How does os.path.getsize() work within a "with" block in Python?

Question

The fun(filename) function creates a new file passed to it as a parameter, adds some text and then returns its size. When print(os.path.getsize(filename) is used within the 'with' block it doesn't work as intended.

import os def fun(filename): with open(filename,'w') as file: file.write("hello") print(os.path.getsize(filename)) fun("newFile2.txt")

It outputs 0. But when print(os.path.getsize(filename) is used outside the 'with' block as

import os def fun(filename): with open(filename,'w') as file: file.write("hello") print(os.path.getsize(filename)) fun("newFile2.txt")

Then it prints the correct output: 5. What's going on behind the scenes?

The file will written completely when it is flushed or closed, like at the end of the with block. — Klaus D.
– Klaus D., Commented Sep 5, 2022 at 18:51
Related: stackoverflow.com/questions/5972277/write-not-working-in-python — mkrieger1
– mkrieger1, Commented Sep 5, 2022 at 19:00

iHowell · Accepted Answer · 2022-09-05 18:51:21Z

This is due to the buffered nature of writing to files. When you close the file, i.e., exit the with block, the rest of the buffered input gets written to the file. You can read more about python buffering modes used in open here: https://docs.python.org/3/library/functions.html#open

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