自定义函数

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User-defined functions are important features, because they significantly extend the expressiveness of Python Table API programs.

Scalar Functions

Scalar Functions

It supports to use Python scalar functions in Python Table API programs. In order to define a Python scalar function, one can extend the base class ScalarFunction in pyflink.table.udf and implement an evaluation method. The behavior of a Python scalar function is defined by the evaluation method which is named eval. The evaluation method can support variable arguments, such as eval(*args).

The following example shows how to define your own Python hash code function, register it in the TableEnvironment, and call it in a query. Note that you can configure your scalar function via a constructor before it is registered:

class HashCode(ScalarFunction):
  def __init__(self):
    self.factor = 12

  def eval(self, s):
    return hash(s) * self.factor

# use StreamTableEnvironment since Python UDF is not supported in the old planner under batch mode
table_env = StreamTableEnvironment.create(env)

# register the Python function
table_env.register_function("hash_code", udf(HashCode(), DataTypes.BIGINT(), DataTypes.BIGINT()))

# use the Python function in Python Table API
my_table.select("string, bigint, bigint.hash_code(), hash_code(bigint)")

# use the Python function in SQL API
table_env.sql_query("SELECT string, bigint, hash_code(bigint) FROM MyTable")

It also supports to use Java/Scala scalar functions in Python Table API programs.

'''
Java code:

// The Java class must have a public no-argument constructor and can be founded in current Java classloader.
public class HashCode extends ScalarFunction {
  private int factor = 12;

  public int eval(String s) {
      return s.hashCode() * factor;
  }
}
'''

table_env = BatchTableEnvironment.create(env)

# register the Java function
table_env.register_java_function("hash_code", "my.java.function.HashCode")

# use the Java function in Python Table API
my_table.select("string.hash_code(), hash_code(string)")

# use the Java function in SQL API
table_env.sql_query("SELECT string, bigint, hash_code(string) FROM MyTable")

There are many ways to define a Python scalar function besides extending the base class ScalarFunction. The following examples show the different ways to define a Python scalar function which takes two columns of bigint as the input parameters and returns the sum of them as the result.

# option 1: extending the base class `ScalarFunction`
class Add(ScalarFunction):
  def eval(self, i, j):
    return i + j

add = udf(Add(), [DataTypes.BIGINT(), DataTypes.BIGINT()], DataTypes.BIGINT())

# option 2: Python function
@udf(input_types=[DataTypes.BIGINT(), DataTypes.BIGINT()], result_type=DataTypes.BIGINT())
def add(i, j):
  return i + j

# option 3: lambda function
add = udf(lambda i, j: i + j, [DataTypes.BIGINT(), DataTypes.BIGINT()], DataTypes.BIGINT())

# option 4: callable function
class CallableAdd(object):
  def __call__(self, i, j):
    return i + j

add = udf(CallableAdd(), [DataTypes.BIGINT(), DataTypes.BIGINT()], DataTypes.BIGINT())

# option 5: partial function
def partial_add(i, j, k):
  return i + j + k

add = udf(functools.partial(partial_add, k=1), [DataTypes.BIGINT(), DataTypes.BIGINT()],
          DataTypes.BIGINT())

# register the Python function
table_env.register_function("add", add)
# use the function in Python Table API
my_table.select("add(a, b)")