Can relying on python3 to automatically close the files result in an unexpected behaviour?

Question

Can something wrong happen with the following implementation?

def ReadFromFile(file_name): return [line for line in open(file_name)] def AppendToFile(new_line, file_name): open(file_name, 'a').write(new_line)

I am not explicitly calling close() method after reading / writing to the file. My understanding is that semantically the program has to behave as if the file is always closed at the end of each function.

Can the following use of these functions give unexpected results, e.g.

original_lines = ReadFromFile("file.txt") for line in original_lines: AppendToFile(line, "file.txt") modified_lines = ReadFromFile("file.txt")

I would expect e.g. len(modified_lines) == len(original_lines) * 2. Can that ever not be the case?

why dont you use with open(...) ? This will make sure file is closed. — balderman
– balderman, Commented May 23, 2021 at 11:26
@balderman Extra identation block required, the code becomes slightly less readable IMO — mercury0114
– mercury0114, Commented May 23, 2021 at 12:45
Don’t sacrifice correctness for brevity…!? The extra block makes it explicit when and how the file is opened and closed… — deceze
– deceze ♦, Commented May 23, 2021 at 12:48
@deceze so my question is: am I sacrificing correctness? Any case where things may go wrong? — mercury0114
– mercury0114, Commented May 23, 2021 at 12:51

Lakshika Parihar · Accepted Answer · 2021-05-23 17:30:28Z

When we write onto a file using any of the write functions. Python holds everything to write in the file in a buffer and pushes it onto the actual file on the storage device either at the end of the python file or if it encounters a close() function.

Also if we opened another file with same file object then the first file will be closed by python for example:

 file_object1 = open(file1,'r') file_object1 = open(file2, 'r')

Here in this scenario also file1 will be automatically closed

So if the file terminates in between then the data is not stored in the file. So I would suggest two options:

use with because as soon as you get out of the block or encounter any exception it closes the file,
```
 with open(filename , file_mode) as file_object: do the file manipulations........ 
```
or you can use the flush() function if you want to force python to write contents of buffer onto storage without closing the file.
```
file_object.flush() 
```

For Reference: https://lerner.co.il/2015/01/18/dont-use-python-close-files-answer-depends/

"either at the end of the python file" - no, when the file object goes out of scope
@mercury0114 you are right if in between we change the value of the filehandle, for example, opened another file with the same file_object then also it closes the file but when the python file ends then the object will automatically go out of scope, so that's not incorrect

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