"""
Using a dictionary for open files.

The file is filled with NULL. Only the last entry is normal.

Result:
00 00 00 00 ... 00 0A 48 65 6C 6C 6F 0A

Ubuntu - 3.8.10
Windows - 3.9.8
"""
fhandles = {}
for f in range(100):
    fh = fhandles.setdefault("suffix", open(r"test.txt", "w"))
    fh.write("\nHello\n")
    fh.flush()
for f in fhandles.values():
    f.close()

import sys
print(sys.version_info)

Every time open(r"test.txt", "w") is called, the existing "test.txt" file gets overwritten in the filesystem as an empty file. For each iteration, setdefault() returns a reference to the first file object, which advances its file pointer with each fh.write("\nHello\n") call. For the last write, the file pointer is at offset 693 (i.e. 7 * 99), and the OS first writes null bytes up to the file pointer. This is normal behavior, not a bug.

I didn't think about calculating the argument before passing it to the function...