On Sat, 30 Nov 2013 14:45:18 -0800, Eamonn Rea wrote: > When opening a file, you'd say whether you want to read or write to a > file. This is fine, but say for example later on in the program I change > my mind and I want to write to a file instead of reading it. Yes, I > could just say 'r+w' when opening the file, but what if I don't know if > I'm going to do both? What if I have the user decide, and then later on > I let the user change this.
I'd say if you're worried about this, your program needs to be redesigned. In general, with the exception of special purpose files like databases (and you're not inventing your own database, I hope) it is good clean programming practice to keep files open only so long as you really need them to be open. - It's only possible to have open some number of files open at a time. That number is typically quite large, but not *that* large. Every time you open a file and keep it open, it impacts other programs and the operating system by a small amount. - Particularly on Windows, once you open a file, nothing else can open it. That means it can't be backed up, scanned for viruses, opened by other programs, deleted or renamed. - When you open a file for writing, and keep it open, the changes you make aren't guaranteed to be written to disk until you actually close the file. You can call the sync method, the longer you keep it open the more likely the risk of data-loss in the case of sudden crash or power interruption. - To avoid data loss, you should try to avoid updating files in place. If the program crashes, you could leave the file in a messed up state. With very few exceptions, I recommend that you avoid this approach: # Don't do this. open file read data process in memory write changes to file more processing write changes to file more processing write changes to file close file exit in favour of this approach: open file; read data; close file process in memory open file; write to file; close file more processing open file; write to file; close file more processing open file; write to file; close file exit > Is it possible to do so without opening the file again and using the > same file object? No. You cannot even re-open a file object using the same file mode. Why do you care? It isn't difficult to throw away the file object and create a new one. That part is cheap. -- Steven -- https://mail.python.org/mailman/listinfo/python-list
