How to Save a VM Snapshot State to a JSON File in Python

Define a dataclass for a VM snapshot and serialize it to a JSON file, then reload it to verify the state.

Easy Python 3.9+ Aug 9, 2026 Automation & scripting 13 views 0 copies

json dataclass files automation vm

Python code

35 lines
Python 3.9+
Run it 
import json
from dataclasses import dataclass, asdict
from pathlib import Path


@dataclass
class VMSnapshot:
    name: str
    memory_mb: int
    disk_gb: int
    state: str = "saved"

    def snapshot_to_file(self, path: Path) -> str:
        """Write snapshot state to a JSON file and return the filename."""
        path.write_text(
            json.dumps(asdict(self), indent=2)
        )
        return path.name


if __name__ == "__main__":
    snapshot = VMSnapshot(
        name="web-server-01",
        memory_mb=8192,
        disk_gb=200,
        state="saved"
    )

    filename = snapshot.snapshot_to_file(Path("vm_snapshot.json"))
    print(f"Snapshot saved to: {filename}")

    # Read back to verify
    with Path("vm_snapshot.json") as f:
        data = json.loads(f.read_text())
        print(f"Verified state: {data['state']} @ {data['memory_mb']} MB")

Output

stdout 
Snapshot saved to: vm_snapshot.json
Verified state: saved @ 8192 MB

How it works

The @dataclass decorator automatically generates an __init__, __repr__ and other methods, reducing boilerplate. asdict() converts the dataclass instance into a plain dictionary, which json.dumps() can serialize with indentation for readability. Writing the JSON to disk with Path.write_text() makes the snapshot durable and easy to inspect. Reloading with json.loads(Path.read_text()) confirms the data round-trips correctly. This pattern is simple, uses only the standard library, and is ideal for lightweight automation scripts.

Common mistakes

  • Forgetting that `Path` objects do not have a context manager — use `with open(path)` or call `read_text()` directly.
  • Using `json.dump` on a dataclass directly without converting to a dict first — it will raise a TypeError.
  • Not setting `indent=2` makes the output a single long line, harder to debug.
  • Assuming the file path exists — always ensure the parent directory is present or use `parents=True` with `mkdir()`.

Variations

  1. Use `Path.write_text(json.dumps(asdict(snapshot)))` without indentation for smaller files.
  2. Add `default=str` to `json.dumps` if your dataclass has non-serializable fields like datetime objects.

Real-world use cases

  • Saving VM configuration snapshots before automated provisioning or rollback in infrastructure scripts.
  • Persisting test environment state in CI pipelines for debugging and reproducibility.
  • Storing application state in a file-based format for simple backup and restore utilities.

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