Python f.seek caused OSError [Errno 22] if manually edit the text file but no error if program output to file

Question

I am trying to get last line from a text file, which I used the solution from What is the most efficient way to get first and last line of a text file?

def read_last_line(filename):
    with open(filename, "rb") as f:
        first = f.readline()
        if f.read(1) == '':
            return first
        f.seek(-2, 2)  # Jump to the second last byte.
        while f.read(1) != b"\n":  # Until EOL is found...
            f.seek(-2, 1)  # ...jump back the read byte plus one more.
        last = f.readline()  # Read last line.
        return last.decode('ascii')

It managed to get the last line of the text file successfully if the file is modified by another script/program, but when I modify the text file using Notepad++ the exact same modification as another script/program, it will throw the following exception:

in read_last_line
    f.seek(-2, 2)
OSError: [Errno 22] Invalid argument

What I am trying to do is, I used watchdog to check if there's file changes, and on modify I will call read_last_line on the modified file.

Sample file

11/26/2020 2:05:12 PM Time Updated: +2ms            Regular Update
11/26/2020 2:06:13 PM Time Updated: +4ms            Regular Update
11/26/2020 2:07:13 PM Time Updated: +1ms            Regular Update

How I am calling the function:

from watchdog.observers import Observer
from watchdog.events import FileSystemEventHandler
import ntpath

class FileEventHandler(FileSystemEventHandler):
    def __init__(self, filetowatch):
        self.file = filetowatch
    
    def on_modified(self, event):
        modified_file = ntpath.basename(event.src_path)
        if modified_file == self.file:
            read_last_line(event.src_path)

if __name__ == "__main__":
    event_handler = FileEventHandler("sample.txt")
    observer = Observer()
    observer.schedule(event_handler, path='C:/Program Files (x86)/SomeTime', recursive=False)
    observer.start()

May I know if anyone know what is causing the error?

Platform: Windows 10, Python 3.7.4

UPDATE - ANSWER

So the error was because of fread(1) == '' which was fixed using falsetru's solution.

The reason why it is not performing the way I expected was because text editor deletes the sample file and create a file using the same file name, therefore fread(1)=='' was triggered (which throws) and using script/program to modify the sample file did not throw simply because I did not delete the file.

Answer 1

If there's only a single (with / without trailing newline), while loop condition will never be met.

That cause f.seek(-2, 1) to try to seek to negative file position which causes the error.

Guard such case (prevent trying to seek beyond the file beginning), using .tell() which let you know current file position:

        while f.tell() >= 1 and f.read(1) != b"\n":

or you can use seek(..) return value:

        while f.read(1) != b"\n":
            if f.seek(-2, 1) == 0:
                break  # prevent going beyond file beginning.

UPDATE

In binary mode <io object>.read() return bytes object. In if condition, the code is comparing the bytes object with a string ''; which will always fail because of type difference. Changing to compare with bytes literal will fix the issue.

        if f.read(1) == b'':
            return first