Atomic writes — how tempfile + os.replace prevent corrupted JSON

What happens if the power cuts out while a process is writing to a config file? Or if antivirus software on Windows briefly locks a file mid-write? If you naively overwrite a file with open(path, 'w') , whatever partial content existed at the moment of interruption is what remains on disk. For JSON, that usually means broken syntax — json.load() throws on the next startup, and the entire configuration is effectively lost. This article walks through a standard technique for preventing that: writing to a temporary file first, then swapping it in atomically. Note: "Atomic" here means an operation either completes entirely or doesn't happen at all — there's no partial, observable in-between state. It's the same sense of the word used for database transactions. Why direct overwrites are dangerous open(path, 'w') effectively truncates the file first and then writes the new content. If the process is interrupted during that window, the file is left empty or holding incomplete content. # Dangerous: a crash mid-write leaves a corrupted file behind with open ( ' config.json ' , ' w ' ) as f : json . dump ( data , f ) # what if this gets interrupted? The causes vary: a kill -9 , a power outage, antivirus software briefly blocking file access on Windows, or a backup tool grabbing the file mid-write. This rarely reproduces during local development, but in a long-running production environment, it will eventually happen with near certainty. The fix: write to a temp file, then swap it in The core idea is simple. Never touch the target file directly. Write the complete new content to a temporary file first, confirm that write fully succeeded, and only then replace the target file with that temp file. import json import os import tempfile def atomic_write_json ( filepath , data ): dirpath = os . path . dirname ( os . path . abspath ( filepath )) or ' . ' fd , tmp_path = tempfile . mkstemp ( dir = dirpath , suffix = ' .json.tmp ' ) try : with os . fdopen ( fd , ' w ' , encoding = ' u