writing-udf-python.md

layout	title
default	Writing user-defined functions in Python

Writing user-defined functions in Python

DeepDive supports user-defined functions (UDFs) for data processing, in addition to the normal derivation rules in DDlog. UDF can be any program that takes tab-separated values (TSV or PostgreSQL's text format) from stdin and prints TSV to stdout.

The following sections describe DeepDive's recommended way of writing UDFs in Python and how they are used in DDlog programs.

Using UDFs in DDlog

To use user-defined functions in DDlog, they must be declared first then called using special syntax.

First, let's define the schema of the two relations for our running example.

article(
    id     int,
    url    text,
    title  text,
    author text,
    words  text[]
).

classification(
    article_id int,
    topic      text
).

In this example, let's suppose we want to write a simple UDF to classify each article into different topics by adding tuples to the relation classification. The two following sections detail how to declare such a function and how to call it in DDlog.

Function declarations

A function declaration includes input/output schema as well as a pointer to its implementation.

function <function_name> over (<input_var_name> <input_var_type>,...)
    returns [(<output_var_name> <output_var_type>,...) | rows like <relation_name>]
    implementation "<executable_path>" handles tsv lines.

In our example, suppose we will use only the author and words of each article to determine the topic identified by its id, and the implementation will be kept in an executable file with relative path udf/classify.py. The exact declaration for such function is shown below.

function classify_articles over (id int, author text, words text[])
    returns (article_id int, topic text)
    implementation "udf/classify.py" handles tsv lines.

Notice that the column definitions of relation classification are repeated in the returns clause. This can be omitted by using the rows like syntax as shown below.

function classify_articles over (id int, author text, words text[])
    return rows like classification
    implementation "udf/classify.py" handles tsv lines.

Also note that the function input is similar to the articles relation, but some columns are missing. This is because the function will not use the rest of the columns as mentioned before, and it is a good idea to drop unnecessary values for efficiency. Next section shows how such input tuples can be derived and fed into a function.

Function call rules

The function declared above can be called to derive tuples for another relation of the output type. The input tuples for the function call are derived using a syntax similar to a normal derivation rule. For example, the rule shown below calls the classify_articles function to fill the classification relation using a subset of columns from the articles relation.

classification += classify_articles(id, author, words) :-
    article(id, _, _, author, words).

Function call rules can be viewed as special cases of normal derivation rules with different head syntax, where += and a function name is appended to the head relation name.

Writing UDFs in Python

DeepDive provides a templated way to write user-defined functions in Python. It provides several Python function decorators to simplify parsing and formatting input and output respectively. The Python generator to be called upon every input row should be decorated with @tsv_extractor, i.e., before the def line @tsv_extractor should be placed. (A Python generator is a Python function that uses yield instead of return to produce an iterable of multiple results per call) The input and output column types expected by the generator can be declared using the argument default values and @returns decorator, respectively. They tell how the input parser and output formatter should behave.

Let's look at a realistic example to describe how exactly they should be used in the code. Below is a near-complete code for the udf/classify.py declared as the implementation for the DDlog function classify_articles.

#!/usr/bin/env python
from deepdive import *  # Required for @tsv_extractor and @returns

compsci_authors = [...]
bio_authors     = [...]
bio_words       = [...]

@tsv_extractor  # Declares the generator below as the main function to call
@returns(lambda # Declares the types of output columns as declared in DDlog
        article_id = "int",
        topic      = "text",
    :[])
def classify(   # The input types can be declared directly on each parameter as its default value
        article_id = "int",
        author     = "text",
        words      = "text[]",
    ):
    """
    Classify articles by assigning topics.
    """
    num_topics = 0

    if author in compsci_authors:
        num_topics += 1
        yield [article_id, "cs"]

    if author in bio_authors:
        num_topics += 1
        yield [article_id, "bio"]
    elif any (word for word in bio_words if word in words):
        num_topics += 1
        yield [article_id, "bio"]

    if num_topics == 0:
        yield [article_id, None]

This simple UDF assigns a topic to articles based on their authors' membership in known categories. If the author is not recognized, we try to look for words that appear in a predefined set. Finally, if nothing matches, we simply put it into another catch-all topic. Note that the topics themselves here are completely user defined.

Notice that to use these Python decorators you'll need to have from deepdive import *. Also notice that the types of input columns can be declared as default values for the generator parameters in the same way as @returns.

@tsv_extractor decorator

The @tsv_extractor decorator should be placed as the first decorator for the main generator that will take one input row at a time and yield zero or more output rows as list of values. This basically lets DeepDive know which function to call when running the Python program.

Caveats

Generally, this generator should be placed at the bottom of the program unless there are some cleanup or tear-down tasks to do after processing all the input rows. Any function or variable used by the decorated generator should be appear before it as the @tsv_extractor decorator will immediately start parsing input and calling the generator. The generator should not print or sys.stdout.write anything as that will corrupt the standard output. Instead, print >>sys.stderr or sys.stderr.write can be used for logging useful information. More information can be found in the debugging-udf section

Parameter default values and @returns decorator

To parse the input TSV lines correctly into Python values and format the values generated by the @tsv_extractor correctly in TSV, the column types need to be written down in the Python program. They should be consistent with the function declaration in DDlog. The types for input columns can be declared directly in the @tsv_extractor generator's signature as parameter default values as shown in the example above. Arguments to the @returns decorator can be either a list of name and type pairs or a function with all parameters having their default values set as its type. The use of lambda is preferred because the list of pairs require more symbols that clutter the declaration. For example, compare above with @returns([("article_id", "int"), ("topic", "text")]). The reason dict(column="type", ... ) or { "column": "type", ... } do not work is because Python forgets the order of the columns with those syntax, which is crucial for the TSV parser and formatter. The passed function is never called so the body can be left as any value, such as empty list ([]). DeepDive also provides an @over decorator for input columns symmetric to @returns, but use of this is not recommended as it incurs redundant declarations.

Running and debugging UDFs

Once a first cut of the UDF is written, it can be run using the deepdive do and deepdive redo commands. For example, the classify_articles function in our running example to derive the classification relation can be run with the following command:

deepdive redo classification

This will invoke the Python program udf/classify.py, giving as input the TSV rows holding three columns of the article table, and taking its output to add rows to the classification table in the database.

There are dedicated pages describing more details about running these UDFs and debugging these UDFs.