Python file write behaves differently in IDLE, PyCharm, and the command line: causes and fixes

Learn why Python file writes flush differently in IDLE, PyCharm, and command line, how interpreter lifetime affects buffers, and how context managers fix it.

Why a simple file write behaves differently in IDLE, PyCharm, and the command line

Problem overview

Consider a small script that opens a file for writing, writes a line, prints a status message, and then exits. In one environment the file is written as expected, and in another the file ends up empty. After the script prints “Completed” in IDLE, a dialog appears saying, “Your program is still running! Do you want to kill it?”

Here is a minimal example of the pattern:

outf = open('test_file.txt', 'w')
outf.write('This is a test file.')
print('Completed')
exit()

When run in PyCharm, the file content is written. When run from the command line, the content is also written. When run inside IDLE, the file can remain empty.

What actually goes wrong

The key difference is interpreter lifetime. IDLE keeps the Python interpreter running after your script finishes so that you can return to the Shell and inspect state. In that state, the file handle is still open and buffered writes may not yet have been pushed to disk. If you fully exit IDLE or restart the Shell (Ctrl-F6), the interpreter exits (or restarts) and the file is flushed and closed.

Other IDEs, such as running from the command line or from PyCharm, end the interpreter when the script completes. That shutdown flushes and closes open files. It’s simply a different design choice.

You can reproduce IDLE-like behavior from the command line with python -i script.py, which leaves the interpreter running after executing the script. The file won’t be flushed until you exit that interactive session.

One more detail: in IDLE, calling exit() triggers the “Your program is still running!” dialog. It kills the interpreter and closes the Shell, but because it’s a kill rather than a clean exit, the file isn’t flushed.

How to fix it reliably

The robust fix is to ensure files are closed deterministically. Using a context manager guarantees that the file is flushed and closed when the block ends, regardless of how the surrounding environment manages the interpreter.

with open('test_file.txt', 'w') as sink:
    sink.write('This is a test file.')
print('Completed')

This removes the dependency on interpreter shutdown for flushing and closes the file as soon as the block finishes. It also avoids the IDLE dialog by not invoking exit().

Why this matters

Even when using the same Python interpreter, running code inside different hosts changes runtime semantics around process lifetime and resource cleanup. Relying on interpreter teardown to flush I/O is fragile and environment-dependent. Keeping file handling explicit makes behavior consistent across IDLE, PyCharm, and the command line, and avoids confusing differences like an empty file in one environment and a populated file in another.

Takeaways

Close files explicitly and prefer context managers for file I/O. If you do keep the interpreter alive after script execution, expect open resources to remain open and buffered writes to be pending until the interpreter actually exits. If you see IDLE’s “Your program is still running!” dialog after calling exit(), remember that it’s killing the interpreter rather than performing a clean shutdown, so pending writes won’t be flushed. Writing code that manages resources directly is the simplest way to make outcomes predictable everywhere.