AWS Lambda, python 2.7 and variables lifetime

Question

I found that python instructions which are not in the main function but in the global scope are run only once, after the function is created/updated (and perhaps after the container replacement but did not test it) which causes not the same behavior of the function first and all subsequent launches. The test code:

assets = []
print "======0. should be [] ========"
print(assets)   

def lambda_handler(event, context):
    global assets

    print "======l. should be [] ========"
    print(assets)   

    assets.append({
            "key": "val"
        })
    print "======2. should be [{'key': 'val'}] ========"
    print(assets)

    assets = {"data":assets}
    print "======3. should be {'data': [{'key': 'val'}]} ========"
    print(assets)

When I run the function the very first time (or after I updated it) I have:

======0. should be [] ========
[]
START RequestId: ecdf063e-51f0-11e8-9783-7f18ff8ee142 Version: $LATEST
======l. should be [] ========
[]
======2. should be [{'key': 'val'}] ========
[{'key': 'val'}]
======3. should be {'data': [{'key': 'val'}]} ========
{'data': [{'key': 'val'}]}
END RequestId: ecdf063e-51f0-11e8-9783-7f18ff8ee142

This is what I expect despite I confused why the output started before 'START'. The issue is: when I run it next time assets variable already have old value:

START RequestId: 66999707-51f1-11e8-b21a-9963eeb4aa56 Version: $LATEST
======l. should be [] ========
{'data': [{'key': 'val'}]}
'dict' object has no attribute 'append': AttributeError
Traceback (most recent call last):
  File "/var/task/test.py", line 11, in lambda_handler
    assets.append({
AttributeError: 'dict' object has no attribute 'append'

END RequestId: 66999707-51f1-11e8-b21a-9963eeb4aa56

As you can see there's no ======0. should be [] ======== output, and the variable already have value.

I'm not a Python guru, so am I doing something wrong or is it a bug?

E.J. Brennan · Accepted Answer · 2018-05-07 12:50:04Z

2

Lambda may or may not re-use the same function when you re-run it, so in other words, it is possible that some state will be left over between runs - you need to make sure you don't assume anything - so if you need something to be initialized, do it in the function itself - not in the global scope.

This article http://rodos.haywood.org/2015/06/lambda-functions-idempotent.html, although about nodejs not python, pretty much explains it.

answered May 7, 2018 at 12:50

E.J. Brennan

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1 Comment

user3292373 Over a year ago

Hi Brenanan, What if I need to have a variable in global scope. My requirement is I am using aws lambda handler which calls athena table. But it calls athena for every 20 sec. I wanted to make it global, so subsequent calls to athena can be avoided I can reuse the resultset dataframe . What I have to do now to make it global . Please help

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