Getting started

Getting started#

fcollections is a library that aims at reading a collections of files. Its primary goal is to combine the selection, reading and concatenation of files within a common model.

Let’s set up a minimal case with stub data for the SWOT altimetry mission.

import tempfile
import numpy as np
import xarray as xr

# Create stub data
path = tempfile.mkdtemp()
ds = xr.Dataset(data_vars={
    "ssha": (('num_lines', 'num_pixels'), np.random.random((9860, 69))),
    "swh": (('num_lines', 'num_pixels'), np.random.random((9860, 69))),})
ds.to_netcdf(f'{path}/SWOT_L2_LR_SSH_Expert_001_011_20240101T000000_20240101T030000_PGC0_01.nc')
ds.to_netcdf(f'{path}/SWOT_L2_LR_SSH_Expert_001_012_20240101T030000_20240101T060000_PGC0_01.nc')

Implementations#

When confronted to a files collection, the first step is to try and find if an implementation matching the data already exists. Such implementation may be found in the catalog

From the catalog, we can see that NetcdfFilesDatabaseSwotLRL2 matches our file names. In case no implementation is available, the user can build its own following creation procedure.

Listing files#

An implementation can be used by simply giving the path to the data. An important endpoint for the implementation is the ability to list files matching given criterias

from fcollections.implementations import NetcdfFilesDatabaseSwotLRL2
fc = NetcdfFilesDatabaseSwotLRL2(path)
fc.list_files(cycle_number=1)

	cycle_number	pass_number	time	level	subset	version	filename
0	1	11	[2024-01-01T00:00:00.000000, 2024-01-01T03:00:...	ProductLevel.L2	ProductSubset.Expert	PGC0_01	/tmp/tmp7xxvaq0o/SWOT_L2_LR_SSH_Expert_001_011...
1	1	12	[2024-01-01T03:00:00.000000, 2024-01-01T06:00:...	ProductLevel.L2	ProductSubset.Expert	PGC0_01	/tmp/tmp7xxvaq0o/SWOT_L2_LR_SSH_Expert_001_012...

Listing files using filters is the first step toward subsetting the files set.

Query data#

Another important endpoint is the ability to read the file contents using the query method.

fc.query()

<xarray.Dataset> Size: 22MB
Dimensions:       (num_lines: 19720, num_pixels: 69)
Dimensions without coordinates: num_lines, num_pixels
Data variables:
    ssha          (num_lines, num_pixels) float64 11MB dask.array<chunksize=(9860, 69), meta=np.ndarray>
    swh           (num_lines, num_pixels) float64 11MB dask.array<chunksize=(9860, 69), meta=np.ndarray>
    cycle_number  (num_lines) uint16 39kB 1 1 1 1 1 1 1 1 1 ... 1 1 1 1 1 1 1 1
    pass_number   (num_lines) uint16 39kB 11 11 11 11 11 11 ... 12 12 12 12 12

The method returns a xarray.Dataset containing the combined data for all files matching the regex specified by the implementation.

It is possible to load only a subset of the data by applying filters in the query. For example, giving the cycle_number and pass_number argument will select one half orbit of our altimetry mission.

fc.query(cycle_number=1, pass_number=11)

<xarray.Dataset> Size: 11MB
Dimensions:       (num_lines: 9860, num_pixels: 69)
Dimensions without coordinates: num_lines, num_pixels
Data variables:
    ssha          (num_lines, num_pixels) float64 5MB dask.array<chunksize=(9860, 69), meta=np.ndarray>
    swh           (num_lines, num_pixels) float64 5MB dask.array<chunksize=(9860, 69), meta=np.ndarray>
    cycle_number  (num_lines) uint16 20kB 1 1 1 1 1 1 1 1 1 ... 1 1 1 1 1 1 1 1
    pass_number   (num_lines) uint16 20kB 11 11 11 11 11 11 ... 11 11 11 11 11

Variable selection is also available to return only part of the data

ds = fc.query(selected_variables=['ssha'])
list(ds.variables)

['ssha', 'cycle_number', 'pass_number']

Each implementation has its own filters. By order of availability, the user should consult:

The Query overview section of the implementation’s Documentation (see the catalog)
The API documentation of the implementation’s method (see the catalog)
The prompted help displayed in a jupyter notebook or Python interpreter

fc.query?

Access metadata#

The database can display information about the variables and attributes contained in the files’ collection using the variables_info method

fc.variables_info(subset='Expert')

Group: /

Dimensions

num_lines	9860
num_pixels	69

Variables

ssha

name	ssha
dtype	float64
dimensions	('num_lines', 'num_pixels')
_FillValue	nan

swh

name	swh
dtype	float64
dimensions	('num_lines', 'num_pixels')
_FillValue	nan

Attributes

It will offer a simple collapsible tree view with multiple levels of nesting depending on the data you manipulate

In order to return consistent metadata, the method ensures that only one homogeneous subset is selected. In case you handle unmixable data (for example Expert and Unsmoothed datasets), you must give proper filters on the subset partitioning keys fc.unmixer.partition_keys. If these filters are missing, an error with the possible choices will be raised.

# Create Unsmoothed file, this will mix Expert and Unsmoothed dataset
ds.to_netcdf(f'{path}/SWOT_L2_LR_SSH_Unsmoothed_001_012_20240101T030000_20240101T060000_PGC0_01.nc')

# This will not work because we don't know if we need to display Expert or
# Unsmoothed metadata
fc.variables_info()

Group: /

Dimensions

num_lines	9860
num_pixels	69

Variables

ssha

name	ssha
dtype	float64
dimensions	('num_lines', 'num_pixels')
_FillValue	nan

swh

name	swh
dtype	float64
dimensions	('num_lines', 'num_pixels')
_FillValue	nan

Attributes

# Use the enumeration name for filtering
fc.variables_info(subset='Expert')

Group: /

Dimensions

num_lines	9860
num_pixels	69

Variables

ssha

name	ssha
dtype	float64
dimensions	('num_lines', 'num_pixels')
_FillValue	nan

swh

name	swh
dtype	float64
dimensions	('num_lines', 'num_pixels')
_FillValue	nan

Attributes

Getting started

Contents

Getting started#

Implementations#

Listing files#

Query data#

Access metadata#