How can I write to a file in Python without overwriting previous content from multiple functions?

General Discussions
1

I’m working with a script where several functions, including recursive ones—need to log to the same file. However, every time I open the file and write using the "w" mode, it overwrites the previous content.

What’s the proper way in Python to write to a file across multiple functions without losing existing data?

2

I’ve run into this issue myself! The ‘w’ mode is the culprit here because it literally means ‘write (and wipe),’ so it clears the file each time. If you want to add to the file without overwriting, you’ll need to use ‘a’ mode. It’s perfect when you’re appending from multiple functions. Here’s a simple example:


with open("output.txt", "a") as f:
    f.write("This will be added to the end of the file.\n")

I rely on this method a lot, especially when logging from recursive or async functions. It’s really reliable and saves a lot of hassle.

3

Ah, that sounds like something I dealt with in one of my CLI tools! Switching to ‘a’ fixed the overwriting issue, but I also ran into another problem, sometimes my logs were getting jumbled when I was calling from different threads. If you’re doing multithreading, I’d recommend using a file lock to avoid that. Or, even better, consider Python’s built-in logging module with FileHandler. It’s thread-safe, which makes things a lot easier to manage, especially when dealing with concurrent writes.

4

I’ve faced something similar as well. For more structured logging, especially when I had specific formatting requirements, I combined data with pandas and logged it all at once. Here’s a neat trick I used:

df.set_index('ID').apply(lambda row: row.tolist(), axis=1).to_dict()

It gives the same result but lets me tweak the format easily. But when it comes to logging messages, I prefer collecting them into a list during execution, then dumping everything into the file in one go at the end using ‘w’. It avoids multiple file I/O operations across functions, which is often more efficient:

log_lines = []

def do_something():
    log_lines.append("Step 1 complete")

# Later
with open("output.txt", "w") as f:
    f.write("\n".join(log_lines))

It just depends on how dynamic your logging needs are!