global external_variables attribute. Multiple external files may be provided, which are searched in random order for the required external variables.

If an external variable is not found in any external files, or is found in multiple external files, then the relevant metadata construct is still created, but without any metadata or data. In this case the construct’s is_external method will return True.

Parameter example:


Parameter example:


Parameter example:

external=('', '')

extra: (sequence of) str, optional

Create extra, independent fields from netCDF variables that correspond to particular types metadata constructs. The extra parameter may be one, or a sequence, of:


Metadata constructs


Field ancillary constructs


Domain ancillary constructs


Dimension coordinate constructs


Auxiliary coordinate constructs


Cell measure constructs

Parameter example:

To create fields from auxiliary coordinate constructs: extra='auxiliary_coordinate' or extra=['auxiliary_coordinate'].

Parameter example:

To create fields from domain ancillary and cell measure constructs: extra=['domain_ancillary', 'cell_measure'].

An extra field construct created via the extra parameter will have a domain limited to that which can be inferred from the corresponding netCDF variable, but without the connections that are defined by the parent netCDF data variable. It is possible to create independent fields from metadata constructs that do incorporate as much of the parent field construct’s domain as possible by using the convert method of a returned field construct, instead of setting the extra parameter.

verbose: int or str or None, optional

If an integer from -1 to 3, or an equivalent string equal ignoring case to one of:

  • 'DISABLE' (0)

  • 'WARNING' (1)

  • 'INFO' (2)

  • 'DETAIL' (3)

  • 'DEBUG' (-1)

set for the duration of the method call only as the minimum cut-off for the verboseness level of displayed output (log) messages, regardless of the globally-configured cfdm.log_level. Note that increasing numerical value corresponds to increasing verbosity, with the exception of -1 as a special case of maximal and extreme verbosity.

Otherwise, if None (the default value), output messages will be shown according to the value of the cfdm.log_level setting.

Overall, the higher a non-negative integer or equivalent string that is set (up to a maximum of 3/'DETAIL') for increasing verbosity, the more description that is printed to convey how the contents of the netCDF file were parsed and mapped to CF data model constructs.

warnings: bool, optional

If True then print warnings when an output field construct is incomplete due to structural non-compliance of the dataset. By default such warnings are not displayed.

warn_valid: bool, optional

If True then print a warning for the presence of valid_min, valid_max or valid_range properties on field contructs and metadata constructs that have data. By default no such warning is issued.

“Out-of-range” data values in the file, as defined by any of these properties, are automatically masked by default, which may not be as intended. See the mask parameter for turning off all automatic masking.

See for details.

New in version (cfdm): 1.8.3

mask: bool, optional

If False then do not mask by convention when reading the data of field or metadata constructs from disk. By default data is masked by convention.

The masking by convention of a netCDF array depends on the values of any of the netCDF variable attributes _FillValue, missing_value, valid_min, valid_max and valid_range.

See for details.

New in version (cfdm): 1.8.2

_implementation: (subclass of) CFDMImplementation, optional

Define the CF data model implementation that provides the returned field constructs.


The field constructs found in the dataset. The list may be empty.


>>> x ='')
>>> print(type(x))
<type 'list'>

Read a file and create field constructs from CF-netCDF data variables as well as from the netCDF variables that correspond to particular types metadata constructs:

>>> f ='', extra='domain_ancillary')
>>> g ='', extra=['dimension_coordinate',
...                                 'auxiliary_coordinate'])

Read a file that contains external variables:

>>> h ='')
>>> i ='', external='')
>>> j ='', external=['', ''])