Python: How to ensure the last line of code has completed?

Question

Hi, I'm doing some kind of complex operation, it needs the last line(s) of code has completed, then proceed to the next step, for example. I need to ensure a file has written on the disk then read it. Always, the next line of code fires while the file haven't written on disk , and thus error came up. How resolve this?

Okay..

picture.save(path, format='png')
time.sleep(1)       #How to ensure the last step has finished
im = Image.open(path)

Answer 1

Seems like you just want to check is that there's a file at path before trying to open it.

You could check for the path's existence before trying to open it and sleep for a bit if it doesn't exist, but that sleep might not be long enough, and you still might have a problem opening the file.

You really should just enclose the open operation in a try-except block:

try:
    im = Image.open(path)
except [whatever exception you are getting]:
    # Handle the fact that you couldn't open the file
    im = None # Maybe like this..

Answer 2

You do not need to do anything unless the previous operation is asynchronous. In your case, you should check picture.save's documentation to see if it specifically define as asynchronous. By default everything is synchronize. Each line will complete before it continues to next operation.