How to receive MQTT payload in JSON in Azure Function subscribed to Event Grid using Python?

Question

How to receive MQTT payload in JSON in Azure Function subscribed to Event Grid using Python?

Pawel Czop 20

I configured Azure Event grid with namespace and enabled MQTT broker.

I want to receive the MQTT messages including JSON payload using Azure Function App in Python language.

The exemplary code provided in the EventGrid trigger example returns topic of the message in "subject" and message Id in "id", but the rest of the fields including "data" are null for EventGridEvent instance passed to function.

However, when I run C# example:

https://learn.microsoft.com/en-us/azure/event-grid/mqtt-routing-to-azure-functions-portal#create-an-azure-function-using-event-grid-trigger

I can also read data properly

Please provide a guidance how to read the JSON payload from published MQTT messages from EventGridEvent object instance in Python?

LeelaRajeshSayana-MSFT 17,676 Reputation points

2025-04-24T22:45:39.9966667+00:00

@Pawel Czop Can you inspect the schema of the MQTT message payload the Event Grid is receiving? If you are trying to access the data from the function app using event.get_json() please make sure your event data has a schema as providing the following sample schema.
Pawel Czop 20 Reputation points

2025-04-25T06:08:47.35+00:00

The schema of MQTT message is JSON, and like:

{ "data": "test" }

and in MQTT protocol does not limit it.

Custom topic is configured with CloudEvent 1.0 standard (as advised in tutorial).

As I also wrote it works in C# example, but there we are able to get CloudEvent object instance. Please provide the equivalent Python code to get CloudEvent and read the JSON payload.

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LeelaRajeshSayana-MSFT 17,676 Reputation points

2025-04-24T22:45:39.9966667+00:00

@Pawel Czop Can you inspect the schema of the MQTT message payload the Event Grid is receiving? If you are trying to access the data from the function app using event.get_json() please make sure your event data has a schema as providing the following sample schema.
Pawel Czop 20 Reputation points

2025-04-25T06:08:47.35+00:00

The schema of MQTT message is JSON, and like:

{ "data": "test" }

and in MQTT protocol does not limit it.

Custom topic is configured with CloudEvent 1.0 standard (as advised in tutorial).

As I also wrote it works in C# example, but there we are able to get CloudEvent object instance. Please provide the equivalent Python code to get CloudEvent and read the JSON payload.

Answer 1

@Pawel Czop The current version of Azure Function Python package does not have an implementation to handle events in the format of CloudEvent You can find the supported Python classes for the Azure functions in Event Grid from the downloaded package azure_functions-1.23.0\azure\functions\_eventgrid.py file. Python version of Azure function only supports EventGridEvent.

The C# code you have referenced does support this CloudEvent format which is why the data from the routing is successful. Since Event Grid Namespace routing to a custom topic only works with Cloud Event Schema type, I would say this is a limitation with Python code.

I will take this limitation to the product team and see if this gets addressed in the future versions.

If this answers your query, please do click Accept Answer and Yes, as it might be beneficial to other community members reading this thread. If you have any further query, do let us know. Thank you

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How to receive MQTT payload in JSON in Azure Function subscribed to Event Grid using Python?

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