PynamoDB is attempt to be a Pythonic interface to DynamoDB that supports all of DynamoDB’s powerful features in both Python 3, and Python 2. This includes support for unicode and binary attributes.
But why stop there? PynamoDB also supports:
- Sets for Binary, Number, and Unicode attributes
- Automatic pagination for bulk operations
- Global secondary indexes
- Local secondary indexes
- Complex queries
It all started when I needed to use Global Secondary Indexes, a new and powerful feature of DynamoDB. I quickly realized that my go to library, dynamodb-mapper, didn’t support them. In fact, it won’t be supporting them anytime soon because dynamodb-mapper relies on another library, boto.dynamodb, which itself won’t support them. In fact, boto doesn’t support Python 3 either. If you want to know more, I blogged about it.
PynamoDB provides three API levels, a
TableConnection, and a
Each API is built on top of the previous, and adds higher level features. Each API level is
fully featured, and can be used directly. Before you begin, you should already have an
Amazon Web Services account, and have your
AWS credentials configured your boto.
Defining a Model¶
The most powerful feature of PynamoDB is the
Model API. You start using it by defining a model
class that inherits from
pynamodb.models.Model. Then, you add attributes to the model that
pynamodb.attributes.Attribute. The most common attributes have already been defined for you.
Here is an example, using the same table structure as shown in Amazon’s DynamoDB Thread example.
The table that your model represents must exist before you can use it. It can be created in this example by calling Thread.create_table(...). Any other operation on a non existent table will cause a TableDoesNotExist exception to be raised.
from pynamodb.models import Model from pynamodb.attributes import ( UnicodeAttribute, NumberAttribute, UnicodeSetAttribute, UTCDateTimeAttribute ) class Thread(Model): class Meta: table_name = 'Thread' forum_name = UnicodeAttribute(hash_key=True) subject = UnicodeAttribute(range_key=True) views = NumberAttribute(default=0) replies = NumberAttribute(default=0) answered = NumberAttribute(default=0) tags = UnicodeSetAttribute() last_post_datetime = UTCDateTimeAttribute()
All DynamoDB tables have a hash key, and you must specify which attribute is the hash key for each
Model you define.
forum_name attribute in this example is specified as the hash key for this table with the
subject attribute is specified as the range key with the
Meta class is required with at least the
table_name class attribute to tell the model which DynamoDB table to use -
Meta can be used to configure the model in other ways too. You can specify which DynamoDB region to use with the
and the URL endpoint for DynamoDB can be specified using the
host attribute. You can also specify the table’s read and write
capacity by adding
Here is an example that specifies both the
host and the
region to use:
from pynamodb.models import Model from pynamodb.attributes import UnicodeAttribute class Thread(Model): class Meta: table_name = 'Thread' # Specifies the region region = 'us-west-1' # Specifies the hostname host = 'http://localhost' # Specifies the write capacity write_capacity_units = 10 # Specifies the read capacity read_capacity_units = 10 forum_name = UnicodeAttribute(hash_key=True)
Defining Model Attributes¶
Model has attributes, which are mapped to attributes in DynamoDB. Attributes are responsible for serializing/deserializing
values to a format that DynamoDB accepts, optionally specifying whether or not an attribute may be empty using the null argument,
and optionally specifying a default value with the default argument. You can specify a default value for any field, and
can even be a function.
DynamoDB will not store empty attributes.
By default, an
Attribute cannot be
None unless you specify
null=True in the
DynamoDB attributes can’t be null and set attributes can’t be empty.
PynamoDB attempts to do the right thing by pruning null attributes when serializing an item to be put into DynamoDB.
By default, PynamoDB attributes can’t be null either - but you can easily override that by adding
null=True to the constructor of the attribute.
When you make an attribute nullable, PynamoDB will omit that value if the value is
None when saving to DynamoDB.
It is not recommended to give every attribute a value if those values can represent null, as those values representing null take up space - which literally costs you money
(DynamoDB pricing is based on reads and writes per second per KB).
Instead, treat the absence of a value as equivalent to being null (which is what PynamoDB does).
The only exception of course, are hash and range keys which must always have a value.
Here is an example of an attribute with a default value:
from pynamodb.models import Model from pynamodb.attributes import UnicodeAttribute class Thread(Model): class Meta: table_name = 'Thread' forum_name = UnicodeAttribute(hash_key=True, default='My Default Value')
Here is an example of an attribute with a default callable value:
from pynamodb.models import Model from pynamodb.attributes import UnicodeAttribute def my_default_value(): return 'My default value' class Thread(Model): class Meta: table_name = 'Thread' forum_name = UnicodeAttribute(hash_key=True, default=my_default_value)
Here is an example of an attribute that can be empty:
from pynamodb.models import Model from pynamodb.attributes import UnicodeAttribute class Thread(Model): class Meta: table_name = 'Thread' forum_name = UnicodeAttribute(hash_key=True) my_nullable_attribute = UnicodeAttribute(null=True)
By default, PynamoDB assumes that the attribute name used on a Model has the same name in DynamoDB. For example, if you define a UnicodeAttribute called ‘username’ then PynamoDB will use ‘username’ as the field name for that attribute when interacting with DynamoDB. If you wish to have custom attribute names, they can be overidden. One such use case is the ability to use human readable attribute names in PynamoDB that are stored in DynamoDB using shorter, terse attribute to save space.
Here is an example of customizing an attribute name:
from pynamodb.models import Model from pynamodb.attributes import UnicodeAttribute class Thread(Model): class Meta: table_name = 'Thread' forum_name = UnicodeAttribute(hash_key=True) # This attribute will be called 'tn' in DynamoDB thread_name = UnicodeAttribute(null=True, attr_name='tn')
PynamoDB comes with several built in attribute types for convenience, which include the following:
All of these built in attributes handle serializing and deserializng themselves, in both Python 2 and Python 3.
Creating the table¶
If your table doesn’t already exist, you will have to create it. This can be done with easily:
>>> if not Thread.exists(): Thread.create_table(read_capacity_units=1, write_capacity_units=1, wait=True)
wait argument tells PynamoDB to wait until the table is ready for use before returning.
Deleting a table¶
Deleting is made quite simple when using a Model:
Using the Model¶
Now that you’ve defined a model (referring to the example above), you can start interacting with your DynamoDB table. You can create a new Thread item by calling the Thread constructor.
>>> thread_item = Thread('forum_name', 'forum_subject')
The first two arguments are automatically assigned to the item’s hash and range keys. You can specify attributes during construction as well:
>>> thread_item = Thread('forum_name', 'forum_subject', replies=10)
The item won’t be added to your DynamoDB table until you call save:
If you want to retrieve an item that already exists in your table, you can do that with get:
>>> thread_item = Thread.get('forum_name', 'forum_subject')
If the item doesn’t exist, Thread.DoesNotExist will be raised.
You can update an item with the latest data from your table:
Updates to table items are supported too, even atomic updates. Here is an example of atomically updating the view count of an item + updating the value of the last post.
>>> thread_item.update(actions=[ Thread.views.set(Thread.views + 1), Thread.last_post_datetime.set(datetime.now()), ])
Update actions use the update expression syntax (see Update Expressions).
Deprecated since version 2.0:
update_item() is replaced with
>>> thread_item.update_item('views', 1, action='add')