• stat
• os.stat

Calling write does not immediately write to disk. The Operating System provides buffering as an optimization to avoid frequent access to the disk. In this case it means the file has not been saved before we already check its size.

import os

for ix in range(10):
    filename = f'data{ix}.txt'
    fh = open(filename, 'w')
    fh.write('hello')
    if ix == 0:
        break
    fh.close()
stat = os.stat(filename)
print(stat.st_size)    # 0,   the file has not been saved yet