By default, Modal functions are executed within a Debian Bullseye Linux container with a basic Python installation of the same minor version that your local Python has.
Oftentimes you might need some third party Python packages, or some other pre-installed dependencies for your function. Modal provides a few different options to customize the container your function runs in.
Additional Python packages
The simplest and most common container modification is to add some third party
Python package, like pandas. To do this you can
create a custom
modal.Image by starting with the
function, and then extending the image by invoking the
pip_install method with
a list of all of the packages you need.
pandas_image = modal.Image.debian_slim().pip_install("pandas", "numpy") def my_function(): import pandas as pd import numpy as np df = pd.DataFrame() ...
Importing Python packages
You might want to use packages inside your Modal code that you don’t have on
your local computer. In the example above, we build a container that uses
pandas. But if we don’t have pandas locally, on the computer launching the
Modal job, we can’t put
import pandas at the top of the script, since it would
The easiest solution to this is to put
import pandas in the function body
instead, as you can see above. This means that
pandas is only imported when
running inside the remote Modal container, which has
If you have a lot of functions and a lot of Python packages, you might want to
keep the imports in the global scope so that every function can use the same
imports. In that case, you can use the
pandas_image = modal.Image.debian_slim().pip_install("pandas", "numpy") if stub.is_inside(): import pandas as pd import numpy as np def my_function(): df = pd.DataFrame()
is_inside is considered beta and its interface may change in the
You can also supply shell commands that should be executed when building the container image. This can be useful for installing additional binary dependencies:
ffmpeg_image = modal.Image.debian_slim().apt_install("ffmpeg") def process_video(): subprocess.call(["ffmpeg", ...])
Or for preloading custom assets into the container:
image_with_model = ( modal.Image.debian_slim().apt_install("curl").run_commands( "curl -O https://raw.githubusercontent.com/opencv/opencv/master/data/haarcascades/haarcascade_frontalcatface.xml", ) ) def find_cats(): content = open("/haarcascade_frontalcatface.xml").read() ...
Using existing Docker Hub images
Docker Hub has many pre-built images for common
software packages. You can use any such image as your function container using
modal.Image.from_dockerhub as long as the image conforms to the following
- Python 3.7 or above is present, and is available as
pipis installed correctly
- The image is built for the
sklearn_image = modal.Image.from_dockerhub("huanjason/scikit-learn") def fit_knn(): from sklearn.neighbors import KNeighborsClassifier ...
pip isn’t set up properly, you can use the
setup_dockerfile_commands to run extra commands before the Modal package is
image = modal.Image.from_dockerhub( "gisops/valhalla:latest", setup_dockerfile_commands=["RUN apt-get update", "RUN apt-get install -y python3-pip"], )
Using Conda instead of pip
Modal provides a pre-built Conda base image, if you would like to use
for package management. The Python version available is whatever version the
official miniconda3 image
currently comes with (
3.9.12 at this time).
pymc_image = modal.Image.conda().conda_install("theano-pymc==1.1.2", "pymc3==3.11.2") def fit(): import pymc3 as pm ...
Using a Dockerfile
Modal also supports using a Dockerfile using the
function. It takes a path to an existing Dockerfile. For instance:
FROM python:3.9 RUN pip install sklearn
dockerfile_image = modal.Image.from_dockerfile("Dockerfile") def fit(): import sklearn ...
Dockerfile command compatibility
Since Modal doesn’t use Docker to build containers, we have our own implementation of the Dockerfile specification. Most Dockerfiles should work out of the box, but there are some differences to be aware of.
First, a few minor Dockerfile commands and flags have not been implemented yet. Please reach out to us if your use case requires any of these.
Next, there are some command-specific things that may be useful when porting a Dockerfile to Modal.
command is supported, there is an additional constraint to the entrypoint script
provided: it must also
exec the arguments passed to it at some point. This is
so that Modal’s own Python entrypoint can run after your own. Most entrypoint
scripts in Docker containers are typically “wrapper” scripts so this is already
If you wish to write your own entrypoint script, you can use the following as a template:
# Your custom startup commands here. exec "$@" # Runs the command passed to the entrypoint script.
If the above file is saved as
/usr/bin/my_entrypoint.sh in your container,
then you can register it as an entrypoint with
ENTRYPOINT ["/usr/bin/my_entrypoint.sh"] in your Dockerfile, or with
dockerfile_commands as an
Image build step.
ENV commands are supported, we don’t currently support
This means you can’t yet do
A work-around is to use
ENTRYPOINT instead, and use regular bash commands to
Running a function as a build step (beta)
Instead of using shell commands, you can also run a Python function as an image
build step using the
Image.run_function method. For
example, you can use this to download model parameters to your image:
def download_models(): import diffusers pipe = diffusers.StableDiffusionPipeline.from_pretrained( model_id, use_auth_token=os.environ["HUGGINGFACE_TOKEN"] ) pipe.save_pretrained("/model") image = ( modal.Image .debian_slim() .pip_install("diffusers[torch]", "transformers", "ftfy", "accelerate") .run_function(download_models, secrets=[modal.Secret.from_name("huggingface")]) )
Any kwargs accepted by
SharedVolumes, and resource requests) can be
supplied to it. Essentially, this is equivalent to running a Modal function and
snapshotting the resulting filesystem as an image.
Please see the reference documentation for an explanation of which changes to your build function trigger image rebuilds.
Forcing an image to rebuild
Modal uses the definition of an image to determine whether it needs to be
rebuilt. In some cases, you may want to force an image to rebuild, even if the
definition hasn’t changed. You can do this by adding the
argument to any of the image build steps.
image = ( modal.Image.debian_slim() .apt_install("git") .pip_install("slack-sdk", force_build=True) .run_commands("echo hi") )
In the above example, both
run_commands("echo hi") will run again, but
apt_install("git") will not.
Remember to remove
force_build=True after you’ve rebuilt the image, otherwise
it will rebuild every time you run your code.