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Declare a resourceโ€‹

A resource is an (optionally async) function that yields data. To create a resource, we add the @dlt.resource decorator to that function.

Commonly used arguments:

  • name The name of the table generated by this resource. Defaults to decorated function name.
  • write_disposition How should the data be loaded at destination? Currently, supported: append, replace and merge. Defaults to append.


@dlt.resource(name='table_name', write_disposition='replace')
def generate_rows():
for i in range(10):
yield {'id':i, 'example_string':'abc'}

def source_name():
return generate_rows

To get the data of a resource, we could do:

for row in generate_rows():

for row in source_name().resources.get('table_name'):

Typically, resources are declared and grouped with related resources within a source function.

Define schemaโ€‹

dlt will infer schema for tables associated with resources from the resource's data. You can modify the generation process by using the table and column hints. Resource decorator accepts following arguments:

  1. table_name the name of the table, if different from resource name.

  2. primary_key and merge_key define name of the columns (compound keys are allowed) that will receive those hints. Used in incremental loading.

  3. columns let's you define one or more columns, including the data types, nullability and other hints. The column definition is a TypedDict: TTableSchemaColumns. In example below, we tell dlt that column tags (containing a list of tags) in user table should have type complex which means that it will be loaded as JSON/struct and not as child table.

    @dlt.resource(name="user", columns={"tags": {"data_type": "complex"}})
    def get_users():

    # the `table_schema` method gets table schema generated by a resource

๐Ÿ’ก You can pass dynamic hints which are functions that take the data item as input and return a hint value. This let's you create table and column schemas depending on the data. See example in next section.

๐Ÿ’ก You can mark some resource arguments as configuration and credentials values so dlt can pass them automatically to your functions.

Put a contract on a tables, columns and dataโ€‹

Use the schema_contract argument to tell dlt how to deal with new tables, data types and bad data types. For example if you set it to freeze, dlt will not allow for any new tables, columns or data types to be introduced to the schema - it will raise an exception. Learn more in on available contract modes here

Define a schema with Pydanticโ€‹

You can alternatively use a Pydantic model to define the schema. For example:

from pydantic import BaseModel

class Address(BaseModel):
street: str
city: str
postal_code: str

class User(BaseModel):
id: int
name: str
tags: List[str]
email: Optional[str]
address: Address
status: Union[int, str]

@dlt.resource(name="user", columns=User)
def get_users():

The data types of the table columns are inferred from the types of the pydantic fields. These use the same type conversions as when the schema is automatically generated from the data.

Pydantic models integrate well with schema contracts as data validators.

Things to note:

  • Fields with an Optional type are marked as nullable
  • Fields with a Union type are converted to the first (not None) type listed in the union. E.g. status: Union[int, str] results in a bigint column.
  • list, dict and nested pydantic model fields will use the complex type which means they'll be stored as a JSON object in the database instead of creating child tables.

You can override this by configuring the Pydantic model

from typing import ClassVar
from dlt.common.libs.pydantic import DltConfig

class UserWithNesting(User):
dlt_config: ClassVar[DltConfig] = {"skip_complex_types": True}

@dlt.resource(name="user", columns=UserWithNesting)
def get_users():

"skip_complex_types" omits any dict/list/BaseModel type fields from the schema, so dlt will fall back on the default behaviour of creating child tables for these fields.

We do not support RootModel that validate simple types. You can add such validator yourself, see data filtering section.

Dispatch data to many tablesโ€‹

You can load data to many tables from a single resource. The most common case is a stream of events of different types, each with different data schema. To deal with this, you can use table_name argument on dlt.resource. You could pass the table name as a function with the data item as an argument and the table_name string as a return value.

For example, a resource that loads GitHub repository events wants to send issue, pull request, and comment events to separate tables. The type of the event is in the "type" field.

# send item to a table with name item["type"]
@dlt.resource(table_name=lambda event: event['type'])
def repo_events() -> Iterator[TDataItems]:
yield item

# the `table_schema` method gets table schema generated by a resource and takes optional
# data item to evaluate dynamic hints
print(repo_events().compute_table_schema({"type": "WatchEvent", id:...}))

In more advanced cases, you can dispatch data to different tables directly in the code of the resource function:

def repo_events() -> Iterator[TDataItems]:
# mark the "item" to be sent to table with name item["type"]
yield dlt.mark.with_table_name(item, item["type"])

Parametrize a resourceโ€‹

You can add arguments to your resource functions like to any other. Below we parametrize our generate_rows resource to generate the number of rows we request:

@dlt.resource(name='table_name', write_disposition='replace')
def generate_rows(nr):
for i in range(nr):
yield {'id':i, 'example_string':'abc'}

for row in generate_rows(10):

for row in generate_rows(20):

You can mark some resource arguments as configuration and credentials values so dlt can pass them automatically to your functions.

Process resources with dlt.transformerโ€‹

You can feed data from a resource into another one. The most common case is when you have an API that returns a list of objects (i.e. users) in one endpoint and user details in another. You can deal with this by declaring a resource that obtains a list of users and another resource that receives items from the list and downloads the profiles.

def users(limit=None):
for u in _get_users(limit):
yield u

# feed data from users as user_item below,
# all transformers must have at least one
# argument that will receive data from the parent resource
def users_details(user_item):
for detail in _get_details(user_item["user_id"]):
yield detail

# just load the user_details.
# dlt figures out dependencies for you.

In the example above, user_details will receive data from default instance of users resource (with limit set to None). You can also use pipe | operator to bind resources dynamically

# you can be more explicit and use a pipe operator.
# with it you can create dynamic pipelines where the dependencies
# are set at run time and resources are parametrized i.e.
# below we want to load only 100 users from `users` endpoint | user_details)

Transformers are allowed not only to yield but also to return values and can decorate async functions and async generators. Below we decorate an async function and request details on two pokemons. Http calls are made in parallel via httpx library.

import dlt
import httpx

async def pokemon(id):
async with httpx.AsyncClient() as client:
r = await client.get(f"{id}")
return r.json()

# get bulbasaur and ivysaur (you need dlt 0.4.6 for pipe operator working with lists)
print(list([1,2] | pokemon()))

Declare a standalone resourceโ€‹

A standalone resource is defined on a function that is top level in a module (not inner function) that accepts config and secrets values. Additionally if standalone flag is specified, the decorated function signature and docstring will be preserved. dlt.resource will just wrap the decorated function and user must call the wrapper to get the actual resource. Below we declare a filesystem resource that must be called before use.

def filesystem(bucket_url=dlt.config.value):
"""list and yield files in `bucket_url`"""

# `filesystem` must be called before it is extracted or used in any other way"s3://my-bucket/reports"), table_name="reports")

Standalone may have dynamic name that depends on the arguments passed to the decorated function. For example::

@dlt.resource(standalone=True, name=lambda args: args["stream_name"])
def kinesis(stream_name: str):

kinesis_stream = kinesis("telemetry_stream")

kinesis_stream resource has a name telemetry_stream

Declare parallel and async resourcesโ€‹

You can extract multiple resources in parallel threads or with async IO. To enable this for a sync resource you can set the parallelized flag to True in the resource decorator:

def get_users():
for u in _get_users():
yield u

def get_orders():
for o in _get_orders():
yield o

# users and orders will be iterated in parallel in two separate threads[get_users(), get_orders()])

Async generators are automatically extracted concurrently with other resources:

async def get_users():
async for u in _get_users(): # Assuming _get_users is an async generator
yield u

Please find more details in extract performance

Customize resourcesโ€‹

Filter, transform and pivot dataโ€‹

You can attach any number of transformations that are evaluated on item per item basis to your resource. The available transformation types:

  • map - transform the data item (resource.add_map).
  • filter - filter the data item (resource.add_filter).
  • yield map - a map that returns iterator (so single row may generate many rows - resource.add_yield_map).

Example: We have a resource that loads a list of users from an api endpoint. We want to customize it so:

  1. We remove users with user_id == "me".
  2. We anonymize user data.

Here's our resource:

import dlt

def users():
users = requests.get(...)
yield users

Here's our script that defines transformations and loads the data:

from pipedrive import users

def anonymize_user(user_data):
user_data["user_id"] = hash_str(user_data["user_id"])
user_data["user_email"] = hash_str(user_data["user_email"])
return user_data

# add the filter and anonymize function to users resource and enumerate
for user in users().add_filter(lambda user: user["user_id"] != "me").add_map(anonymize_user):

Reduce the nesting level of generated tablesโ€‹

You can limit how deep dlt goes when generating child tables. By default, the library will descend and generate child tables for all nested lists, without limit.


max_table_nesting is optional so you can skip it, in this case dlt will use it from the source if it is specified there or fallback to default value which has 1000 as maximum nesting level.

import dlt

def my_resource():
yield {
"id": 1,
"name": "random name",
"properties": [
"name": "customer_age",
"type": "int",
"label": "Age",
"notes": [
"text": "string",
"author": "string",

In the example above we want only 1 level of child tables to be generated (so there are no child tables of child tables). Typical settings:

  • max_table_nesting=0 will not generate child tables at all and all nested data will be represented as json.
  • max_table_nesting=1 will generate child tables of top level tables and nothing more. All nested data in child tables will be represented as json.

You can achieve the same effect after the resource instance is created:

from my_resource import my_awesome_module

resource = my_resource()
resource.max_table_nesting = 0

Several data sources are prone to contain semi-structured documents with very deep nesting i.e. MongoDB databases. Our practical experience is that setting the max_nesting_level to 2 or 3 produces the clearest and human-readable schemas.

Sample from large dataโ€‹

If your resource loads thousands of pages of data from a REST API or millions of rows from a db table, you may want to just sample a fragment of it in order i.e. to quickly see the dataset with example data and test your transformations etc. In order to do that, you limit how many items will be yielded by a resource by calling resource.add_limit method. In the example below we load just 10 first items from and infinite counter - that would otherwise never end.

r = dlt.resource(itertools.count(), name="infinity").add_limit(10)
assert list(r) == list(range(10))

๐Ÿ’ก We are not skipping any items. We are closing the iterator/generator that produces data after limit is reached.

๐Ÿ’ก You cannot limit transformers. They should process all the data they receive fully to avoid inconsistencies in generated datasets.

๐Ÿ’ก If you are paremetrizing the value of add_limit and sometimes need it to be disabled, you can set None or -1 to disable the limiting. You can also set the limit to 0 for the resource to not yield any items.

Set table name and adjust schemaโ€‹

You can change the schema of a resource, be it standalone or as a part of a source. Look for method named apply_hints which takes the same arguments as resource decorator. Obviously you should call this method before data is extracted from the resource. Example below converts an append resource loading the users table into merge resource that will keep just one updated record per user_id. It also adds "last value" incremental loading on created_at column to prevent requesting again the already loaded records:

tables = sql_database()

To just change a name of a table to which resource will load data, do the following:

tables = sql_database()
tables.users.table_name = "other_users"

Adjust schema when you yield dataโ€‹

You can set or update the table name, columns and other schema elements when your resource is executed and you already yield data. Such changes will be merged with the existing schema in the same way apply_hints method above works. There are many reason to adjust schema at runtime. For example when using Airflow, you should avoid lengthy operations (ie. reflecting database tables) during creation of the DAG so it is better do do it when DAG executes. You may also emit partial hints (ie. precision and scale for decimal types) for column to help dlt type inference.

def sql_table(credentials, schema, table):
# create sql alchemy engine
engine = engine_from_credentials(credentials)
metadata = MetaData(schema=schema)
# reflect the table schema
table_obj = Table(table, metadata, autoload_with=engine)

for idx, batch in enumerate(table_rows(engine, table_obj)):
if idx == 0:
# emit first row with hints, table_to_columns and get_primary_key are helpers that extract dlt schema from
# SqlAlchemy model
yield dlt.mark.with_hints(
dlt.mark.make_hints(columns=table_to_columns(table_obj), primary_key=get_primary_key(table_obj)),
# just yield all the other rows
yield batch

In the example above we use dlt.mark.with_hints and dlt.mark.make_hints to emit columns and primary key with the first extracted item. Table schema will be adjusted after the batch is processed in the extract pipeline but before any schema contracts are applied and data is persisted in load package.


You can emit columns as Pydantic model and use dynamic hints (ie. lambda for table name) as well. You should avoid redefining Incremental this way.

Duplicate and rename resourcesโ€‹

There are cases when you your resources are generic (ie. bucket filesystem) and you want to load several instances of it (ie. files from different folders) to separate tables. In example below we use filesystem source to load csvs from two different folders into separate tables:

def filesystem(bucket_url):
# list and yield files in bucket_url

def csv_reader(file_item):
# load csv, parse and yield rows in file_item

# create two extract pipes that list files from the bucket and send to them to the reader.
# by default both pipes will load data to the same table (csv_reader)
reports_pipe = filesystem("s3://my-bucket/reports") | load_csv()
transactions_pipe = filesystem("s3://my-bucket/transactions") | load_csv()

# so we rename resources to load to "reports" and "transactions" tables
[reports_pipe.with_name("reports"), transactions_pipe.with_name("transactions")]

with_name method returns a deep copy of the original resource, its data pipe and the data pipes of a parent resources. A renamed clone is fully separated from the original resource (and other clones) when loading: it maintains a separate resource state and will load to a table

Load resourcesโ€‹

You can pass individual resources or list of resources to the dlt.pipeline object. The resources loaded outside the source context, will be added to the default schema of the pipeline.

@dlt.resource(name='table_name', write_disposition='replace')
def generate_rows(nr):
for i in range(nr):
yield {'id':i, 'example_string':'abc'}

pipeline = dlt.pipeline(
# load individual resource
# load a list of resources[generate_rows(10), generate_rows(20)])

Do a full refreshโ€‹

To do a full refresh of an append or merge resources you temporarily change the write disposition to replace. You can use apply_hints method of a resource or just provide alternative write disposition when loading:, write_disposition="replace")

This demo works on codespaces. Codespaces is a development environment available for free to anyone with a Github account. You'll be asked to fork the demo repository and from there the README guides you with further steps.
The demo uses the Continue VSCode extension.

Off to codespaces!


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