# Use a cache for pip

Using a download cache speeds up image builds.

Including this element in an image build causes $HOME/.cache/image-create/pip to be bind mounted as /tmp/pip inside the image build chroot. The $PIP_DOWNLOAD_CACHE environment variable is then defined as /tmp/pip, which causes pip to cache all downloads to the defined location.

Note that pip and its use of $PIP_DOWNLOAD_CACHE is not concurrency safe. Running multiple instances of diskimage-builder concurrently can cause issues. Therefore, it is advised to only have one instance of diskimage-builder that includes the pip-cache element running at a time.

The pip concurrency issue is being tracked upstream at https://github.com/pypa/pip/issues/1141