File Access

If you want to pass data in and out of the Sandbox during execution, you can use our filesystem API to easily read and write files. The API supports reading files up to 100 MiB and writes up to 16 MiB in size.

import modal

app = modal.App.lookup("sandbox-fs-demo", create_if_missing=True)

sb = modal.Sandbox.create(app=app)

with sb.open("test.txt", "w") as f:
    f.write("Hello World\n")

f = sb.open("test.txt", "rb")
print(f.read())
f.close()

The filesystem API is similar to Python’s built-in io.FileIO and supports many of the same methods, including read, readline, readlines, write, flush, seek, and close.

In addition, we also provide the special methods replace_bytes and delete_bytes, which may be useful for LLM-generated code.

from modal.file_io import delete_bytes, replace_bytes

with sb.open("example.txt", "w") as f:
    f.write("The quick brown fox jumps over the lazy dog")

with sb.open("example.txt", "r+") as f:
    # The quick brown fox jumps over the lazy dog
    print(f.read())

    # The slow brown fox jumps over the lazy dog
    replace_bytes(f, b"slow", start=4, end=9)

    # The slow red fox jumps over the lazy dog
    replace_bytes(f, b"red", start=9, end=14)

    # The slow red fox jumps over the dog
    delete_bytes(f, start=32, end=37)

    f.seek(0)
    print(f.read())

sb.terminate()

Syncing files outside the Sandbox

Modal Volumes or CloudBucketMounts can also be attached to Sandboxes for file syncing outside the Sandbox. If you want to give the caller access to files written by the Sandbox, you could create an ephemeral Volume that will be garbage collected when the App finishes:

with modal.Volume.ephemeral() as vol:
    sb = modal.Sandbox.create(
        volumes={"/cache": vol},
        app=my_app,
    )
    p = sb.exec("bash", "-c", "echo foo > /cache/a.txt")
    p.wait()
    sb.terminate()
    for data in vol.read_file("a.txt"):
        print(data)

Alternatively, if you want to persist files between Sandbox invocations (useful if you’re building a stateful code interpreter, for example), you can use create a persisted Volume with a dynamically assigned label:

session_id = "example-session-id-123abc"
vol = modal.Volume.from_name(f"vol-{session_id}", create_if_missing=True)
sb = modal.Sandbox.create(
    volumes={"/cache": vol},
    app=my_app,
)
p = sb.exec("bash", "-c", "echo foo > /cache/a.txt")
p.wait()
sb.terminate()
for data in vol.read_file("a.txt"):
    print(data)

File syncing behavior differs between Volumes and CloudBucketMounts. For Volumes, files are only synced back to the Volume when the Sandbox terminates. For CloudBucketMounts, files are synced automatically.