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Templates capture setup once so every sandbox can start from a ready environment. 
from nullspace import Sandbox, Template, default_build_logger

builder = (
    Template()
    .from_python_image("3.12")
    .apt_install(["git", "curl"])
    .pip_install(["fastapi", "uvicorn"])
    .copy("./app.py", "/workspace/app.py")
    .set_workdir("/workspace")
)

build = Template.build(
    builder,
    name="fastapi-agent",
    tags=["stable"],
    on_log_entry=default_build_logger(),
)

with Sandbox.create(template=build.canonical_ref) as sandbox:
    result = sandbox.commands.run("python3 --version", shell=True)
    print(result.stdout)
Use Template.build_in_background() when a build should continue while your client disconnects or polls progress later. 
build = Template.build_in_background(builder, name="fastapi-agent", tags=["next"])
finished = build.wait_until_terminal()
print(finished.build.status)