How to upload JSON data to Azure storage blob using Azure Blob storage bindings for Azure Functions in Python

Question

I want to upload JSON data as a .json file to Azure storage blobs using Azure Functions in Python.

Because I am using Azure Functions and not an actual server, I do not want to (and probably cannot) create a temporary file in local memory and upload that file to Azure blob storage using the Azure Blob storage client library v2.1 for Python (reference link here). Therefore, I want to use output blob storage bindings for Azure Functions (reference link here).

I am testing this out in an Azure Functions with an HTTP Trigger. I take in an Azure blob through an input blob storage binding (which is working just fine), process it, and update it by uploading a new Azure blob that overrides it (which is what I need help with). My function.json file looks like this:

{
  "scriptFile": "__init__.py",
  "bindings": [
    {
      "authLevel": "anonymous",
      "type": "httpTrigger",
      "direction": "in",
      "name": "req",
      "methods": [
        "get",
        "post"
      ]
    },
    {
      "name": "inputblob",
      "type": "blob",
      "path": "{containerName}/{blobName}.json",
      "connection": "MyStorageConnectionAppSetting",
      "direction": "in"
    },
    {
      "name": "outputblob",
      "type": "blob",
      "path": "{containerName}/{blobName}.json",
      "connection": "MyStorageConnectionAppSetting",
      "direction": "out"
    },
    {
      "type": "http",
      "direction": "out",
      "name": "$return"
    }
  ]
}

And my Python code looks like this:

import logging
import azure.functions as func
import azure.storage.blob
from azure.storage.blob import BlobServiceClient, BlobClient, ContainerClient
import json, os

def main(req: func.HttpRequest, inputblob: func.InputStream, outputblob: func.Out[func.InputStream]) -> func.HttpResponse:
    logging.info('Python HTTP trigger function processed a request.')

    # Initialize variable for tracking any changes
    anyChanges= False

    # Read JSON file
    jsonData= json.loads(inputblob.read())

    # Make changes to jsonData (omitted for simplicity) and update anyChanges

    # Upload new JSON file
    if anyChanges:
        outputblob.set(jsonData)

    return func.HttpResponse(f"Input data: {jsonData}. Any changes: {anyChanges}.")

However, this isn't working at all, with the following error being thrown (screenshot):

Value 'func.Out' is unsubscriptable

What am I missing?

evilSnobu evilSnobu · Accepted Answer · 2019-12-10T11:50:46

You want bytes or str, not InputStream, like this:

def main(inputblob: func.InputStream, outputblob: func.Out[bytes], context: func.Context):
    ...
    # Write to output blob
    outputblob.set(jsonData)

InputStream represents an input blob.

See this sample for str and this one for bytes.

Later update:

The call to json.loads() returns a dictionary and for some reason outputblob.set() freaks out and needs some handholding, like this:

def main(req: func.HttpRequest,
         inputblob: func.InputStream,
         outputblob: func.Out[bytes]) -> func.HttpResponse:

    jsonData = json.loads(inputblob.read())
    outputblob.set(str(jsonData))

    return func.HttpResponse(f"Input JSON: {jsonData}")

That should work (well it works on func version 2.7.1948 at least).

How to upload JSON data to Azure storage blob using Azure Blob storage bindings for Azure Functions in Python

1 Answers