"_csv.Error: line contains NULL byte" after truncating a csv log file that is being piped to by anot

How do I truncate a csv log file that is being used as std out pipe destination from another process without generating a _csv.Error: line contains NULL byte error?

I have one process running rtlamr > log/readings.txt that is piping radio signal data to readings.txt. I don't think it matters what is piping to the file--any long-running pipe process will do.

I have a file watcher using watchdog (Python file watcher) on that file, which triggers a function when the file is changed. The function read the files and updates a database.

Then I try to truncate readings.txt so that it doesn't grow infinitely (or back it up).

file = open(dir_path+'/log/readings.txt', "w")
file.truncate()
file.close()

This corrupts readings.txt and generates the error (the start of the file contains garbage characters).

I tried moving the file instead of truncating it, in the hopes that rtlamr will recreate a fresh file, but that only has the effect of stopping the pipe.

EDIT I noticed that the charset changes from us-ascii to binary but attempting to truncate the file with file = open(dir_path+'/log/readings.log', "w",encoding="us-ascii") does not do anything.