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sparkql: Apache Spark SQL DataFrame schema management for sensible humans

Project description

sparkql ✨

PyPI version License: MIT CI codecov

Python Spark SQL DataFrame schema management for sensible humans.

Don't sweat it... sparkql it ✨

Why use sparkql

sparkql takes the pain out of working with DataFrame schemas in PySpark. It makes schema definition more Pythonic. And it's particularly useful you're dealing with structured data.

In plain old PySpark, you might find that you write schemas like this:

CITY_SCHEMA = StructType()
CITY_NAME_FIELD = "name"
CITY_SCHEMA.add(StructField(CITY_NAME_FIELD, StringType(), False))
CITY_LAT_FIELD = "latitude"
CITY_SCHEMA.add(StructField(CITY_LAT_FIELD, FloatType()))
CITY_LONG_FIELD = "longitude"
CITY_SCHEMA.add(StructField(CITY_LONG_FIELD, FloatType()))

CONFERENCE_SCHEMA = StructType()
CONF_NAME_FIELD = "name"
CONFERENCE_SCHEMA.add(StructField(CONF_NAME_FIELD, StringType(), False))
CONF_CITY_FIELD = "city"
CONFERENCE_SCHEMA.add(StructField(CONF_CITY_FIELD, CITY_SCHEMA))

And then plain old PySpark makes you deal with nested fields like this:

dframe.withColumn("city_name", df[CONF_CITY_FIELD][CITY_NAME_FIELD])

Instead, with sparkql, schemas become a lot more literate:

class City(Struct):
    name = String(nullable=False)
    latitude = Float()
    longitude = Float()

class Conference(Struct):
    name = String(nullable=False)
    city = City()

As does dealing with nested fields:

dframe.withColumn("city_name", Conference.city.name.COL)

Here's a summary of sparkql's features.

  • ORM-like class-based Spark schema definitions.
  • Automated field naming: The attribute name of a field as it appears in its Struct is (by default) used as its field name. This name can be optionally overridden.
  • Programatically reference nested fields in your structs with the PATH and COL special properties. Avoid hand-constructing strings (or Columns) to reference your nested fields.
  • Validate that a DataFrame matches a sparkql schema.
  • Reuse and build composite schemas with inheritance, includes, and implements.
  • Get a human-readable Spark schema representation with pretty_schema.
  • Create an instance of a schema as a dictionary, with validation of the input values.

Read on for documentation on these features.

Defining a schema

Each Spark atomic type has a counterpart sparkql field:

PySpark type sparkql field
ByteType Byte
IntegerType Integer
LongType Long
ShortType Short
DecimalType Decimal
DoubleType Double
FloatType Float
StringType String
BinaryType Binary
BooleanType Boolean
DateType Date
TimestampType Timestamp

Array (counterpart to ArrayType in PySpark) allows the definition of arrays of objects. By creating a subclass of Struct, we can define a custom class that will be converted to a StructType.

For example, given the sparkql schema definition:

from sparkql import Struct, String, Array

class Article(Struct):
    title = String(nullable=False)
    tags = Array(String(), nullable=False)
    comments = Array(String(nullable=False))

Then we can build the equivalent PySpark schema (a StructType) with:

from sparkql import schema

pyspark_struct = schema(Article)

Pretty printing the schema with the expression sparkql.pretty_schema(pyspark_struct) will give the following:

StructType(List(
    StructField(title,StringType,false),
    StructField(tags,
        ArrayType(StringType,true),
        false),
    StructField(comments,
        ArrayType(StringType,false),
        true)))

Features

Many examples of how to use sparkql can be found in examples.

Automated field naming

By default, field names are inferred from the attribute name in the struct they are declared.

For example, given the struct

class Geolocation(Struct):
    latitude = Float()
    longitude = Float()

the concrete name of the Geolocation.latitude field is latitude.

Names also be overridden by explicitly specifying the field name as an argument to the field

class Geolocation(Struct):
    latitude = Float(name="lat")
    longitude = Float(name="lon")

which would mean the concrete name of the Geolocation.latitude field is lat.

Field paths and nested objects

Referencing fields in nested data can be a chore. sparkql simplifies this with path referencing.

For example, if we have a schema with nested objects:

class Address(Struct):
    post_code = String()
    city = String()


class User(Struct):
    username = String(nullable=False)
    address = Address()


class Comment(Struct):
    message = String()
    author = User(nullable=False)


class Article(Struct):
    title = String(nullable=False)
    author = User(nullable=False)
    comments = Array(Comment())

We can use the special PATH property to turn a path into a Spark-understandable string:

author_city_str = Article.author.address.city.PATH
"author.address.city"

COL is a counterpart to PATH that returns a Spark Column object for the path, allowing it to be used in all places where Spark requires a column.

Function equivalents path_str, path_col, and name are also available. This table demonstrates the equivalence of the property styles and the function styles:

Property style Function style Result (both styles are equivalent)
Article.author.address.city.PATH sparkql.path_str(Article.author.address.city) "author.address.city"
Article.author.address.city.COL sparkql.path_col(Article.author.address.city) Column pointing to author.address.city
Article.author.address.city.NAME sparkql.name(Article.author.address.city) "city"

For paths that include an array, two approaches are provided:

comment_usernames_str = Article.comments.e.author.username.PATH
"comments.author.username"

comment_usernames_str = Article.comments.author.username.PATH
"comments.author.username"

Both give the same result. However, the former (e) is more type-oriented. The e attribute corresponds to the array's element field. Although this looks strange at first, it has the advantage of being inspectable by IDEs and other tools, allowing goodness such as IDE auto-completion, automated refactoring, and identifying errors before runtime.

DataFrame validation

Struct method validate_data_frame will verify if a given DataFrame's schema matches the Struct. For example, if we have our Article struct and a DataFrame we want to ensure adheres to the Article schema:

dframe = spark_session.createDataFrame([{"title": "abc"}])

class Article(Struct):
    title = String()
    body = String()

Then we can can validate with:

validation_result = Article.validate_data_frame(dframe)

validation_result.is_valid indicates whether the DataFrame is valid (False in this case), and validation_result.report is a human-readable string describing the differences:

Struct schema...

StructType(List(
    StructField(title,StringType,true),
    StructField(body,StringType,true)))

DataFrame schema...

StructType(List(
    StructField(title,StringType,true)))

Diff of struct -> data frame...

  StructType(List(
-     StructField(title,StringType,true)))
+     StructField(title,StringType,true),
+     StructField(body,StringType,true)))

For convenience,

Article.validate_data_frame(dframe).raise_on_invalid()

will raise a InvalidDataFrameError (see sparkql.exceptions) if the
DataFrame is not valid.

Creating an instance of a schema

sparkql simplifies the process of creating an instance of a struct. You might need to do this, for example, when creating test data, or when creating an object (a dict or a row) to return from a UDF.

Use Struct.make_dict(...) to instantiate a struct as a dictionary. This has the advantage that the input values will be correctly validated, and it will convert schema property names into their underlying field names.

For example, given some simple Structs:

class User(Struct):
    id = Integer(name="user_id", nullable=False)
    username = String()

class Article(Struct):
    id = Integer(name="article_id", nullable=False)
    title = String()
    author = User()
    text = String(name="body")

Here are a few examples of creating dicts from Article:

Article.make_dict(
    id=1001,
    title="The article title",
    author=User.make_dict(
        id=440,
        username="user"
    ),
    text="Lorem ipsum article text lorem ipsum."
)

# generates...
{
    "article_id": 1001,
    "author": {
        "user_id": 440,
        "username": "user"},
    "body": "Lorem ipsum article text lorem ipsum.",
    "title": "The article title"
}
Article.make_dict(
    id=1002
)

# generates...
{
    "article_id": 1002,
    "author": None,
    "body": None,
    "title": None
}

See this example for an extended example of using make_dict.

Composite schemas

It is sometimes useful to be able to re-use the fields of one struct in another struct. sparkql provides a few features to enable this:

  • inheritance: A subclass inherits the fields of a base struct class.
  • includes: Incorporate fields from another struct.
  • implements: Enforce that a struct must implement the fields of another struct.

See the following examples for a better explanation.

Using inheritance

For example, the following:

class BaseEvent(Struct):
    correlation_id = String(nullable=False)
    event_time = Timestamp(nullable=False)

class RegistrationEvent(BaseEvent):
    user_id = String(nullable=False)

will produce the following RegistrationEvent schema:

StructType(List(
    StructField(correlation_id,StringType,false),
    StructField(event_time,TimestampType,false),
    StructField(user_id,StringType,false)))

Using an includes declaration

For example, the following:

class EventMetadata(Struct):
    correlation_id = String(nullable=False)
    event_time = Timestamp(nullable=False)

class RegistrationEvent(Struct):
    class Meta:
        includes = [EventMetadata]
    user_id = String(nullable=False)

will produce the RegistrationEvent schema:

StructType(List(
    StructField(user_id,StringType,false),
    StructField(correlation_id,StringType,false),
    StructField(event_time,TimestampType,false)))

Using an implements declaration

implements is similar to includes, but does not automatically incorporate the fields of specified structs. Instead, it is up to the implementor to ensure that the required fields are declared in the struct.

Failing to implement a field from an implements struct will result in a StructImplementationError error.

For example:

class LogEntryMetadata(Struct):
    logged_at = Timestamp(nullable=False)

class PageViewLogEntry(Struct):
    class Meta:
        implements = [LogEntryMetadata]
    page_id = String(nullable=False)

# the above class declaration will fail with the following StructImplementationError error:
#   Struct 'RegistrationEvent' does not implement field 'correlation_id' required by struct 'EventMetadata'

Prettified Spark schema strings

Spark's stringified schema representation isn't very user friendly, particularly for large schemas:

StructType(List(StructField(name,StringType,false),StructField(city,StructType(List(StructField(name,StringType,false),StructField(latitude,FloatType,true),StructField(longitude,FloatType,true))),true)))

The function pretty_schema will return something more useful:

StructType(List(
    StructField(name,StringType,false),
    StructField(city,
        StructType(List(
            StructField(name,StringType,false),
            StructField(latitude,FloatType,true),
            StructField(longitude,FloatType,true))),
        true)))

Merge two Spark StructType types

It can be useful to build a composite schema from two StructTypes. sparkql provides a merge_schemas function to do this.

For example:

schema_a = StructType([
    StructField("message", StringType()),
    StructField("author", ArrayType(
        StructType([
            StructField("name", StringType())
        ])
    ))
])

schema_b = StructType([
    StructField("author", ArrayType(
        StructType([
            StructField("address", StringType())
        ])
    ))
])

merged_schema = merge_schemas(schema_a, schema_b) 

results in a merged_schema that looks like:

StructType(List(
    StructField(message,StringType,true),
    StructField(author,
        ArrayType(StructType(List(
            StructField(name,StringType,true),
            StructField(address,StringType,true))),true),
        true)))

Contributing

Contributions are very welcome. Developers who'd like to contribute to this project should refer to CONTRIBUTING.md.

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