The procedure to call a Python decode method for the different cases described above is basically the same. It involves the following steps:

The first step is the creation of a decode message buffer object. An Asn1JsonDecodeBuffer can be created on any io.TextIOBase object, or you can use the convenience class method from_text to create one from a string of JSON text. For example:

# Create buffer on a JSON text file          
decbuf = Asn1JsonDecodeBuffer(open(filename, 'r'))

# Create a buffer on JSON text
decbuf = Asn1JsonDecodeBuffer.from_text(json_text)
       

The decode method itself is then invoked. In the simple case of a primitive type with no generated class, this involves invoking one of the methods defined in the decode buffer class to decode a primitive value. For example, to decode an integer, do:

value = decbuf.read_int()

This would decode the content at the current decode buffer cursor position, which is expected to be a JSON number representing an integer.

The procedure for invoking a static method is similar. The general form is <classname>.json_decode(decbuf). For example, a class named EmployeeNumber would be generated for the following definition:

EmployeeNumber ::= INTEGER(0..100)

To decode an employee number, do the following:

value = EmployeeNumber.json_decode(decbuf)

This would decode the JSON number and check to ensure the constraint is satisfied. If successful, the decoded integer value will assigned to value.

Finally, to invoke an instance method in a class generated for a constructed type, first create an instance of the class and then invoke the generated decode method:

jSmithName = Name()
jSmithName.json_decode(decbuf)

If successful, the attributes of the jSmithName instance will be populated with the decoded values.

A complete example showing how to invoke a decode method is as follows:

try:   
    # Step 1: create a decode buffer.  This example uses a file for input.           
    f = open(in_filename, 'r')
    decbuf = Asn1JsonDecodeBuffer(f)
    
    # Step 2: create an object of the generated type and invoke the decode
    # method.
    personnelRecord = PersonnelRecord()
    personnelRecord.json_decode(decbuf)
    
    f.close()
     
    personnelRecord.print_value("personnelRecord", 0)
     
except Exception:
    print(traceback.format_exc())
    sys.exit(-1)