Spec API (pipeline.yaml)

This section describes how to specify pipelines using a pipeline.yaml.

** Note:** This document assumes you are already familiar with Ploomber’s core concepts (DAG, product, task, and upstream). If you’re not, check out this guide: Basic concepts.

** Note:** The projects repository contains several pipeline.yaml examples.

Quick reference

A typical pipeline.yaml looks like this:

    extract_product: False

    - source: functions.get_raw_data
      product: output/raw.csv

    - source: scripts/plot.py
        nb: output/plots.html
        data: output/clean.csv

For each task, source specifies the source code for the task, while product says where to save the output (relative to the location of the pipeline.yaml file).

When your pipeline executes, Ploomber extracts upstream dependencies from the source code and passes corresponding values in product and upstream variables.


Under some situations, you might want to change the default configuration; a common scenario happens when using SQL scripts: you have to configure a client to connect to the database.

The complete schema is shown below (with default values), most commonly used sections are shown first:

# (optional section)
    # load task sources (.tasks[*].source) using a ploomber.SourceLoader
    # See section below for details
        # Example:
        # Load sources from my_module...
        module: my_module
        # [optional] ...use this path inside my_module
        path: path/to/sources/

    # Include tasks defined in a different file (must be a list where each
    # element is a valid Task. Useful for composing pipelines.
    # See section below for details
    import_tasks_from: /path/to/tasks.yaml

    # Reload your pipeline every time you open a Jupyter notebook. May
    # affect performance Jupyter's file loading performance if pipeline
    # has many tasks
    jupyter_hot_reload: True

    # Show function tasks as notebooks in jupyter
    jupyter_functions_as_notebooks: False

    # Default product class key for a given task class. Names should
    # match (case-sensitive) the names in the Python API. These are rarely
    # changed, except for SQLScript, see the section below for details
        SQLScript: SQLRelation
        SQLDump: File
        NotebookRunner: File
        ShellScript: File
        PythonCallable: File

    # Extract upstream dependencies from source code. If False, tasks
    # must declare dependencies using the "upstream" key
    extract_upstream: True

    # Extract product from source code. If False, tasks must have a "product" key
    extract_product: False

    # Make paths in File products relative to their sources, otherwise
    # they are relative to the pipeline.yaml parent folder
    product_relative_to_source: False

# execute tasks serially or in parallel (defaults to "serial")
executor: serial # or parallel

# DAG configuration (optional section)
    # For allowed keys and values see ploomber.DAGConfigurator
    {config-key}: {config-value}

# DAG clients (optional section)
    # Clients for connecting to databases
    {task or product class name}: {dotted.path.to.function}
    # Example (calls db.get_client without arguments)
    SQLScript: db.get_client
    # Call with arguments:
        dotted_path: db.get_client
        some_keyword_arg: value

# DAG-level serializer/unserializer for Python functions (both optional)
# see section below for details
serializer: {dotted.path.to.serializer, optional}
unserializer: {dotted.path.to.unserializer}

# DAG-level hooks execute a function on certain events
# (see section below for more details)
on_render: {dotted.path.to.hook, optional}
# executes upon successful execution (all tasks succeed)
on_finish: {dotted.path.to.hook, optional}
# executes upon failure (at least one task failed)
on_failure: {dotted.path.to.hook, optional}

# (this section is required)
    - {task dictionary, see next section for full details}

    # Example (notebook task)
    - source: clean_data.py
      # assuming meta.extract_product: False
      # and meta.extract_upstream: True
        nb: output/clean_data.ipynb
        data: output/clean.csv
      # params for the task, see section below for details
        some_param: some_value

        # resources is a special section, changes to the *contents* of
        # the files listed here cause the task to execute again, see the
        # corresponding section for details
            my_resource: file.json
      # grid of params (can be a dictionary or a list), see the section
      # below for details
        some_param: some_value


These are the default Task or Product clients to use. It allows you to specify a single client for all Tasks/Products with a given class, instead of one per Task or Product. The most common use case is SQL database configuration.

Keys must be valid ploomber.tasks or ploomber.products names, values must be dotted paths to functions that return a ploomber.clients instance.

Here’s an example that uses clients to configure Task and Product clients.

Other scenarios are ploomber.products.File clients, which Ploomber can use to backup pipeline results (say, for example, you run a job that trains several models and want to save output results. You can use ploomber.clients.GCloudStorageClient for that.

on_{render, finish, failure}

These are hooks that execure when specific events happen:

  1. on_render: executes after verifiying there are no errors in your pipeline declaration (e.g., a task that doesn’t exist declared as upstream dependency)

  2. on_finish: executes upon successful pipeline run

  3. on_failure: executes upon failed pipeline run

They all are optional and take a dotted path as an argument. For example, assume your hooks.py looks like this:

def on_render():
    print('finished rendering!')

def on_finish():
    print('finished executing!')

def on_failure():
    print('error when executing!')

You may add those hooks to your pipeline.yaml like this:

on_render: hooks.on_render
on_finish: hooks.on_finish
on_failure: hooks.on_failure

If your hook takes arguments, you may call it like this:

# to call any hook with arguments
    dotted_path: {dotted.path.to.hook}
    argument: value

Calling with arguments is useful when they are pipeline parameters.

serializer and unserializer

By default, tasks whose source is a function (i.e., ploomber.tasks.PythonCallable). Receive input paths (in upstream) and output paths (in product) when the function executes. Saving interim results allows Ploomber to provide incremental builds (What are incremental builds?).

However, in some cases, we might want to provide a pipeline that performs all operations in memory (e.g., to do online serving). ploomber.OnlineDAG can convert a file-based pipeline into an in-memory one without code changes, allowing you to re-use your feature engineering code for training and serving. The only requisite is for tasks to configure a serializer and unserializer. Click here to see an example.

Normally, a task whose source is a function looks like this:

import pandas as pd

def my_task(product, upstream):
    df_upstream = pd.read_csv(upstream['name'])
    # process data...
    # save product

And you use the product parameter to save any task output.

However, if you add a serializer, product isn’t passed, and you must return the product object:

import pandas as pd

def my_task(upstream):
    df_upstream = pd.read_csv(upstream['name'])
    # process data...
    return df_product

The serializer function is called with the returned object as its first argument and product (output path) as the second argument:

serializer(df_product, product)

A similar logic applies to unserializer; when present, the function is called for each upstream dependency with the product as the argument:


In your task function, you receive objects (instead of paths):

import pandas as pd

def my_task(upstream):
    # no need to call pd.read_csv here
    df_upstream = upstream['name']
    return df_product

If you want to provide a Task-level serializer/unserializer pass it directly to the task, if you set a DAG-level serializer/unserializer and wish to exclude specific task pass serializer: null or unserializer: null in the selected task.


If you package your project (i.e., add a setup.py), source_loader offers a convenient way to load sources inside such package.

For example, if your package is named my_package and you want to load from the folder my_sources/ within the package:

        module: my_package
        path: my_sources

    # this is loaded from my_package (my_sources directory)
    - source: script.sql
      # task definition continues...

To find out the location used, you can execute the following in a Python session:

import my_package; print(my_package) # print package location

The above should print something like path/to/my_package/__init__.py. Using the configuration above, it implies that source loader will load the file from path/to/my_package/my_sources/script.sql.

** Note:** this only applies to tasks whose source is a relative path. Dotted paths and absolute paths are not affected.

For details, see ploomber.SourceLoader, which is the underlying Python implementation. Here’s an example that uses source_loader.

SQLScript product class

By default, SQL scripts use ploomber.products.SQLRelation as product class. Such product doesn’t save product’s metadata; required for incremental builds (What are incremental builds?). If you want to use them, you need to change the default value and configure the product’s client.

Here’s an example that uses product_default_class to configure a SQLite pipeline with incremental builds.

For more information on product clients, see: FAQ and Glossary.


When training a Machine Learning pipeline, we obtain raw data, generate features, and train a model. When serving, we receive new observations, create features, and make predictions. Only the first and last parts change, but what happens in the middle remains the same (i.e., feature engineering). import_tasks_from allows you to compose pipelines for training and serving.

For example, you may define all your feature engineering code in a pipeline-features.yaml file. Then import those tasks (using import_tasks_from) in a training pipeline (pipeline.yaml) and a serving pipeline (pipeline-serving.yaml).

Click here to see a batch serving example.

Click here to see an online serving example.

Loading from a factory

The CLI looks for a pipeline.yaml by default, if you’re using the Python API, and want to save some typing, you can specify a pipeline.yaml like this:

# pipeline.yaml
location: {dotted.path.to.factory}

With such configuration, commands such as ploomber build will work.

task schema

# Task source code location, see section below for details
source: {path/to/source/file or dotted.path.to.function}

# Task product. Required if meta.extract_product=False,
# see section below for details
product: {str or dict}

# Task name. If missing, inferred from the task's source
name: {task name, optional}

# Function to execute when the task finishes successfully
on_finish: {dotted.path.to.function, optional}
# Function to execute when the task fails
on_failure: {dotted.path.to.function, optional}
# Function to execute after checking task declaration has no errors
on_render: {dotted.path.to.function, optional}

# Task parameters. See section below for details
    {key}: {value}

# Dotted path to a function that returns the task client.
# See section below for details.
client: {dotted.path.to.function, optional}

# Dotted path to a function that returns the product client.
# See section below for details.
product_client: {dotted.path.to.function, optional}

# Task class to use (any class from ploomber.tasks)
# You rarely have to set this, since it is inferred from "source".
# (e.g., NotebookRunner for .py and .ipynb files, SQLScript for .sql,
# PythonCallable for dotted paths)
class: {task class, optional}

# Product class (any class from ploomber.products)
# You rarely have to set this, since values from meta.product_default_class
# contain the typical cases
product_class: {str, optional}

# Dependencies for this task. Only required if meta.extract_upstream=True
upstream: {str or list, optional}

# All remaining values are passed to the task constructor as keyword
# arguments. See ploomber.tasks documentation for details


Indicates where the source code for a task is. This can be a path to a files if using scripts/notebooks or dotted paths if using a function.

By default, paths are relative to the pipeline.yaml parent folder (absolute paths are not affected), unless source_loader is configured; in such situation, paths are relative to the location configured in the SourceLoader object. See the source_loader section for more details.

For example, if your pipeline is located at project/pipeline.yaml, and you have:

    - source: scripts/my_script.py
      # task definition continues...

Ploomber will expect your script to be located at project/scripts/my_script.py

If using a function, the dotted path should be importable. for example, if you have:

    - source: my_package.my_module.my_function
      # task definition continues...

Ploomber runs a code equivalent to:

from my_package.my_module import my_function


Indicates output(s) generated by the task. This can be either a File(s) or SQL relation(s) (table or view). The exact type depends on the source value for the given task: SQL scripts generate SQL relations, everything else generates files.

When generating files, paths are relative to the pipeline.yaml parent directory. For example, if your pipeline is located at project/pipeline.yaml, and you have:

    - source: scripts/my_script.py
      product: output/my_output.csv

Ploomber will save your output to project/output/my_output.csv

When generating SQL relations, the format is different:

    - source: scripts/my_script.sql
      # list with three elements (last one can be table or view)
      product: [schema, name, table]
      # schema is optional, it can also be: [name, table]

If the task generates multiple products, pass a dictionary:

    - source: scripts/my_script.py
        nb: output/report.html
        data: output/data.csv

The mechanism to make product available when exeuting your task depends on the type of task.

SQL tasks receive a {{product}} placeholder:

-- {{product}} is replaced by "schema.name" or "name" if schema is empty
CREATE TABLE {{product}} AS
SELECT * FROM my_table WHERE my_column > 10

If product is a dictionary, use {{product['key']}}

Python/R scripts/notebooks receive a product variable in the “injected-parameters” cell:

# + tags=["parameters"]
product = None

# + tags=["injected-parameters"]
product = '/path/to/output/data.csv'

# your code...

If product is a dictionary, this becomes product = {'key': '/path/to/output/data.csv', ...}

Python functions receive the product argument:

import pandas as pd

def my_task(product):
    # process data...

If product is a dictionary, use product['key'].

The same logic applies when making upstream dependencies available to tasks, but in this case. upstream is always a dictionary: SQL scripts can refer to their upstream dependencies using {{upstream['key']}}. While Python scripts and notebooks receive upstream in the “injected-parameters” cell, and Python functions are called with an upstream argument.


Use this section to pass arbitrary parameters to a task. The exact mechanism depends on the task type. Assume you have the following:

    - source: {some-source}
      product: {some-product}
        my_param: 42

SQL tasks receive them as placeholders.

-- {{my_param}} is replaced by 42
SELECT * FROM my_table WHERE my_column > {{my_param}}

Python/R scripts/notebooks receive them in the “injected-parameters” cell:

# + tags=["parameters"]
my_param = None

# + tags=["injected-parameters"]
my_param = 42

# your code...

Python functions receive them as arguments:

# function is called with my_param=42
def my_task(product, my_param):

tasks[*].on_{render, finish, failure}

These are hooks that execute under certain events. They are equivalent to dag-level hooks, except they apply to a specific task. See the dag-level hooks documentation for details.

# to call without arguments
{hook-name}: hooks.some_function

# to call a function with arguments
    dotted_path: hooks.some_function
    argument: value


The params section contains an optional section called resources_ (Note the trailing underscore). By default, Ploomber marks tasks as outdated when their parameters change; however, parameters in the resources_ section work differently: they’re marked as outdated when the contents of the file change. For example, suppose you’re using a JSON file as a configuration source for a given task, and want to make Ploomber re-run a task if such file changes, you can do something like this:

    - source: scripts/my-script.py
      product: report.html
            # whenever the JSON file changes, my-script.py runs again
            file: my-config-file.json


Sometimes, you may want to run the same task over a set of parameters, grid allows you to do so. For example, say you want to train multiple models, each one with a different set of parameters:

  - source: random-forest.py
    # name is required when using grid
    name: random-forest-
    product: random-forest.html
        n_estimators: [5, 10, 20]
        criterion: [gini, entropy]

The spec above generates nine tasks for each combination of parameters with products random-forest-X.html where X goes from 0 to 8. Task names also include a suffix.

You may include multiple sets of parameters:

  - source: train-model.py
    name: train-model-
    product: train-model.html
      - model_type: random-forest
        n_estimators: [5, 10, 20]
        criterion: [gini, entropy]

      - model_type: [ada-boost]
        n_estimators: [1, 3, 5]
        learning_rate: [1, 2]

To create a task that depends on all tasks generated by the grid, you can use a wildcard (e.g., train-model-*).

You can use params and grid in the same task. Values in params are constant across the grid.

Click here to go to the sample code.


Task client to use. By default, the class-level client in the clients section is used. This task-level value overrides it. Required for some tasks (e.g., SQLScript), optional for others (e.g., File).

Can be a string (call without arguments):

client: clients.get_db_client

Or a dictionary (to call with arguments):

    dotted_path: clients.get_db_client
    kwarg_1: value_1
    kwarg_k: value_k


Product client to use (to save product’s metadata). Only required if you want to enable incremental builds (What are incremental builds?) if using SQL products. It can be a string or a dictionary (API is the same as tasks[*].client).

More information on product clients: FAQ and Glossary.

Custom task parameters

Parametrizing with env.yaml

In some situations, it’s useful to parametrize a pipeline. For example, you could run your pipeline with a sample of the data as a smoke test; to make sure it runs before triggering a run with the full dataset, which could take several hours to finish.

To add parameters to your pipeline, create and env.yaml file next to your pipeline.yaml:

my_param: my_value
    param: another_value

Then use placeholders in your pipeline.yaml file:

    - source: module.function
        my_param: '{{my_param}}'
        my_second_param: '{{nested.param}}'

In the previous example, module.function is called with my_param='my_value' and my_second_param='another_value'.

A common pattern is to use a pipeline parameter to change the location of tasks[*].product. For example:

    - source: module.function
      # path determined by a parameter
      product: '{{some_directory}}/output.csv'

    - source: my_script.sql
      # schema and prefix determined by a parameter
      product: ['{{some_schema}}', '{{some_prefix}}_name', table]

This can help you keep products generated by runs with different parameters in different locations.

These are the most common use cases, but you can use placeholders anywhere in your pipeline.yaml values (not keys):

    - source: module.function
      # doesn't work
      '{{placeholder}}': value

Setting parameters from the CLI

Once you define pipeline parameters, you can switch them from the command line:

ploomber {command} --env--param value # note the double dash

For example:

ploomber build --env--param value

Default placeholders

There are a few default placeholders you can use in your pipeline.yaml, even if not defined in the env.yaml (or if you don’t have a env.yaml altogether)

  • {{here}}: Absolute path to the parent folder of pipeline.yaml

  • {{cwd}}: Absolute path to the current working directory

  • {{root}}: Absolute path to project’s root folder. It is usually the same as {{here}}, except when the project is a package (i.e., it has setup.py file), in such a case, it points to the parent directory of the setup.py file.

  • {{user}}: Current username

A common use case for this is when passing paths to files to scripts/notebooks. For example, let’s say your script has to read a file from a specific location. Using {{here}} turns path into absolute so you can ready it when using Jupyter, even if the script is in a different location than your pipeline.yaml.

By default, paths in tasks[*].product are interpreted relative to the parent folder of pipeline.yaml. You can use {{cwd}} or {{root}} to override this behavior:

    - source: scripts/my-script.py
        nb: products/report.html
        data: product/data.csv
        # make this an absolute file so you can read it when opening
        # scripts/my-script.py in tJupyter
        input_path: '{{here}}/some/path/file.json'

For more on parametrized pipelines, check out the guide: Parametrized pipelines.