AWS lambda python multiple files application can't import one from another

Question

I have the following structure of my AWS lambda project:

module
  app.py
  b.py

app.py is my default aws lambda function with lambda_handler, it works fine. I decided to pull all the heavy calculations out of it to function calc of b.py.

Then, I imported it to app.py:

from module.b import calc

Now, when I run it locally with sam local invoke Function --event events/event.json, it raises an error:

{"errorType":"Runtime.ImportModuleError","errorMessage":"Unable to import module 'app': No module named 'module'"}

It seems to me that when it prepares the code to run, it moves the files to some other directory, so the imports break. To fix this, I tried to use relative import:

from .b import calc

But it also raised an error:

{"errorType":"Runtime.ImportModuleError","errorMessage":"Unable to import module 'app': attempted relative import with no known parent package"}

How do I setup a multi-file python application on aws lambda?

Answer 1

This is how me resolve that problem.

First your root folder need to seem like this:

lambda_folder
    lambda_function.py // Or your main.py.... this file have the method lambda_handler

Now... When I use multiple files... I always use a lib folder. Like this:

lambda_folder
    lib
        lib1.py
        lib2.py
        lib3.py
    lambda_function.py

IMPORTANT

Inside your lib folder you always need an __init__.py or you can't see the files inside.

lambda_folder
    lib
        lib1.py
        lib2.py
        lib3.py
        __init__.py
    lambda_function.py

NOTE: the __init__.py needs to have the two underscores before and after init.

EXAMPLE

lib1.py

def sum(a,b):
    return a+b

lambda_function.py

from lib import lib1
import json

def lambda_handler(event, context):
    result = lib.sum(5,4)

    return {
        "statusCode": 200,
        "body": "hi " + result
    }

And that's all.