Python script freezes when writing large files

Symptoms

Python script appears to hang or freeze while writing large files
Output file stops growing before completion
No error message is shown in the terminal
High CPU or memory usage during the freeze
Script works normally with small files

Why This Happens

Writing large files stresses several system resources at once: memory, disk I/O, buffering, and sometimes the Python runtime itself. A freeze usually indicates the program is blocked waiting for a resource rather than crashing. Common causes include: - Writing very large data to disk in a single operation - Output buffers filling faster than they can flush - Running out of available memory - Slow or failing storage devices - File systems with strict write limits Because Python abstracts file operations, these problems can appear as a “silent” freeze.

Step-by-Step Troubleshooting

Step 1: Verify Disk Space and Disk Health

Confirm there is sufficient free disk space.
Test writing a large dummy file manually.
Check whether the drive is external or network-based. Disk bottlenecks often show up only during large writes.

Step 2: Write Data in Chunks

Instead of writing everything at once: - Break output into smaller chunks. - Write and flush periodically. - This prevents buffers from blocking.

Step 3: Monitor Memory Usage

Watch memory consumption during execution.
Large in-memory objects can stall garbage collection.
Reduce memory footprint where possible.

Step 4: Explicitly Flush Output

Call file.flush() periodically.
This forces buffered data to disk incrementally.

Step 5: Test Storage Speed

Write to a different drive.
Avoid slow USB drives or network shares.

When This Topic Is Limited

Once chunked writes and memory usage are addressed, most freezes disappear. Remaining issues are usually hardware-related.

Summary

Python scripts freezing during large file writes are almost always blocked on I/O or memory. Writing data incrementally and monitoring system resources resolves most cases.