How to use env variable for path argument in blob trigger python

Question

Hi team,

I'd like to define the path to the blob from env variable in my blob trigger function:

import logging

import azure.functions as func
import azurefunctions.extensions.bindings.blob as blob


app = func.FunctionApp()

@app.blob_trigger(arg_name="client",
                  path="PG_DUMP_CONTAINER_NAME",
                  connection="PG_DUMP_CONTAINER_CONNECTION_STRING",
                  run_on_startup=False
                  )
def function(client: blob.BlobClient, context: func.Context):
    logging.info(
        f"Python blob trigger function processed blob \n"
        f"Properties: {client.get_blob_properties()}"
    )

it doesn't work. I need to hardcode the value of the path. On the other hand connection argument is taken from env variable and it works. Are there any limitations?

Thanks for your help,
Maria

Answer 1

Hi ,

Thanks for reaching out to Microsoft Q&A.

Yes, you are running into a known limitation in azure functions.

In the @blob_trigger, the path parameter does not support env variable substitution, unlike the connection parameter which does. That is why the connection string is working fine from your environment variable, but path="PG_DUMP_CONTAINER_NAME" is not being resolved.

Why?

• connection is handled as a configuration setting and resolved by the azure functions runtime.
• path is treated as a literal string at the time the function bindings are processed. There is no interpolation from environment variables or app settings in the path parameter.

Workaround

Unfortunately, you must hardcode the container and blob path in the path parameter like this:

@app.blob_trigger(arg_name="client",
                  path="my-container-name/{name}",
                  connection="PG_DUMP_CONTAINER_CONNECTION_STRING")

If you really need to make it dynamic:

Hack (not recommended for prod loads)!

You could try:

Generating the function code dynamically using Python before startup.

Or using a generic Event Grid trigger to listen to blob events, and handle paths in code.

Example using Event Grid trigger:

@app.function_name(name="BlobEventGridTrigger")
@app.event_grid_trigger(arg_name="event")
def blob_event_handler(event: func.EventGridEvent):
    blob_url = event.get_json()["data"]["url"]
    # Use blob_url to dynamically process the blob

1. path does not support env vars, limitation of Azure Functions.
2. connection does support env vars.
3. You must hardcode the path or switch to using Event Grid triggers for dynamic scenarios.

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Answer 2

Hi @Maria Dąbrowiecka,

You can use environment variable in path by using % as below:

local.settings.json:

{

  "IsEncrypted": false,

  "Values": {

    "FUNCTIONS_WORKER_RUNTIME": "python",

    "AzureWebJobsFeatureFlags": "EnableWorkerIndexing",

    "AzureWebJobsStorage":"storageconnectionstring",

    "containerName":"mycontainer"

  }

}

function_app.py:

import azure.functions as func

import logging

app = func.FunctionApp()

@app.blob_trigger(arg_name="myblob", path="%containerName%",

                               connection="rithwik82ba_STORAGE") 

def blob_trigger(myblob: func.InputStream):

    logging.info(f"Python blob trigger function processed blob"

                f"Name: {myblob.name}"

                f"Blob Size: {myblob.length} bytes")

Here used path as %pathvariable%.

Output:

After Uploading a blob:

Function gets Triggered:

Hope this helps.

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