Constructors

Every Dagger module has a constructor. The default one is generated automatically and has no arguments.

It's possible to write a custom constructor. The mechanism to do this is SDK-specific.

This is a simple way to accept module-wide configuration, or just to set a few attributes without having to create setter functions for them.

important

Dagger modules have only one constructor. Constructors of custom types are not registered; they are constructed by the function that chains them.

Simple constructor

The default constructor for a module can be overridden by registering a custom constructor. Its parameters are available as flags in the dagger command directly.

important

If you plan to use constructor fields in other module functions, ensure that they are declared as public (in Go and TypeScript). This is because Dagger stores fields using serialization and private fields are omitted during the serialization process. As a result, if a field is not declared as public, calling methods that use it will produce unexpected results.

Here is an example module with a custom constructor:

Go

// A Dagger module for saying hello world!

package main

import (
	"fmt"
)

func New(
	// +default="Hello"
	greeting string,
	// +default="World"
	name string,
) *MyModule {
	return &MyModule{
		Greeting: greeting,
		Name:     name,
	}
}

type MyModule struct {
	Greeting string
	Name     string
}

func (hello *MyModule) Message() string {
	return fmt.Sprintf("%s, %s!", hello.Greeting, hello.Name)
}

Python

from dagger import function, object_type


@object_type
class MyModule:
    greeting: str = "Hello"
    name: str = "World"

    @function
    def message(self) -> str:
        return f"{self.greeting}, {self.name}!"

important

Dagger @object_type classes are Python data classes that become exposed to the Dagger API. This means that you can adjust the generation of the constructor using standard data class features, such as fields and post-init processing. Refer to the official dataclasses documentation to learn more.

TypeScript

import { object, func } from "@dagger.io/dagger"

@object()
class MyModule {
  greeting: string
  name: string

  constructor(greeting = "Hello", name = "World") {
    this.greeting = greeting
    this.name = name
  }

  @func()
  message(): string {
    return `${this.greeting} ${this.name}`
  }
}

PHP

<?php

declare(strict_types=1);

namespace DaggerModule;

use Dagger\Attribute\{DaggerObject, DaggerFunction};

#[DaggerObject]
class MyModule
{
    #[DaggerFunction]
    public function __construct(
        private readonly string $greeting = 'Hello',
        private readonly string $name = 'World',
    ) {
    }

    #[DaggerFunction]
    public function message(): string
    {
        return "{$this->greeting} {$this->message}";
    }
}

info

In the PHP SDK the constructor must be the magic method __construct. As with any method, only public methods with the #[DaggerFunction] attribute will be registered with Dagger.

Java

package io.dagger.modules.mymodule;

import static io.dagger.client.Dagger.dag;

import io.dagger.client.Container;
import io.dagger.client.DaggerQueryException;
import io.dagger.client.Directory;
import io.dagger.module.annotation.Function;
import io.dagger.module.annotation.Ignore;
import io.dagger.module.annotation.Object;
import java.util.concurrent.ExecutionException;

@Object
public class MyModule {
  @Function
  public Container foo(@Ignore({"*", "!src/**/*.java", "!pom.xml"}) Directory source)
      throws ExecutionException, DaggerQueryException, InterruptedException {
    return dag().container().from("alpine:latest").withDirectory("/src", source).sync();
  }
}

info

In the Java SDK, the constructor must be public. A public empty constructor is also required in order to create the object from the serialized data.

Here is an example call for this Dagger Function:

System shell

dagger -c '. --name=Foo | message'

Dagger Shell

First type 'dagger' for interactive mode.

. --name=Foo | message

Dagger CLI

dagger call --name=Foo message

The result will be:

Hello, Foo!

Default values for complex types

Constructors can be passed both simple and complex types (such as Container, Directory, Service etc.) as arguments. Default values can be assigned in both cases.

important

Explicitly declare the type even when it can be inferred, so that the Dagger SDK can extend the GraphQL schema correctly.

Here is an example of a Dagger module with a default constructor argument of type Container:

Go

package main

import (
	"context"

	"main/internal/dagger"
)

func New(
	// +optional
	ctr *dagger.Container,
) *MyModule {
	if ctr == nil {
		ctr = dag.Container().From("alpine:3.14.0")
	}
	return &MyModule{
		Ctr: *ctr,
	}
}

type MyModule struct {
	Ctr dagger.Container
}

func (m *MyModule) Version(ctx context.Context) (string, error) {
	c := m.Ctr
	return c.
		WithExec([]string{"/bin/sh", "-c", "cat /etc/os-release | grep VERSION_ID"}).
		Stdout(ctx)
}

Here is an example call for this Dagger Function:

System shell

dagger -c version

Dagger Shell

First type 'dagger' for interactive mode.

version

Dagger CLI

dagger call version

The result will be:

VERSION_ID=3.14.0

Python

import dagger
from dagger import dag, function, object_type


@object_type
class MyModule:
    ctr: dagger.Container = dag.container().from_("alpine:3.14.0")

    @function
    async def version(self) -> str:
        return await self.ctr.with_exec(
            ["/bin/sh", "-c", "cat /etc/os-release | grep VERSION_ID"]
        ).stdout()

Here is an example call for this Dagger Function:

System shell

dagger -c version

Dagger Shell

First type 'dagger' for interactive mode.

version

Dagger CLI

dagger call version

The result will be:

VERSION_ID=3.14.0

Dagger objects are Python data classes

Since @object_type classes are Python data classes, you can fine tune the generation of the constructor in different ways.

Here is a more complex example:

from typing import Annotated

import dagger
from dagger import (
    DefaultPath,
    Doc,
    dag,
    field,
    object_type,
)


@object_type
class Workspace:
    source: Annotated[
        dagger.Directory,
        Doc("The context for the workspace"),
        DefaultPath("/"),
    ]

    token: Annotated[
        dagger.Secret | None,
        Doc("GitHub API token"),
    ] = None

    container: Annotated[
        dagger.Container,
        Doc("The container for the workspace"),
    ] = field(init=False)

    def __post_init__(self):
        self.container = (
            dag.container()
            .from_("python:3.11")
            .with_workdir("/app")
            .with_directory("/app", self.source)
            .with_mounted_cache("/root/.cache/pip", dag.cache_volume("python-pip"))
            .with_exec(["pip", "install", "-r", "requirements.txt"])
        )

A few important notes:

• Data class fields become constructor arguments by default unless specified as field(init=False). So, even though the container field is not added as a constructor argument it's still a required field that must be initialized either with a default value or in post-init processing.
• The __post_init__ function is called at the end of the auto-generated constructor which can be used, for example, to initialize field values that depend on other field values.
• Setting a default value like None in an attribute is the same as setting dataclasses.field(default=None). dataclasses.field() is only required to specify other non-default values, like init or to set the default using a factory function.
• The dagger.field descriptor wraps dataclasses.field, adding two important differences:
  • dagger.field() is used to expose the attribute as a Dagger Function in the API (a simple getter), but it also becomes a part of the object's state.
  • Not all arguments from dataclasses.field() can be set with dagger.field() which sets opinionated defaults, but you can specify both simple values and factory functions via the same default argument while with dataclasses.field() you need to use separate default and default_factory arguments.

Factory function

For default values that are more complex, dynamic or just mutable, use a factory function without arguments in dataclasses.field(default_factory=...) or dagger.field(default=...):

"""An example module using default factory functions."""

import dataclasses

import dagger
from dagger import dag, function, object_type


@object_type
class MyModule:
    base: dagger.Container = dataclasses.field(
        default_factory=lambda: dag.container().from_("python:alpine"),
    )
    packages: list[str] = dataclasses.field(default_factory=list)

    @function
    def container(self) -> dagger.Container:
        return self.base.with_exec(["apk", "add", "git", *self.packages])

Asynchronous usage

If a constructor argument needs an asynchronous call to set the default value, it's possible to replace the default constructor function from __init__() to a factory class method named create, as in the following code listing:

warning

This factory class method must be named create.

"""An example module using a factory constructor"""

from typing import Annotated

from dagger import Doc, dag, function, object_type


@object_type
class MyModule:
    """Functions for testing a project"""

    parallelize: int

    @classmethod
    async def create(
        cls,
        parallelize: Annotated[
            int | None, Doc("Number of parallel processes to run")
        ] = None,
    ):
        if parallelize is None:
            parallelize = int(
                await dag.container().from_("alpine").with_exec(["nproc"]).stdout()
            )
        return cls(parallelize=parallelize)

    @function
    def debug(self) -> str:
        """Check the number of parallel processes"""
        return f"Number of parallel processes: {self.parallelize}"

TypeScript

import { dag, Container, object, func } from "@dagger.io/dagger"

@object()
class MyModule {
  ctr: Container

  constructor(ctr?: Container) {
    this.ctr = ctr ?? dag.container().from("alpine:3.14.0")
  }

  @func()
  async version(): Promise<string> {
    return await this.ctr
      .withExec(["/bin/sh", "-c", "cat /etc/os-release | grep VERSION_ID"])
      .stdout()
  }
}

This default value can also be assigned directly in the field:

import { dag, Container, object, func } from "@dagger.io/dagger"

@object()
class MyModule {
  ctr: Container = dag.container().from("alpine:3.14.0")

  constructor(ctr?: Container) {
    this.ctr = ctr ?? this.ctr
  }

  @func()
  async version(): Promise<string> {
    return await this.ctr
      .withExec(["/bin/sh", "-c", "cat /etc/os-release | grep VERSION_ID"])
      .stdout()
  }
}

important

When assigning default values to complex types in TypeScript, it is necessary to use the ?? notation for this assignment. It is not possible to use the classic TypeScript notation for default arguments because the argument in this case is not a TypeScript primitive.

Here is an example call for this Dagger Function:

System shell

dagger -c version

Dagger Shell

First type 'dagger' for interactive mode.

version

Dagger CLI

dagger call version

The result will be:

VERSION_ID=3.14.0

PHP

<?php

declare(strict_types=1);

namespace DaggerModule;

use Dagger\Attribute\{DaggerObject, DaggerFunction};
use Dagger\Container;

use function Dagger\dag;

#[DaggerObject]
class MyModule
{
    private Container $ctr;

    #[DaggerFunction]
    public function __construct(
        ?Container $ctr,
    ) {
        $this->ctr = $ctr ?? dag()->container()->from('alpine:3.14.0');
    }

    #[DaggerFunction]
    public function version(): string
    {
        return $this->ctr
            ->withExec(['/bin/sh', '-c', 'cat /etc/os-release | grep VERSION_ID'])
            ->stdout();
    }
}

Here is an example call for this Dagger Function:

System shell

dagger -c version

Dagger Shell

First type 'dagger' for interactive mode.

version

Dagger CLI

dagger call version

The result will be:

VERSION_ID=3.14.0

Java

package io.dagger.modules.mymodule;

import static io.dagger.client.Dagger.dag;

import io.dagger.client.Container;
import io.dagger.client.DaggerQueryException;
import io.dagger.module.annotation.Function;
import io.dagger.module.annotation.Object;
import java.util.List;
import java.util.Optional;
import java.util.concurrent.ExecutionException;

@Object
public class MyModule {
  public Container ctr;

  public MyModule() {}

  public MyModule(Optional<Container> ctr) {
    this.ctr = ctr.orElseGet(() -> dag().container().from("alpine:3.14.0"));
  }

  @Function
  public String version() throws ExecutionException, DaggerQueryException, InterruptedException {
    return ctr.withExec(List.of("/bin/sh", "-c", "cat /etc/os-release | grep VERSION_ID")).stdout();
  }
}

Here is an example call for this Dagger Function:

System shell

dagger -c version

Dagger Shell

First type 'dagger' for interactive mode.

version

Dagger CLI

dagger call version

The result will be:

VERSION_ID=3.14.0