Learn Python On AWS Workshop > Persisting Data > Amazon DynamoDB > Step 3 - Create, Read, Update, and Delete an Item

Step 3 - Create, Read, Update, and Delete an Item

In this step, you perform read and write operations on an item in the Movies table.

To learn more about reading and writing data, see Working with Items and Attributes.

Create a New Item

In this step, you add a new item to the Movies table.

Copy the following program and paste it into a file named MoviesItemOps01.py. Don’t forget to change the region name to the same region as the Cloud9 instance.

import boto3
import json
import decimal

# Helper class to convert a DynamoDB item to JSON.
class DecimalEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, decimal.Decimal):
            if abs(o) % 1 > 0:
                return float(o)
            else:
                return int(o)
        return super(DecimalEncoder, self).default(o)

dynamodb = boto3.resource('dynamodb', region_name='us-west-2')

table = dynamodb.Table('Movies')

title = "The Big New Movie"
year = 2015

response = table.put_item(
   Item={
        'year': year,
        'title': title,
        'info': {
            'plot':"Nothing happens at all.",
            'rating': decimal.Decimal(0)
        }
    }
)

print("PutItem succeeded:")
print(json.dumps(response, indent=4, cls=DecimalEncoder))

The primary key is required. This code adds an item that has primary key (year, title) and info attributes. The info attribute stores sample JSON that provides more information about the movie. The DecimalEncoder class is used to print out numbers stored using the Decimal class. The Boto SDK uses the Decimal class to hold Amazon DynamoDB number values.

To run the program, enter the following command.

python MoviesItemOps01.py

Read an Item

In the previous program, you added the following item to the table.

{
   year: 2015,
   title: "The Big New Movie",
   info: { 
        plot: "Nothing happens at all.",
        rating: 0
   }
}

You can use the get_itemmethod to read the item from the Movies table. You must specify the primary key values so that you can read any item from Movies if you know its year and title.

Copy the following program and paste it into a file named MoviesItemOps02.py.

import boto3
import json
import decimal
from boto3.dynamodb.conditions import Key, Attr
from botocore.exceptions import ClientError

# Helper class to convert a DynamoDB item to JSON.
class DecimalEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, decimal.Decimal):
            if o % 1 > 0:
                return float(o)
            else:
                return int(o)
        return super(DecimalEncoder, self).default(o)

dynamodb = boto3.resource("dynamodb", region_name='us-west-2')

table = dynamodb.Table('Movies')

title = "The Big New Movie"
year = 2015

try:
    response = table.get_item(
        Key={
            'year': year,
            'title': title
        }
    )
except ClientError as e:
    print(e.response['Error']['Message'])
else:
    item = response['Item']
    print("GetItem succeeded:")
    print(json.dumps(item, indent=4, cls=DecimalEncoder))

To run the program, enter the following command.

python MoviesItemOps02.py

Update an Item

You can use the update_item method to modify an existing item. You can update values of existing attributes, add new attributes, or remove attributes.

In this example, you perform the following updates:

Change the value of the existing attributes (rating, plot).

Add a new list attribute (actors) to the existing info map.

The following shows the existing item.

{
   year: 2015,
   title: "The Big New Movie",
   info: { 
        plot: "Nothing happens at all.",
        rating: 0
   }
}

The item is updated as follows.

{
   year: 2015,
   title: "The Big New Movie",
   info: { 
           plot: "Everything happens all at once.",
           rating: 5.5,
           actors: ["Larry", "Moe", "Curly"]
   }
}

Copy the following program and paste it into a file named MoviesItemOps03.py.

import boto3
import json
import decimal

# Helper class to convert a DynamoDB item to JSON.
class DecimalEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, decimal.Decimal):
            if o % 1 > 0:
                return float(o)
            else:
                return int(o)
        return super(DecimalEncoder, self).default(o)

dynamodb = boto3.resource('dynamodb', region_name='us-west-2')

table = dynamodb.Table('Movies')

title = "The Big New Movie"
year = 2015

response = table.update_item(
    Key={
        'year': year,
        'title': title
    },
    UpdateExpression="set info.rating=:r, info.plot=:p, info.actors=:a",
    ExpressionAttributeValues={
        ':r': decimal.Decimal(5.5),
        ':p': "Everything happens all at once.",
        ':a': ["Larry", "Moe", "Curly"]
    },
    ReturnValues="UPDATED_NEW"
)

print("UpdateItem succeeded:")
print(json.dumps(response, indent=4, cls=DecimalEncoder))

This program uses UpdateExpression to describe all updates you want to perform on the specified item. The ReturnValues parameter instructs DynamoDB to return only the updated attributes (UPDATED_NEW).

To run the program, enter the following command.

python MoviesItemOps03.py

Increment an Atomic Counter

DynamoDB supports atomic counters, which use the update_item method to increment or decrement the value of an existing attribute without interfering with other write requests. (All write requests are applied in the order in which they are received.)

The following program shows how to increment the rating for a movie. Each time you run the program, it increments this attribute by one.

Copy the following program and paste it into a file named MoviesItemOps04.py.

import boto3
import json
import decimal

# Helper class to convert a DynamoDB item to JSON.
class DecimalEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, decimal.Decimal):
            if o % 1 > 0:
                return float(o)
            else:
                return int(o)
        return super(DecimalEncoder, self).default(o)

dynamodb = boto3.resource('dynamodb', region_name='us-west-2')

table = dynamodb.Table('Movies')

title = "The Big New Movie"
year = 2015

response = table.update_item(
    Key={
        'year': year,
        'title': title
    },
    UpdateExpression="set info.rating = info.rating + :val",
    ExpressionAttributeValues={
        ':val': decimal.Decimal(1)
    },
    ReturnValues="UPDATED_NEW"
)

print("UpdateItem succeeded:")
print(json.dumps(response, indent=4, cls=DecimalEncoder))

To run the program, enter the following command.

python MoviesItemOps04.py

Update an Item (Conditionally)

The following program shows how to use UpdateItem with a condition. If the condition evaluates to true, the update succeeds; otherwise, the update is not performed.

In this case, the item is updated only if there are more than three actors.

Copy the following program and paste it into a file named MoviesItemOps05.py.

import boto3
from botocore.exceptions import ClientError
import json
import decimal

# Helper class to convert a DynamoDB item to JSON.
class DecimalEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, decimal.Decimal):
            if o % 1 > 0:
                return float(o)
            else:
                return int(o)
        return super(DecimalEncoder, self).default(o)

dynamodb = boto3.resource('dynamodb', region_name='us-west-2')

table = dynamodb.Table('Movies')

title = "The Big New Movie"
year = 2015

# Conditional update (will fail)
print("Attempting conditional update...")

try:
    response = table.update_item(
        Key={
            'year': year,
            'title': title
        },
        UpdateExpression="remove info.actors[0]",
        ConditionExpression="size(info.actors) > :num",
        ExpressionAttributeValues={
            ':num': 3
        },
        ReturnValues="UPDATED_NEW"
    )
except ClientError as e:
    if e.response['Error']['Code'] == "ConditionalCheckFailedException":
        print(e.response['Error']['Message'])
    else:
        raise
else:
    print("UpdateItem succeeded:")
    print(json.dumps(response, indent=4, cls=DecimalEncoder))

To run the program, enter the following command.

python MoviesItemOps05.py

The program should fail with the following message.

The conditional request failed

The program fails because the movie has three actors in it, but the condition is checking for greater than three actors.

Modify the program so that the ConditionExpression looks like the following.

ConditionExpression="size(info.actors) >= :num",

The condition is now greater than or equal to 3 instead of greater than 3.

Run the program again. The UpdateItem operation should now succeed.

Delete an Item

You can use the delete_item method to delete one item by specifying its primary key. Optionally, you can provide a ConditionExpression to prevent the item from being deleted if the condition is not met.

In the following example, you try to delete a specific movie item if its rating is 5 or less.

Copy the following program and paste it into a file named MoviesItemOps06.py.

import boto3
from botocore.exceptions import ClientError
import json
import decimal

# Helper class to convert a DynamoDB item to JSON.
class DecimalEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, decimal.Decimal):
            if o % 1 > 0:
                return float(o)
            else:
                return int(o)
        return super(DecimalEncoder, self).default(o)

dynamodb = boto3.resource('dynamodb', region_name='us-west-2')

table = dynamodb.Table('Movies')

title = "The Big New Movie"
year = 2015

print("Attempting a conditional delete...")

try:
    response = table.delete_item(
        Key={
            'year': year,
            'title': title
        },
        ConditionExpression="info.rating <= :val",
        ExpressionAttributeValues= {
            ":val": decimal.Decimal(5)
        }
    )
except ClientError as e:
    if e.response['Error']['Code'] == "ConditionalCheckFailedException":
        print(e.response['Error']['Message'])
    else:
        raise
else:
    print("DeleteItem succeeded:")
    print(json.dumps(response, indent=4, cls=DecimalEncoder))

To run the program, enter the following command.

python MoviesItemOps06.py

The program should fail with the following message.

The conditional request failed

The program fails because the rating for this particular move is greater than 5.

Modify the program to remove the condition in table.delete_item.

response = table.delete_item(
    Key={
        'year': year,
        'title': title
    }
)

Run the program. Now, the delete succeeds because you removed the condition.