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Version: 2.0.0

Transformations Python API

This is the additional API documentation for mgp.py which contains definitions of the public Transformation Python API provided by Memgraph. At the core, this is a wrapper around the C API. This source file can be found in the Memgraph installation directory, under python_support. On the standard Debian installation, this will be under /usr/lib/memgraph/python_support.

NOTE: This part of the documentation is still under development.

An updated version will soon be available.

For an example of how to implement transformation modules in Python,

check out this guide.

mgp.transformation(func)​

Transformation modules in Python have to follow certain rules in order to work:

  1. The transformation module is a Python function
  2. The function has to be decorated with a @mgp.transformation decorator
  3. The function can have 1 or 2 arguments
  • one of type mgp.Messages (required)
  • one of type mgp.TransCtx (optional)
  1. The function has to return an mgp.Record in the following form:
  • mgp.Record(query=str, parameters=mgp.Nullable[mgp.Map])
  • the return type can also be an iterable of mgp.Records, but not a generator

Examples​

import mgp

@mgp.transformation
def transformation(context: mgp.TransCtx,
                   messages: mgp.Messages
                   ) -> mgp.Record(query=str, parameters=mgp.Nullable[mgp.Map]):
    result_queries = []

    for i in range(messages.total_messages()):
        message = messages.message_at(i)
        payload_as_str = message.payload().decode("utf-8")
        result_queries.append(mgp.Record(
            query=f"CREATE (n:MESSAGE {{timestamp: '{message.timestamp()}', payload: '{payload_as_str}', topic: '{message.topic_name()}'}})",
            parameters=None))

    return result_queries

This transformation extracts the interesting members of each mgp.Message and stores them in query Record, which wraps a CREATE clause with all the interesting members (timestamp, payload, etc.) and an empty parameter list.

Any errors can be reported by raising an Exception.

class mgp.Message(message)​

Bases: object

Represents a single message. You shouldn't store a Message globally.

is_valid()​

Returns true if the underlying mgp.message object is valid and can be accessed.

payload()​

Returns the payload of the message. Raises an InvalidMessageError if is_valid() is false.

topic_name()​

Returns the topic name of the underlying mgp.message. Raises an InvalidMessageError if is_valid() is false.

key()​

Returns the key of the underlying mgp.message as bytes. Raises an InvalidMessageError if is_valid() is false.

timestamp()​

Returns the timestamp of the underlying mgp.message. Raises an InvalidMessageError if is_valid() is false.

class mgp.Messages(messages)​

Bases: object

Represents a list of messages passed to a transformation. You shouldn't store messages globally .

is_valid()​

Returns true if the underlying mgp.messages object is valid and can be accessed.

total_messages()​

Returns the number of mgp.messages contained. Raises InvalidMessagesError if is_valid() is false.

message_at(id)​

Returns the underlying mgp.message at index id. Raises InvalidMessagesError if is_valid() is false.

class mgp.TransCtx(graph)​

Bases: object

Context of a transformation being executed.

Access to a TransCtx is only valid during a single execution of a transformation. You shouldn't store a TransCtx globally.

graph()​

Raise InvalidContextError if context is invalid.

is_valid()​

Returns true if the context is valid and can be accessed.