Package: avro-gen
AVRO-GEN========[![Build Status](https://travis-ci.org/rbystrit/avro_gen.svg?branch=master)](https://travis-ci.org/rbystrit/avro_gen)[![codecov](https://codecov.io/gh/rbystrit/avro_gen/branch/master/graph/badge.svg)](https://codecov.io/gh/rbystrit/avro_gen)##### Avro record class and specific record reader generator.Current Avro implementation in Python is completely typelss and operates on dicts.While in many cases this is convenient and pythonic, not being able to discover the schemaby looking at the code, not enforcing schema during record constructions, and not having anycontext help from the IDE could hamper developer performance and introduce bugs.This project aims to rectify this situation by providing a generator for constructing concreterecord classes and constructing a reader which wraps Avro DatumReader and returns concrete classesinstead of dicts. In order not to violate Avro internals, this functionality is built strictlyon top of the DatumReader and all the specific record classes dict wrappers which define accessorproperties with proper type hints for each field in the schema. For this exact reason thegenerator does not provide an overloaded DictWriter; each specific record appears just to be aregular dictionary.##### Usage:schema_json = "....."output_directory = "....."from avrogen import write_schema_fileswrite_schema_files(schema_json, output_directory)The generator will create output directory if it does not exist and put generated files there.The generated files will be:>  OUTPUT_DIR>  + \_\_init\_\_.py>  + schema_classes.py>  + submodules*In order to deal with Avro namespaces, since python doesn't support circular imports, the generatorwill emit all records into schema_classes.py as nested classes. The top level class there will beSchemaClasses, whose children will be classes representing namespaces. Each namespace class willin turn contain classes for records belonging to that namespace.Consider following schema:{"type": "record", "name": "tweet", "namespace": "com.twitter.avro", "fields": [{"name": "ID", "type": "long" }Then schema_classes.py would contain:class SchemaClasses(object):class com(object):class twitter(object):class acro(object):class tweetClass(DictWrapper):def __init__(self, inner_dict=None):....@propertydef ID(self):""":rtype: long"""return self._inner_dict.get('ID', None)@ID.setterdef ID(self, value):#"""#:param long value:#"""self._inner_dict['ID'] = valueIn order to map specific record types and namespaces to modules, so that proper importing canbe supported, there generator will create a sub-module under the output directory for each namespacewhich will export names of all types contained in that namespace. Types declared with emptynamespace will be exported from the root module.So for the example above, output directory will look as follows:>  OUTPUT_DIR>  + \_\_init\_\_.py>  + schema_classes.py>  + com>   + twitter>     + avro>       + \_\_init\_\_.pyThe contents of OUTPUT_DIR/com/twitter/avro/\_\_init\_\_.py will be:from ....schema_classes import SchemaClassestweet = SchemaClasses.com.twitter.avro.tweetSo in your code you will be able to say:from OUTPUT_DIR.com.twitter.avro import tweetfrom OUTPUT_DIR import SpecificDatumReader as TweetReader, SCHEMA as your_schemafrom avro import datafile, iomy_tweet = tweet()my_tweet.ID = 1with open('somefile', 'w+b') as f:writer = datafile.DataFileWriter(f,io.DatumWriter(), your_schema)writer.append(my_tweet)writer.close()with open('somefile', 'rb') as f:reader = datafile.DataFileReader(f,TweetReader(readers_schema=your_schema))my_tweet1 = reader.next()reader.close()### Avro protocol supportAvro protocol support is implemented the same way as schema support. To generate classesfor a protocol:protocol_json = "....."output_directory = "....."from avrogen import write_protocol_fileswrite_protocol_files(protocol_json, output_directory)The structure of the generated code will be exactly same as for schema, but in addition toregular types, *Request types will be generated in the root namespace of the protocol for eacheach message defined.### Logical types supportAvrogen implements logical types on top of standard avro package and supports generation ofclasses thus typed. To enable logical types support, pass **use_logical_types=True** to schemaand protocol generators. If custom logical types are implemented and such types map to typesother than simple types or datetime.* or decimal.* then pass **custom_imports** parameter togenerator functions so that your types are imported. Types implemented out of the box are:- decimal (using string representation only)- date- time-millis- time-micros- timestamp-millis- timestamp-microsTo register your custom logical type, inherit from avrogen.logical.LogicalTypeProcessor, implementabstract methods, and add an instance to avrogen.logical.DEFAULT_LOGICAL_TYPES dictionary under thename of your logical type. A sample implementation looks as follows:class DateLogicalTypeProcessor(LogicalTypeProcessor):_matching_types = {'int', 'long', 'float', 'double'}def can_convert(self, writers_schema):return isinstance(writers_schema, schema.PrimitiveSchema) and writers_schema.type == 'int'def validate(self, expected_schema, datum):return isinstance(datum, datetime.date)def convert(self, writers_schema, value):if not isinstance(value, datetime.date):raise Exception("Wrong type for date conversion")return (value - EPOCH_DATE).total_seconds() // SECONDS_IN_DAYdef convert_back(self, writers_schema, readers_schema, value):return EPOCH_DATE + datetime.timedelta(days=int(value))def does_match(self, writers_schema, readers_schema):if isinstance(writers_schema, schema.PrimitiveSchema):if writers_schema.type in DateLogicalTypeProcessor._matching_types:return Truereturn Falsedef typename(self):return 'datetime.date'def initializer(self, value=None):return (('logical.DateLogicalTypeProcessor().convert_back(None, None, %s)' % value) if value is not Noneelse 'datetime.datetime.today().date()')To read/write data with logical type support, use generated SpecificDatumReaderand a LogicalDatumWriter from avro.logical.