cfdm.write(fields, filename, fmt='NETCDF4', mode='w', overwrite=True, global_attributes=None, variable_attributes=None, file_descriptors=None, external=None, Conventions=None, datatype=None, least_significant_digit=None, endian='native', compress=0, fletcher32=False, shuffle=True, string=True, verbose=None, warn_valid=True, group=True, coordinates=False, omit_data=None, _implementation=<CFDMImplementation: >)[source]

Write field and domain constructs to a netCDF file.

File format

See the fmt parameter for details on which output netCDF file formats are supported.

NetCDF variable and dimension names

These names are stored within constructs read a from dataset, or may be set manually. They are used when writing a field construct to the file. If a name has not been set then one will be constructed (usually based on the standard name if it exists). The names may be modified internally to prevent duplication in the file.

Each construct, or construct component, that corresponds to a netCDF variable has the following methods to get, set and remove a netCDF variable name: nc_get_variable, nc_set_variable and nc_del_variable method

The domain axis construct has the following methods to get, set and remove a netCDF dimension name: nc_get_dimension, nc_set_dimension and nc_del_dimension.

NetCDF attributes

Field construct properties may be written as netCDF global attributes and/or netCDF data variable attributes. See the file_descriptors, global_attributes and variable_attributes parameters for details.

External variables

Metadata constructs marked as external are omitted from the file and referred to via the netCDF external_variables global attribute. However, omitted constructs may be written to an external file (see the external parameter for details).

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 with well defined rules for resolving references to out-of-group netCDF variables and dimensions. The group structure defined by a field construct’s netCDF interface will, by default, be recreated in the output dataset. See the group parameter for details.

NetCDF4 HDF chunksizes

HDF5 chunksizes may be set on construct’s data. See the nc_hdf5_chunksizes, nc_clear_hdf5_chunksizes and nc_set_hdf5_chunksizes methods of a Data instance.

New in version (cfdm): 1.7.0

See also


fields: (sequence of) Field or Domain

The field and domain constructs to write to the file.

filename: str

The output netCDF file name. Various type of expansion are applied to the file names.

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/../'.

fmt: str, optional

The format of the output file. One of:


Output file type


NetCDF4 format file. This is the default.


NetCDF4 classic format file (see below)


NetCDF3 classic format file (limited to file sizes less than 2GB).


NetCDF3 64-bit offset format file


An alias for 'NETCDF3_64BIT_OFFSET'


NetCDF3 64-bit offset format file with extensions (see below)

By default the format is 'NETCDF4'.

All formats support large files (i.e. those greater than 2GB) except 'NETCDF3_CLASSIC'.

'NETCDF3_64BIT_DATA' is a format that requires version 4.4.0 or newer of the C library (use cfdm.environment to see which version if the netCDF-C library is in use). It extends the 'NETCDF3_64BIT_OFFSET' binary format to allow for unsigned 64 bit integer data types and 64-bit dimension sizes.

'NETCDF4_CLASSIC' files use the version 4 disk format (HDF5), but omits features not found in the version 3 API. They can be read by HDF5 clients. They can also be read by netCDF3 clients only if they have been re-linked against the netCDF4 library.

'NETCDF4' files use the version 4 disk format (HDF5) and use the new features of the version 4 API.

mode: str, optional

Specify the mode of write access for the output file. One of:




Open a new file for writing to. If it exists and overwrite is True then the file is deleted prior to being recreated.


Open an existing file for appending new information to. The new information will be incorporated whilst the original contents of the file will be preserved.

In practice this means that new fields will be created, whilst the original fields will not be edited at all. Coordinates on the fields, where equal, will be shared as standard.

For append mode, note the following:

  • Global attributes on the file will remain the same as they were originally, so will become inaccurate where appended fields have incompatible attributes. To rectify this, manually inspect and edit them as appropriate after the append operation using methods such as nc_clear_global_attributes and nc_set_global_attribute.

  • Fields with incompatible featureType to the original file cannot be appended.

  • At present fields with groups cannot be appended, but this will be possible in a future version. Groups can however be cleared, the fields appended, and groups re-applied, via methods such as nc_clear_variable_groups and nc_set_variable_groups, to achieve the same for now.

  • At present domain ancillary constructs of appended fields may not be handled correctly and can appear as extra fields. Set them on the resultant fields using set_domain_ancillary and similar methods if required.


Alias for 'a'.

By default the file is opened with write access mode 'w'.

overwrite: bool, optional

If False then raise an error if the output file pre-exists. By default a pre-existing output file is overwritten.

Conventions: (sequence of) str, optional

Specify conventions to be recorded by the netCDF global Conventions attribute. By default the current conventions are always included, but if an older CF conventions is defined then this is used instead.

Parameter example:


Parameter example:


Parameter example:

Conventions=['CMIP-6.2', 'UGRID-1.0']

Parameter example:


Parameter example:

Conventions=['CF-1.7', 'UGRID-1.0']

Note that if the Conventions property is set on a field construct then it is ignored.

file_descriptors: dict, optional

Create description of file contents netCDF global attributes from the specified attributes and their values.

If any field construct has a property with the same name then it will be written as a netCDF data variable attribute, even if it has been specified by the global_attributes parameter, or has been flagged as global on any of the field constructs (see cfdm.Field.nc_global_attributes for details).

Identification of the conventions being adhered to by the file are not specified as a file descriptor, but by the Conventions parameter instead.

Parameter example:

file_attributes={'title': 'my data'}

Parameter example:

file_attributes={'history': 'created 2019-01-01', 'foo': 'bar'}

global_attributes: (sequence of) str, optional

Create netCDF global attributes from the specified field construct properties, rather than netCDF data variable attributes.

These attributes are in addition to the following field construct properties, which are created as netCDF global attributes by default:

  • the description of file contents properties (as defined by the CF conventions), and

  • properties flagged as global on any of the field constructs being written (see cfdm.Field.nc_global_attributes for details).

Note that it is not possible to create a netCDF global attribute from a property that has different values for different field constructs being written. In this case the property will not be written as a netCDF global attribute, even if it has been specified by the global_attributes parameter or is one of the default properties, but will appear as an attribute on the netCDF data variable corresponding to each field construct that contains the property.

Any global attributes that are also specified as file descriptors will not be written as netCDF global variables, but as netCDF data variable attributes instead.

Parameter example:


Parameter example:


Parameter example:

global_attributes=['project', 'experiment']

variable_attributes: (sequence of) str, optional

Create netCDF data variable attributes from the specified field construct properties.

By default, all field construct properties that are not created as netCDF global properties are created as attributes netCDF data variables. See the global_attributes parameter for details.

Any field construct property named by the variable_attributes parameter will always be created as a netCDF data variable attribute

Parameter example:


Parameter example:


Parameter example:

variable_attributes=['project', 'doi']

external: str, optional

Write metadata constructs that have data and are marked as external to the named external file. Ignored if there are no such constructs.

datatype: dict, optional

Specify data type conversions to be applied prior to writing data to disk. This may be useful as a means of packing, or because the output format does not support a particular data type (for example, netCDF3 classic files do not support 64-bit integers). By default, input data types are preserved. Any data type conversion is only applied to the arrays on disk, and not to the input field constructs themselves.

Data types conversions are defined by numpy.dtype objects in a dictionary whose keys are input data types with values of output data types.

Parameter example:

To convert 64-bit integers to 32-bit integers: datatype={numpy.dtype('int64'): numpy.dtype('int32')}.

endian: str, optional

The endian-ness of the output file. Valid values are 'little', 'big' or 'native'. By default the output is native endian. See the netCDF4 package for more details.

Parameter example:


compress: int, optional

Regulate the speed and efficiency of compression. Must be an integer between 0 and 9. 0 means no compression; 1 is the fastest, but has the lowest compression ratio; 9 is the slowest but best compression ratio. The default value is 0. An error is raised if compression is requested for a netCDF3 output file format. See the netCDF4 package for more details.

Parameter example:


least_significant_digit: int, optional

Truncate the input field construct data arrays, but not the data arrays of metadata constructs. For a given positive integer, N the precision that is retained in the compressed data is 10 to the power -N. For example, a value of 2 will retain a precision of 0.01. In conjunction with the compress parameter this produces ‘lossy’, but significantly more efficient, compression. See the netCDF4 package for more details.

Parameter example:


fletcher32: bool, optional

If True then the Fletcher-32 HDF5 checksum algorithm is activated to detect compression errors. Ignored if compress is 0. See the netCDF4 package for details.

shuffle: bool, optional

If True (the default) then the HDF5 shuffle filter (which de-interlaces a block of data before compression by reordering the bytes by storing the first byte of all of a variable’s values in the chunk contiguously, followed by all the second bytes, and so on) is turned off. By default the filter is applied because if the data array values are not all wildly different, using the filter can make the data more easily compressible. Ignored if the compress parameter is 0 (which is its default value). See the netCDF4 package for more details.

string: bool, optional

By default string-valued construct data are written as netCDF arrays of type string if the output file format is 'NETCDF4', or of type char with an extra dimension denoting the maximum string length for any other output file format (see the fmt parameter). If string is False then string-valued construct data are written as netCDF arrays of type char with an extra dimension denoting the maximum string length, regardless of the selected output file format.

New in version (cfdm): 1.8.0

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 constructs map to output netCDF dimensions, variables and attributes.

warn_valid: bool, optional

If True (the default) then print a warning when writing “out-of-range” data, as indicated by the values, if present, of any of the valid_min, valid_max or valid_range properties on field and metadata constructs that have data. If False the warning is not printed.

The consequence of writing out-of-range data values is that, by default, these values will be masked when the file is subsequently read.

Parameter example:

If a construct has valid_max property with value 100 and data with maximum value 999, then the resulting warning may be suppressed by setting warn_valid=False.

New in version (cfdm): 1.8.3

group: bool, optional

If False then create a “flat” netCDF file, i.e. one with only the root group, regardless of any group structure specified by the field constructs. By default any groups defined by the netCDF interface of the field constructs and its components will be created and populated.

New in version (cfdm): 1.8.6

coordinates: bool, optional

If True then include CF-netCDF coordinate variable names in the ‘coordinates’ attribute of output data variables. By default only auxiliary and scalar coordinate variables are included.

New in version (cfdm):

omit_data: (sequence of) str, optional

Do not write the data of the named construct types.

This does not affect the amount of netCDF variables and dimensions that are written to the file, nor the netCDF variables’ attributes, but does not create data on disk for the requested variables. The resulting file will be smaller than it otherwise would have been, and when the new file is read then the data of these variables will be represented by an array of all missing data.

The omit_data parameter may be one, or a sequence, of:


Construct types


Field constructs


Field ancillary constructs


Domain ancillary constructs


Dimension coordinate constructs


Auxiliary coordinate constructs


Cell measure constructs


All of the above constructs

Parameter example:

To omit the data from only field constructs: omit_data='field' or omit_data=['field'].

Parameter example:

To omit the data from domain ancillary and cell measure constructs: omit_data=['domain_ancillary', 'cell_measure'].

New in version (cfdm):

_implementation: (subclass of) CFDMImplementation, optional

Define the CF data model implementation that defines field and metadata constructs and their components.




>>> cfdm.write(f, '')
>>> cfdm.write(f, '', fmt='NETCDF3_CLASSIC')
>>> cfdm.write(f, '', external='')
>>> cfdm.write(f, '', Conventions='CMIP-6.2')