Tutorial

Exploring EGADS

The simplest way to start working with EGADS is to run it from the Python command line. To load EGADS into the Python name-space, simply import it:

>>> import egads

You may then begin working with any of the algorithms and functions contained in EGADS.

There are several useful methods to explore the routines contained in EGADS. The first is using the Python built-in dir() command:

>>> dir(egads)

returns all the classes and subpackages contained in EGADS. EGADS follows the naming conventions from the Python Style Guide (http://www.python.org/dev/peps/pep-0008), so classes are always MixedCase, functions and modules are generally lowercase or lowercase_with_underscores. As a further example,

>>> dir(egads.input)

would returns all the classes and subpackages of the egads.input module.

Another way to explore EGADS is by using tab completion, if supported by your Python installation. Typing

>>> egads.

then hitting TAB will return a list of all available options.

Python has built-in methods to display documentation on any function known as docstrings. The easiest way to access them is using the help() function:

>>> help(egads.input.NetCdf)

or

>>> egads.input.NetCdf?

will return all methods and their associated documentation for the NetCdf class.

Simple operations with EGADS

To have a list of file in a directory, use the following function:

>>> egads.input.get_file_list('path/to/all/netcdf/files/*.nc')

The EgadsData class

At the core of the EGADS package is a data class intended to handle data and associated metadata in a consistent way between files, algorithms and within the framework. This ensures that important metadata is not lost when combining data form various sources in EGADS.

Additionally, by subclassing the Quantities and Numpy packages, EgadsData incorporates unit comprehension to reduce unit-conversion errors during calculation, and supports broad array manipulation capabilities. This section describes how to employ the EgadsData class in the EGADS program scope.

Creating EgadsData instances

The EgadsData class takes four basic arguments:

  • value
    Value to assign to EgadsData instance. Can be scalar, array, or other EgadsData instance.
  • units
    Units to assign to EgadsData instance. Should be string representation of units, and can be a compound units type such as ‘g/kg’, ‘m/s^2’, ‘feet/second’, etc.
  • variable metadata
    An instance of the VariableMetadata type or dictionary, containing keywords and values of any metadata to be associated with this EgadsData instance.
  • other attributes
    Any other attributes added to the class are automatically stored in the VariableMetadata instance associated with the EgadsData instance.

The following are examples of creating EgadsData instances:

>>> x = egads.EgadsData([1,2,3], 'm')
>>> a = [1,2,3,4]
>>> b = egads.EgadsData(a, 'km', b_metadata)
>>> c = egads.EgadsData(28, 'degC', long_name="current temperature")

If, during the call to EgadsData, no units are provided, but a variable metadata instance is provided with a units property, this will then be used to define the EgadsData units:

>>> x_metadata = egads.core.metadata.VariableMetadata({'units':'m', 'long_name':'Test Variable'})
>>> x = egads.EgadsData([1,2,3], x_metadata)
>>> print x.units
m
>>> print x.metadata
{'units': 'm', 'long_name': 'Test Variable'}

The EgadsData is a subclass of the Quantities and Numpy packages. Thus it allows different kind of operations like addition, substraction, slicing, and many more. For each of those operations, a new EgadsData class is created with all their attributes.

Note

With mathematical operands, as metadata define an EgadsData before an operation, and may not reflect the new EgadsData, it has been decided to not keep the metadata attribute. It’s the responsability of the user to add a new VariableMetadata instance or a dictionary to the new EgadsData object. It is not true if a user wants to slice an EgadsData. In that case, metadata are automatically attributed to the new EgadsData.

Metadata

The metadata object used by EgadsData is an instance of VariableMetadata, a dictionary object containing methods to recognize, convert and validate known metadata types. It can reference parent metadata objects, such as those from an algorithm or data file, to enable users to track the source of a particular variable.

When reading in data from a supported file type (NetCDF, NASA Ames), or doing calculations with an EGADS algorithm, EGADS will automatically populate the associated metadata and assign it to the output variable. However, when creating an EgadsData instance manually, the metadata must be user-defined.

As mentioned, VariableMetadata is a dictionary object, thus all metadata are stored as keyword:value pairs. To create metadata manually, simply pass in a dictionary object containing the desired metadata:

>>> var_metadata_dict = {'long_name':'test metadata object',
                         '_FillValue':-9999}
>>> var_metadata = egads.core.metadata.VariableMetadata(var_metadata_dict)

To take advantage of its metadata recognition capabilities, a conventions keyword can be passed with the variable metadata to give a context to these metadata.

>>> var_metadata = egads.core.metadata.VariableMetadata(var_metadata_dict, conventions='CF-1.0')

If a particular VariableMetadata object comes from a file or algorithm, the class attempts to assign the conventions automatically. While reading from a file, for example, the class attempts to discover the conventions used based on the “Conventions” keyword, if present.

Working with units

EgadsData subclasses Quantities, thus all of the latter’s unit comprehension methods are available when using EgadsData. This section will outline the basics of unit comprehension. A more detailed tutorial of the unit comprehension capabilities can be found at http://packages.python.org/quantities/

In general, units are assigned to EgadsData instances when they are being created.

>>> a = egads.EgadsData([1,2,3], 'm')
>>> b = egads.EgadsData([4,5,6], 'meters/second')

Once a unit type has been assigned to an EgadsData instance, it will remain that class of unit and can only be converted between other types of that same unit. The rescale method can be used to convert between similar units, but will give an error if an attempt is made to convert to non-compatible units.

>>> a = egads.EgadsData([1,2,3], 'm')
>>> a_km = a.rescale('km')
>>> print a_km
['EgadsData', array([0.001, 0.002, 0.003]), 'km']
>>> a_grams = a.rescale('g')
ValueError: Unable to convert between units of "m" and "g"

Likewise, arithmetic operations between EgadsData instances are handled using the unit comprehension provided by Quantities, and behave . For example adding units of a similar type is permitted:

>>> a = egads.EgadsData(10, 'm')
>>> b = egads.EgadsData(5, 'km')
>>> a + b
['EgadsData', array(5010.0), 'm']

But, non-compatible types cannot be added. They can, however, be multiplied or divided:

>>> distance = egads.EgadsData(10, 'm')
>>> time = egads.EgadsData(1, 's')
>>> distance + time
ValueError: Unable to convert between units of "s" and "m"
>>> distance/time
['EgadsData', array(10), 'm/s']

Working with raw text files

EGADS provides the egads.input.text_file_io.EgadsFile class as a simple wrapper for interacting with generic text files. EgadsFile can read, write and display data from text files, but does not have any tools for automatically formatting input or output data.

Opening

To open a text file the EgadsFile class, use the open(filename, permissions)() method:

>>> import egads
>>> f = egads.input.EgadsFile()
>>> f.open('/pathname/filename.txt','r')

Valid values for permissions are:

  • r – Read: opens file for reading only. Default value if nothing is provided.
  • w – Write: opens file for writing, and overwrites data in file.
  • a – Append: opens file for appending data.
  • r+ – Read and write: opens file for both reading and writing.

File Manipulation

The following methods are available to control the current position in the file and display more information about the file.

  • f.display_file() – Prints contents of file out to standard output.
  • f.get_position() – Returns current position in file as integer.
  • f.seek(location, from_where) – Seeks to specified location in file. location is an integer specifying how far to seek. Valid options for from_where are b to seek from beginning of file, c to seek from current position in file and e to seek from the end of the file.
  • f.reset() – Resets position to beginning of file.

Reading Data

Reading data is done using the read(size) method on a file that has been opened with r or r+ permissions:

>>> import egads
>>> f = egads.input.EgadsFile()
>>> f.open('myfile.txt','r')
>>> single_char_value = f.read()
>>> multiple_chars = f.read(10)

If the size parameter is not specified, the read() function will input a single character from the open file. Providing an integer value n as the size parameter to read(size) will return n characters from the open file.

Data can be read line-by-line from text files using read_line():

>>> line_in = f.read_line()

Writing Data

To write data to a file, use the write(data) method on a file that has been opened with w, a or r+ permissions:

>>> import egads
>>> f = egads.input.EgadsFile()
>>> f.open('myfile.txt','a')
>>> data = 'Testing output data to a file.\n This text will appear on the 2nd line.'
>>> f.write(data)

Closing

To close a file, simply call the close() method:

>>> f.close()

Tutorial

Here is a basic ASCII file, created by EGADS:

# The current file has been created with EGADS
# Institution: My Institution
# Author(s): John Doe
time    sea level    corr sea level
1.0    5.0    1.0
2.0    2.0    3.0
3.0    -2.0    -1.0
4.0    0.5    2.5
5.0    4.0    6.0

This file has been created with the following commands:

  • import EGADS module:

    >>> import egads
    
  • create two main variables, following the official EGADS convention:

    >>> data1 = egads.EgadsData(value=[5.0,2.0,-2.0,0.5,4.0], units='mm', name='sea level', scale_factor=1., add_offset=0., _FillValue=-9999)
    >>> data2 = egads.EgadsData(value=[1.0,3.0,-1.0,2.5,6.0], units='mm', name='corr sea level', scale_factor=1., add_offset=0., _FillValue=-9999)
    
  • create an independant variable, still by following the official EGADS convention:

    >>> time = egads.EgadsData(value=[1.0,2.0,3.0,4.0,5.0], units='seconds since 19700101T00:00:00', name='time')
    
  • create a new EgadsFile instance:

    >>> f = egads.input.EgadsFile()
    
  • use the following function to open a new file:

    >>> f.open('main_raw_file.dat', 'w')
    
  • prepare the headers if necessary:

    >>> headers = '# The current file has been created with EGADS\n# Institution: My Institution\n# Author(s): John Doe\n'
    >>> headers += time.metadata["long_name"] + '    ' + data1.metadata["long_name"] + '    ' + data2.metadata["long_name"] + '\n''My institution')
    
  • prepare an object to receive all data:

    >>> data = ''
    >>> for i, _ in enumerate(time.value):
        ... data += str(time.value[i]) + '    ' + str(data1.value[i]) + '    ' + str(data2.value[i]) + '\n'
    
  • write the headers and data into the file

    >>> f.write(headers)
    >>> f.write(data)
    
  • and do not forget to close the file:

    >>> f.close()
    

Working with CSV files

egads.input.text_file_io.EgadsCsv is designed to easily input or output data in CSV format. Data input using EgadsCsv is separated into a list of arrays, which each column a separate array in the list.

Opening

To open a text file the EgadsCsv class, use the open(pathname, permissions, delimiter, quotechar) method:

>>> import egads
>>> f = egads.input.EgadsCsv()
>>> f.open('/pathname/filename.txt','r',',','"')

Valid values for permissions are:

  • r – Read: opens file for reading only. Default value if nothing is provided.
  • w – Write: opens file for writing, and overwrites data in file.
  • a – Append: opens file for appending data.
  • r+ – Read and write: opens file for both reading and writing.

The delimiter argument is a one-character string specifying the character used to separate fields in the CSV file to be read; the default value is ,. The quotechar argument is a one-character string specifying the character used to quote fields containing special characters in the CSV file to to be read; the default value is ".

File Manipulation

The following methods are available to control the current position in the file and display more information about the file.

  • f.display_file() – Prints contents of file out to standard output.
  • f.get_position() – Returns current position in file as integer.
  • f.seek(location, from_where) – Seeks to specified location in file. location is an integer specifying how far to seek. Valid options for from_where are b to seek from beginning of file, c to seek from current position in file and e to seek from the end of the file.
  • f.reset() – Resets position to beginning of file.

Reading Data

Reading data is done using the read(lines, format) method on a file that has been opened with r or r+ permissions:

>>> import egads
>>> f = egads.input.EgadsCsv()
>>> f.open('mycsvfile.csv','r')
>>> single_line_as_list = f.read(1)
>>> all_lines_as_list = f.read()

read(lines, format) returns a list of the items read in from the CSV file. The arguments lines and format are optional. If lines is provided, read(lines, format) will read in the specified number of lines, otherwise it will read the whole file. format is an optional list of characters used to decompose the elements read in from the CSV files to their proper types. Options are:

  • i – int
  • f – float
  • l – long
  • s – string

Thus to read in the line:

FGBTM,20050105T143523,1.5,21,25

the command to input with proper formatting would look like this:

>>> data = f.read(1, ['s','s','f','f'])

Writing Data

To write data to a file, use the write(data) method on a file that has been opened with w, a or r+ permissions:

>>> import egads
>>> f = egads.input.EgadsCsv()
>>> f.open('mycsvfile.csv','a')
>>> titles = ['Aircraft ID','Timestamp','Value1','Value2','Value3']
>>> f.write(titles)

where the titles parameter is a list of strings. This list will be output to the CSV, with each strings separated by the delimiter specified when the file was opened (default is ,).

To write multiple lines out to a file, writerows(data) is used:

>>> data = [['FGBTM','20050105T143523',1.5,21,25],['FGBTM','20050105T143524',1.6,20,25.6]]
>>> f.writerows(data)

Closing

To close a file, simply call the close() method:

>>> f.close()

Tutorial

Here is a basic CSV file, created by EGADS:

time,sea level,corrected sea level
1.0,5.0,1.0
2.0,2.0,3.0
3.0,-2.0,-1.0
4.0,0.5,2.5
5.0,4.0,6.0

This file has been created with the following commands:

  • import EGADS module:

    >>> import egads
    
  • create two main variables, following the official EGADS convention:

    >>> data1 = egads.EgadsData(value=[5.0,2.0,-2.0,0.5,4.0], units='mm', name='sea level', scale_factor=1., add_offset=0., _FillValue=-9999)
    >>> data2 = egads.EgadsData(value=[1.0,3.0,-1.0,2.5,6.0], units='mm', name='corr sea level', scale_factor=1., add_offset=0., _FillValue=-9999)
    
  • create an independant variable, still by following the official EGADS convention:

    >>> time = egads.EgadsData(value=[1.0,2.0,3.0,4.0,5.0], units='seconds since 19700101T00:00:00', name='time')
    
  • create a new EgadsFile instance:

    >>> f = egads.input.EgadsCsv()
    
  • use the following function to open a new file:

    >>> f.open('main_csv_file.csv','w',',','"')
    
  • prepare the headers if necessary:

    >>> headers = ['time', 'sea level', 'corrected sea level']
    
  • prepare an object to receive all data:

    >>> data = [time.value, data1.value, data2.value]
    
  • write the headers and data into the file

    >>> f.write(headers)
    >>> f.write(data)
    
  • and do not forget to close the file:

    >>> f.close()
    

Working with NetCDF files

EGADS provides two classes to work with NetCDF files. The simplest, egads.input.netcdf.NetCdf, allows simple read/write operations to NetCDF files. The other, egads.input.netcdf.EgadsNetCdf, is designed to interface with NetCDF files conforming to the EUFAR Standards & Protocols data and metadata regulations. This class directly reads or writes NetCDF data using instances of the EgadsData class.

Opening

To open a NetCDF file, simply create a NetCdf() instance and then use the open(pathname, permissions) command:

>>> import egads
>>> f = egads.input.NetCdf()
>>> f.open('/pathname/filename.nc','r')

Valid values for permissions are:

  • r – Read: opens file for reading only. Default value if nothing is provided.
  • w – Write: opens file for writing, and overwrites data in file.
  • a – Append: opens file for appending data.
  • r+ – Same as a.

Getting info

  • f.get_dimension_list() – returns a list of all dimensions and their sizes
  • f.get_dimension_list(var_name)var_name is optional and if provided, the function returns a list of all dimensions and their sizes attached to var_name
  • f.get_attribute_list() – returns a list of all top-level attributes
  • f.get_attribute_list(var_name)var_name is optional and if provided, the function returns a list of all attributes attached to var_name
  • f.get_variable_list() – returns list of all variables
  • f.get_filename() – returns filename for currently opened file
  • f.get_perms() – returns the current permissions on the file that is open

Reading data

To read data from a file, use the read_variable() function:

>>> data = f.read_variable(var_name, input_range)

where var_name is the name of the variable to read in, and input_range (optional) is a list of min/max values.

If using the egads.input.NetCdf() class, an array of values contained in var_name will be returned. If using the egads.input.EgadsNetCdf() class, an instance of the EgadsData class will be returned containing the values and attributes of var_name.

Writing data

The following describe how to add dimensions or attributes to a file.

  • f.add_dim(dim_name, dim_size) – add dimension to file
  • f.add_attribute(attr_name, attr_value) – add attribute to file
  • f.add_attribute(attr_name, attr_value, var_name)var_name is optional and if provided, the function add attribute to var_name

Data can be output to variables using the write_variable() function as follows:

>>> f.write_variable(data, var_name, dims, type)

where var_name is a string for the variable name to output, dims is a tuple of dimension names (not needed if the variable already exists), and type is the data type of the variable. The default value is double, other valid options are float, int, short, char and byte.

If using NetCdf, values for data passed into write_variable must be scalar or array. Otherwise, if using EgadsNetCdf, an instance of EgadsData must be passed into write_variable. In this case, any attributes that are contained within the EgadsData instance are applied to the NetCDF variable as well.

Conversion from NetCDF to NASA/Ames file format

The conversion is only possible on opened NetCDF files. If modifications have been made and haven’t been saved, the conversion won’t take into account those modifications. Actually, the only File Format Index supported by the conversion in the NASA/Ames format is 1001. Consequently, if variables depend on multiple independant variables (e.g. data is function of time, longitude and latitude), the file won’t be converted and the function will raise an exception. On the contrary, if multiple independant variables (or dimensions) exist, and if each variable depend on only one independant variable (e.g. data is only function of time), the file will be converted and the function will generate one file per independant variable. If the user needs to convert a complex file with variables depending on multiple independant variables, the conversion should be done manually by creating a NASA/Ames instance and a NASA/Ames dictionary, by populating the dictionary and by saving the file.

To convert a NetCDF file, simply use:

  • f.convert_to_nasa_ames() – convert the currently opened NetCDF file to NASA/Ames file format
  • f.convert_to_nasa_ames(na_file, requested_ffi, float_format, delimiter, no_header)na_file, requested_ffi, float_format, delimiter and no_header are optional parameters ; na_file is the name of the output file once it has been converted, by default the name of the NetCDF file will be used with the extension .na ; requested_ffi is not used actually, but will be functional in a next version of EGADS ; float_format is the formatting string used for formatting floats when writing to output file, by default %g ; delimiter is a character or a sequence of character for use between data items in the data file, by default ‘ ‘ (four spaces) ; if no_header is set to True, then only the data blocks are written to file, by default False

To convert a NetCDF file to NASA/Ames CSV format, simply use:

  • f.convert_to_csv() – convert the currently opened NetCDF file to NASA/Ames CSV format
  • f.convert_to_csv(csv_file, float_format, no_header)csv_file, float_format and no_header are optional parameters ; csv_file is the name of the output file once it has been converted, by default the name of the NetCDF file will be used with the extension .csv ; float_format is the formatting string used for formatting floats when writing to output file, by default %g ; if no_header is set to True, then only the data blocks are written to file, by default False

Other operations

  • f.get_attribute_value(attr_name) – returns the value of a global attribute
  • f.get_attribute_value(attr_name, var_name)var_name is optional and if provided, the function returns the value of an attribute attached to var_name
  • f.change_variable_name(var_name, new_name) – change the variable name in currently opened NetCDF file

Closing

To close a file, simply use the close() method:

>>> f.close()

Note

The EGADS NetCdf and EgadsNetCdf use the official NetCDF I/O routines, therefore, as described in the NetCDF documentation, it is not possible to remove a variable or more, and to modify the values of a variable. As attributes, global and those linked to a variable, are more dynamic, it is possible to remove, rename, or replace them.

Tutorial

Here is a NetCDF file, created by EGADS, and viewed by the command ncdump -h ....:

=> ncdump -h main_netcdf_file.nc 
    netcdf main_netcdf_file {
    dimensions:
        time = 5 ;
    variables:
        double time(time) ;
            time:units = "seconds since 19700101T00:00:00" ;
            time:long_name = "time" ;
        double sea_level(time) ;
            sea_level:_FillValue = -9999. ;
            sea_level:category = "TEST" ;
            sea_level:scale_factor = 1. ;
            sea_level:add_offset = 0. ;
            sea_level:long_name = "sea level" ;
            sea_level:units = "mm" ;
        double corrected_sea_level(time) ;
            corrected_sea_level:_FillValue = -9999. ;
            corrected_sea_level:units = "mm" ;
            corrected_sea_level:add_offset = 0. ;
            corrected_sea_level:scale_factor = 1. ;
            corrected_sea_level:long_name = "corr sea level" ;

    // global attributes:
            :Conventions = "CF-1.0" ;
            :history = "the netcdf file has been created by EGADS" ;
            :comments = "no comments on the netcdf file" ;
            :institution = "My institution" ;
    }

This file has been created with the following commands:

  • import EGADS module:

    >>> import egads
    
  • create two main variables, following the official EGADS convention:

    >>> data1 = egads.EgadsData(value=[5.0,2.0,-2.0,0.5,4.0], units='mm', name='sea level', scale_factor=1., add_offset=0., _FillValue=-9999)
    >>> data2 = egads.EgadsData(value=[1.0,3.0,-1.0,2.5,6.0], units='mm', name='corr sea level', scale_factor=1., add_offset=0., _FillValue=-9999)
    
  • create an independant variable, still by following the official EGADS convention:

    >>> time = egads.EgadsData(value=[1.0,2.0,3.0,4.0,5.0], units='seconds since 19700101T00:00:00', name='time')
    
  • create a new EgadsNetCdf instance with a file name:

    >>> f = egads.input.EgadsNetCdf('main_netcdf_file.nc', 'w')
    
  • add the global attributes to the NetCDF file:

    >>> f.add_attribute('Conventions', 'CF-1.0')
    >>> f.add_attribute('history', 'the netcdf file has been created by EGADS')
    >>> f.add_attribute('comments', 'no comments on the netcdf file')
    >>> f.add_attribute('institution', 'My institution')
    
  • add the dimension(s) of your variable(s), here it is time:

    >>> f.add_dim('time', len(time))
    
  • write the variable(s), it is a good practice to write at the first place the independant variable time:

    >>> f.write_variable(time, 'time', ('time',), 'double')
    >>> f.write_variable(data1, 'sea_level', ('time',), 'double')
    >>> f.write_variable(data2, 'corrected_sea_level', ('time',), 'double')
    
  • and do not forget to close the file:

    >>> f.close()
    

Working with NASA Ames files

To work with NASA Ames files, EGADS Lineage incorporates its own library. Thus, from EGADS, NASA Ames files can be accessed via the egads.input.nasa_ames_io.NasaAmes class. Actually, only the FFI 1001 has been interfaced with EGADS.

Opening

To open a NASA Ames file, simply create a NasaAmes() instance and then use the open(pathname, permissions) command:

>>> import egads
>>> f = egads.input.NasaAmes()
>>> f.open('/pathname/filename.na','r')

Valid values for permissions are:

  • r – Read: opens file for reading only. Default value if nothing is provided.
  • w – Write: opens file for writing, and overwrites data in file.
  • a – Append: opens file for appending data.
  • r+ – Same as a.

Once a file has been opened, a dictionary of NASA/Ames format elements is loaded into memory. That dictionary will be used to overwrite the file or to save to a new file.

Getting info

  • f.get_attribute_list() – returns a list of all top-level attributes
  • f.get_attribute_list(var_name, var_type, na_dict)var_name is optional and if provided, the function returns list of all attributes attached to var_name ; if var_type is provided the function will search in the variable type var_type by default ; na_dict is optional if provided, will return a list of all top-level attributes, or all var_name attributes, in the NASA/Ames dictionary na_dict
  • f.get_attribute_value(attr_name) – returns the value of a global attribute named attr_name
  • f.get_attribute_value(attr_name, var_name, var_type, na_dict)var_name, var_type and na_dict are optional ; if var_name is provided, returns the value of an attribute named attr_name attached to a variable named var_name ; if var_type is provided, the function will search in the variable type var_type by default ; if na_dict is provided, returns the attribute value from the NASA/Ames dictionary na_dict
  • f.get_dimension_list() – returns a list of all variable dimensions
  • f.get_dimension_list(na_dict, var_type)var_type is optional, if provided, the function returns a list of all variable dimensions based on the var_type by default ; na_dict is optional and will returns the dimension list from the NASA/Ames dictionary na_dict ;
  • f.get_variable_list() – returns list of all variables ;
  • f.get_variable_list(na_dict)na_dict is optional and if provided, will return a list of all variables in the NASA/Ames dictionary na_dict
  • f.get_filename() – returns filename for currently opened file

Reading data

To read data from a file, use the read_variable() function:

>>> data = f.read_variable(var_name)

where var_name is the name of the variable to read in. The data will be read in to an instance of the EgadsData class, containing the values and attributes of var_name.

Writing data

To write data to the current file or to a new file, the user must save a dictionary of NASA/Ames elements. Few functions are available to help him to prepare the dictionary:

  • f.create_na_dict – create a new dictionary populated with standard NASA/Ames keys.
  • f.write_attribute_value(attr_name, attr_value) – write or replace a specific attribute (from the official NASA/Ames attribute list) in the currently opened dictionary
  • f.write_attribute_value(attr_name, attr_value, var_name, var_type, na_dict)var_name and var_type are optional, if provided, write or replace a specific attribute linked to the variable var_name (var_type is by default equal to ‘main’) in the currently opened dictionary ; ccepted attributes for a variable are ‘name’, ‘units’, ‘_FillValue’ and ‘scale_factor’, other attributes will be refused and should be passed as ‘special comments’ ; na_dict is optional and if provided the function will write the attribute in the NASA/Ames dictionary na_dict
  • f.write_variable(data, var_name) – write or replace a variable ; the function will search if data is already in the dictionary by comparing varname with other variable names in the dictionary, if it is found, data will replace the old variable, if not data is considered as a new variable ; data can be an EgadsData or a vector/matrix.
  • f.write_variable(data, var_name, var_type, attr_dict, na_dict)var_type, attr_dict and na_dict are optional ; attr_dict (a dictionary of standard NASA/ames variable attributes: ‘name’, ‘units’, ‘_FillValue’ and ‘scale_factor’) must be provided if data is not an EgadsData (in that case, variable attributes are retrieve from the EgadsData.metadata dictionary) ; if na_dict is provided, the function saves the variable in the NASA/Ames dictionary na_dict

Saving a file

Once a dictionary is ready, use the save_na_file() function to save the file:

>>> data = f.save_na_file(file_name, na_dict, float_format):

where file_name is the name of the new file or the name of the current file, na_dict the name of the dictionary to be saved (optional, if not provided, the current dictionary will be used), and float_format the format of the floating numbers in the file (by deffault, two decimal places).

Conversion from NASA/Ames file format to NetCDF

When a NASA/Ames file is opened, all metadata and data are read and stored in memory in a dedicated dictionary. The conversion will convert that dictionary to generate a NetCDF file. If modifications are made to the dictionary, the conversion will take into account those modifications. Actually, the only File Format Index supported by the conversion in the NASA/Ames format is 1001. Consequently, if variables depend on multiple independant variables (e.g. data is function of time, longitude and latitude), the file won’t be converted and the function will raise an exception. If the user needs to convert a complex file with variables depending on multiple independant variables, the conversion should be done manually by creating a NetCDF instance and by populating the NetCDF files with NASA/Ames data and metadata.

To convert a NASA/Ames file, simply use:

  • f.convert_to_netcdf() – convert the currently opened NASA/Ames file to NetCDF format.
  • f.convert_to_netcdf(nc_file)nc_file is an optional parameter ; na_file is the name of the output file once it has been converted, by default the name of the NASA/Ames file will be used with the extension .nc

Other operations

  • f.read_na_dict() – returns a deep copy of the current opened file dictionary
  • f.na_format_information() – returns a text explaining the structure of a NASA/Ames file to help the user to modify or to create his own dictionary

Closing

To close a file, simply use the close() method:

>>> f.close()

Tutorial

Here is a NASA/Ames file:

23    1001
John Doe
An institution
tide gauge
ATESTPROJECT
1    1
2017 1 30    2017 1 30
0.0
time (seconds since 19700101T00:00:00)
2
1    1
-9999    -9999
sea level (mm)
corr sea level (mm)
3
========SPECIAL COMMENTS===========
this file has been created with egads
=========END=========
4
========NORMAL COMMENTS===========
headers:
time    sea level   corrected sea level
=========END=========
1.00    5.00    1.00
2.00    2.00    3.00
3.00    -2.00    -1.00
4.00    0.50    2.50
5.00    4.00    6.00

This file has been created with the following commands:

  • import EGADS module:

    >>> import egads
    
  • create two main variables, following the official EGADS convention:

    >>> data1 = egads.EgadsData(value=[5.0,2.0,-2.0,0.5,4.0], units='mm', name='sea level', scale_factor=1, _FillValue=-9999)
    >>> data2 = egads.EgadsData(value=[1.0,3.0,-1.0,2.5,6.0], units='mm', name='corr sea level', scale_factor=1, _FillValue=-9999)
    
  • create an independant variable, still by following the official EGADS convention:

    >>> time = egads.EgadsData(value=[1.0,2.0,3.0,4.0,5.0], units='seconds since 19700101T00:00:00', name='time')
    
  • create a new NASA/Ames empty instance:

    >>> f = egads.input.NasaAmes()
    
  • initialize a new NASA/Ames dictionary:

    >>> na_dict = f.create_na_dict()
    
  • prepare the normal and special comments if needed, in a list, one cell for each line, or only one string with lines separated by \n:

    >>> scom = ['========SPECIAL COMMENTS===========','this file has been created with egads','=========END=========']
    >>> ncom = ['========NORMAL COMMENTS===========','headers:','time    sea level   corrected sea level','=========END=========']
    or
    >>> scom = '========SPECIAL COMMENTS===========\nthis file has been created with egads\n=========END========='
    >>> ncom = '========NORMAL COMMENTS===========\nheaders:\ntime    sea level   corrected sea level\n=========END========='
    
  • populate the main NASA/Ames attributes:

    >>> f.write_attribute_value('ONAME', 'John Doe', na_dict = na_dict) # ONAME is the name of the author(s)
    >>> f.write_attribute_value('ORG', 'An institution', na_dict = na_dict) # ORG is tne name of the organization responsible for the data
    >>> f.write_attribute_value('SNAME', 'tide gauge', na_dict = na_dict) # SNAME is the source of data (instrument, observation, platform, ...)
    >>> f.write_attribute_value('MNAME', 'ATESTPROJECT', na_dict = na_dict) # MNAME is the name of the mission, campaign, programme, project dedicated to data
    >>> f.write_attribute_value('DATE', [2017, 1, 30], na_dict = na_dict) # DATE is the date at which the data recorded in this file begin (YYYY MM DD)
    >>> f.write_attribute_value('NIV', 1, na_dict = na_dict) # NIV is the number of independent variables
    >>> f.write_attribute_value('NSCOML', 3, na_dict = na_dict) # NSCOML is the number of special comments lines or the number of elements in the SCOM list
    >>> f.write_attribute_value('NNCOML', 4, na_dict = na_dict) # NNCOML is the number of special comments lines or the number of elements in the NCOM list
    >>> f.write_attribute_value('SCOM', scom, na_dict = na_dict) # SCOM is the special comments attribute
    >>> f.write_attribute_value('NCOM', ncom, na_dict = na_dict) # NCOM is the normal comments attribute
    
  • write each variable in the dictionary:

    >>> f.write_variable(time, 'time', vartype="independant", na_dict = na_dict)
    >>> f.write_variable(data1, 'sea level', vartype="main", na_dict = na_dict)
    >>> f.write_variable(data2, 'corrected sea level', vartype="main", na_dict = na_dict)
    
  • and finally, save the dictionary to a NASA/Ames file:

    >>> f.save_na_file('na_example_file.na', na_dict)
    

Converting between file formats

Two functions have been introduced to allow a conversion from NetCDF to NASA/Ames format, and from NASA/Ames format to NetCDF. Please read the section about NetCDF and NASA/Ames file handling to learn how to convert between those formats.

Working with algorithms

Algorithms in EGADS are stored in the egads.algorithms module, and separated into sub-modules by category (microphysics, thermodynamics, radiation, etc). Each algorithm follows a standard naming scheme, using the algorithm’s purpose and source:

{CalculatedParameter}{Detail}{Source}

For example, an algorithm which calculates static temperature, which was provided by CNRM would be named:

TempStaticCnrm

Getting algorithm information

There are several methods to get information about each algorithm contained in EGADS. The EGADS Algorithm Handbook is available for easy reference outside of Python. In the handbook, each algorithm is described in detail, including a brief algorithm summary, descriptions of algorithm inputs and outputs, the formula used in the algorithm, algorithm source and links to additional references. The handbook also specifies the exact name of the algorithm as defined in EGADS. The handbook can be found on the EGADS website.

Within Python, usage information on each algorithm can be found using the help() command:

>>> help(egads.algorithms.thermodynamics.VelocityTasCnrm)

>>> Help on class VelocityTasCnrm in module egads.algorithms.thermodynamics.
    velocity_tas_cnrm:

class VelocityTasCnrm(egads.core.egads_core.EgadsAlgorithm)
 |  FILE        velocity_tas_cnrm.py
 |
 |  VERSION     $Revision: 104 $
 |
 |  CATEGORY    Thermodynamics
 |
 |  PURPOSE     Calculate true airspeed
 |
 |  DESCRIPTION Calculates true airspeed based on static temperature,
 |              static pressure and dynamic pressure using St Venant's
 |              formula.
 |
 |  INPUT       T_s         vector  K or C      static temperature
 |              P_s         vector  hPa         static pressure
 |              dP          vector  hPa         dynamic pressure
 |              cpa         coeff.  J K-1 kg-1  specific heat of air (dry
 |                                              air is 1004 J K-1 kg-1)
 |              Racpa       coeff.  ()          R_a/c_pa
 |
 |  OUTPUT      V_p         vector  m s-1       true airspeed
 |
 |  SOURCE      CNRM/GMEI/TRAMM
 |
 |  REFERENCES  "Mecanique des fluides", by S. Candel, Dunod.
 |
 |               Bulletin NCAR/RAF Nr 23, Feb 87, by D. Lenschow and
 |               P. Spyers-Duran
 |
...

Calling algorithms

Algorithms in EGADS generally accept and return arguments of EgadsData type, unless otherwise noted. This has the advantages of constant typing between algorithms, and allows metadata to be passed along the whole processing chain. Units on parameters being passed in are also checked for consistency, reducing errors in calculations, and rescaled if needed. However, algorithms will accept any normal data type, as well. They can also return non-EgadsData instances, if desired.

To call an algorithm, simply pass in the required arguments, in the order they are described in the algorithm help function. An algorithm call, using the VelocityTasCnrm in the previous section as an example, would therefore be the following:

>>> V_p = egads.algorithms.thermodynamics.VelocityTasCnrm().run(T_s, P_s, dP,
    cpa, Racpa)

where the arguments T_s, P_s, dP, etc are all assumed to be previously defined in the program scope. In this instance, the algorithm returns an EgadsData instance to V_p. To run the algorithm, but return a standard data type (scalar or array of doubles), set the return_Egads flag to false.

>>> V_p = egads.algorithms.thermodynamics.VelocityTasCnrm(return_Egads=false).
    run(T_s, P_s, dP, cpa, Racpa)

Note

When injecting a variable in an EgadsAlgorithm, the format of the variable should follow closely the documentation of the algorithm. If the variable is a scalar, and the algorithm needs a vector, the scalar should be surrounded by brackets: 52.123 -> [52.123].

Scripting

The recommended method for using EGADS is to create script files, which are extremely useful for common or repetitive tasks. This can be done using a text editor of your choice. The example script belows shows the calculation of density for all NetCDF files in a directory.

#!/usr/bin/env python

# import egads package
import egads			
# import thermodynamic module and rename to simplify usage
import egads.algorithms.thermodynamics as thermo

# get list of all NetCDF files in 'data' directory
filenames = egads.input.get_file_list('data/*.nc')
f = egads.input.EgadsNetCdf()   # create EgadsNetCdf instance

for name in filenames:          # loop through files

    f.open(name, 'a')            # open NetCdf file with append permissions
    T_s = f.read_variable('T_t') # read in static temperature
    P_s = f.read_variable('P_s') # read in static pressure from file
    rho = thermo.DensityDryAirCnrm().run(P_s, T_s)  # calculate density
    f.write_variable(rho, 'rho', ('Time',))      # output variable

    f.close()                                    # close file

Scripting Hints

When scripting in Python, there are several important differences from other programming languages to keep in mind. This section outlines a few of these differences.

Importance of white space

Python differs from C++ and Fortran in how loops or nested statements are signified. Whereas C++ uses brackets (‘{‘ and ‘}’) and FORTRAN uses end statements to signify the end of a nesting, Python uses white space. Thus, for statements to nest properly, they must be set at the proper depth. As long as the document is consistent, the number of spaces used doesn’t matter, however, most conventions call for 4 spaces to be used per level. See below for examples:

FORTRAN:

X = 0
DO I = 1,10
  X = X + I
  PRINT I
END DO
PRINT X

Python:

x = 0
for i in range(1,10):
    x = x + i
    print i
print x

Using the GUI

Since September 2016, a Graphical User Interface is available at https://github.com/eufarn7sp/egads-gui. It gives the user the possibility to explore data, apply/create algorithms, display and plot data. Still in beta state, the user will have the possibility in the future to work on a batch of file. For now, EGADS GUI comes as a simple python script and need to be launch from the terminal, if EGADS is installed, and once in the EGADS GUI directory:

>>> python egads_gui.py

It will be available soon as a stand alone (imbedding a version of EGADS CORE or using an already installed EGADS package).

Note

As for EGADS, the Graphical User Interface is available from two branches: master and Lineage. The Lineage one is only compatible with Python 3 and the earlier versions of EGADS Lineage.