Python relative imports in AWS Lambda fail with `attempted relative import with no known parent package`

aws-lambda-relative-import-no-known-parent-package.md

Python relative imports in AWS Lambda fail with attempted relative import with no known parent package

The Problem

In AWS Lambda if I attempt an explicit relative import like this

.
├── lambda_file.py
└── example.py

# lambda_file.py
from .example import lambda_handler

# example.py
def lambda_handler(event, context):
    return True

And I configure AWS Lambda's handler to lambda_file.lambda_handler I get the following errors

• Python 2.7 : Attempted relative import in non-package
• Python 3.7 : attempted relative import with no known parent package

Why use explicit relative imports

PEP008 says :

Implicit relative imports should never be used and have been removed in Python 3.

How to workaround by using implicit relative imports

If I change lambda_file.py to contain the following, it works, but no longer uses explicit relative imports

# lambda_file.py
from example import lambda_handler

How to correctly solve the problem

The solution is to ensure that the "Handler" value that you configure in AWS Lambda contain at least 2 . periods. To achieve this you need to put your code within a directory in your AWS Lambda code zip file and make that directory a module by adding an empty __init__.py file. The resulting structure looks like this

.
├── app
│   ├── __init__.py
│   ├── lambda_file.py
│   └── example.py

And you now change the "Handler" value from lambda_file.lambda_handler to app.lambda_file.lambda_handler

Additional Notes

• AWS Lambda loads your handler as a module import not as a top-level script
• This solution came after reading this explanation of explicit relative imports by Brendan Barnwell

Deploying with container images
If you are Deploying container images, you may also need to make these modifications.
This is before breaking into multiple files.

.
├── app.py
|── Dockerfile
└── requirements.txt

After with the lambda_handler in the __init__.py

.
├── app
│   ├── __init__.py
│   ├── models.py
│   ├── exceptions.py
│   └── utils.py
|── Dockerfile
└── requirements.txt

You will also need to rework the Dockerfile COPY commands from

COPY app.py requirements.txt ./

RUN python3.8 -m pip install -r requirements.txt -t .

to

COPY requirements.txt ./
RUN python3.8 -m pip install -r requirements.txt -t .

COPY app/ ./app/

Via this method, you can keep the CMD command the same CMD ["app.lambda_handler"]
This is what I had to do to get it to work for me.

@AustinGilkison
Thanks for your input, it works perfectly fine for files that exist inside container.
Let say If I want to call the handler file located in EFS file system and its mounted to Lambda function with the mount path "/mnt/xxxfolder" .

I have copied the 'init.py' in all the folder structure defined till handler function, but Its still failing with

[ERROR] TypeError: the 'package' argument is required to perform a relative import for '.mnt.xxxfolder.yyfolder.lambda_handler'
Traceback (most recent call last):
  File "/var/lang/lib/python3.9/importlib/init.py", line 122, in import_module
    raise TypeError(msg.format(name))
    
 Appreciate your comments. Thanks.

And you now change the "Handler" value from lambda_file.lambda_handler to app.lambda_file.lambda_handler

Worked a treat. I also had to stick a bunch of __init__.py, due to deeply nested structure of some of the lambdas.

Thanks so much, this quickly helped me solve my problem!

Thanks .

Thanks a lot!

Thank you!

Thanks a bunch! I spent too much time figuring this out.