How can I achieve test coverage for `ClientError` exceptions raised in my unit tests for a Python script that utilizes boto3 and moto?

Question

I have this python script snippet with mocked aws infrastructure for a todo list application:

import os
import boto3
import time
import uuid
import json
import functools
from botocore.exceptions import ClientError


def get_table(dynamodb=None):
    if not dynamodb:
        URL = os.environ['ENDPOINT_OVERRIDE']
        if URL:
            print('URL dynamoDB:'+URL)
            boto3.client = functools.partial(boto3.client, endpoint_url=URL)
            boto3.resource = functools.partial(boto3.resource,
                                               endpoint_url=URL)
        dynamodb = boto3.resource("dynamodb")
    # fetch todo from the database
    table = dynamodb.Table(os.environ['DYNAMODB_TABLE'])
    return table

def put_item(text, dynamodb=None):
    table = get_table(dynamodb)
    timestamp = str(time.time())
    print('Table name:' + table.name)
    item = {
        'id': str(uuid.uuid1()),
        'text': text,
        'checked': False,
        'createdAt': timestamp,
        'updatedAt': timestamp,
    }
    try:
        # write the todo to the database
        table.put_item(Item=item)
        # create a response
        response = {
            "statusCode": 200,
            "body": json.dumps(item)
        }

    except ClientError as e:
        print(e.response['Error']['Message'])
    else:
        return response

Which is to be unit tested with this one:

import warnings
import unittest
import boto3
from moto import mock_dynamodb
import sys
import os
import json

@mock_dynamodb
class TestDatabaseFunctions(unittest.TestCase):
    def setUp(self):
        print ('---------------------')
        print ('Start: setUp')
        warnings.filterwarnings(
            "ignore",
            category=ResourceWarning,
            message="unclosed.*<socket.socket.*>")
        warnings.filterwarnings(
            "ignore",
            category=DeprecationWarning,
            message="callable is None.*")
        warnings.filterwarnings(
            "ignore",
            category=DeprecationWarning,
            message="Using or importing.*")
        """Create the mock database and table"""
        self.dynamodb = boto3.resource('dynamodb', region_name='us-east-1')
        self.is_local = 'true'
        self.uuid = "123e4567-e89b-12d3-a456-426614174000"
        self.text = "Aprender DevOps y Cloud en la UNIR"

        from src.todoList import create_todo_table
        self.table = create_todo_table(self.dynamodb)
        #self.table_local = create_todo_table()
        print ('End: setUp')

    def test_put_todo(self):
        print ('---------------------')
        print ('Start: test_put_todo')
        # Testing file functions
        from src.todoList import put_item
        # Table local
        response = put_item(self.text, self.dynamodb)
        print ('Response put_item:' + str(response))
        self.assertEqual(200, response['statusCode'])
        # Table mock
        #self.assertEqual(200, put_item(self.text, self.dynamodb)[
        #                 'ResponseMetadata']['HTTPStatusCode'])
        print ('End: test_put_todo')

    def test_put_todo_error(self):
        print ('---------------------')
        print ('Start: test_put_todo_error')
        # Testing file functions
        from src.todoList import put_item
        # Table mock
        self.assertRaises(Exception, put_item("", self.dynamodb))
        self.assertRaises(Exception, put_item("", self.dynamodb))
        print ('End: test_put_todo_error')

And I cannot for the life of me cover the following lines from the first script with the tests on the second one:

    except ClientError as e:
        print(e.response['Error']['Message'])

Thank you in advance for your wisdom.

I wasn't able to cover it as I am a neofite in this subject.

score 0 · Answer 1 · answered Jun 01 '23 at 10:33

The easiest way to trigger an error scenario would be to call put_item without creating the table first. That would result in a ResourceNotFoundException, which triggers the ClientError.

From your example, it looks like you can simply comment out create_todo_table to test this scenario.

To give some background information:

AWS will throw a ClientError for the following exceptions:

DynamoDB.Client.exceptions.ConditionalCheckFailedException
DynamoDB.Client.exceptions.ProvisionedThroughputExceededException
DynamoDB.Client.exceptions.ResourceNotFoundException
DynamoDB.Client.exceptions.ItemCollectionSizeLimitExceededException
DynamoDB.Client.exceptions.TransactionConflictException
DynamoDB.Client.exceptions.RequestLimitExceeded
DynamoDB.Client.exceptions.InternalServerError

Moto currently only supports the following scenarios:

DynamoDB.Client.exceptions.ConditionalCheckFailedException
DynamoDB.Client.exceptions.ResourceNotFoundException
DynamoDB.Client.exceptions.ItemCollectionSizeLimitExceededException

Or in other words, Moto will throw an exception if:

the put_item contains an invalid ConditionExpression
the provided table does not exist
the size of the item is too large (>5MB, out the top of my head?)

This is awesome information, thank you. My follow up query would be, wouldn't commenting that line mess with the rest of the tests which rely on having that table created? I also need to cover the "if not dynamodb:" branch, I'm guessing it would be covered by dynamodb variable not existing, can that be done? — javier, Jun 01 '23 at 13:09
Other tests would fail, yes. You can either move the `create_todo_table` into the `test_put_todo`-method, or simply create a new class specifically to test errors, something like `class TestDatabaseFunctionErrors` - whichever is easier — Bert Blommers, Jun 01 '23 at 20:53

How can I achieve test coverage for `ClientError` exceptions raised in my unit tests for a Python script that utilizes boto3 and moto?

1 Answers1