How to decipher text coded files
We don't want to delete the existing messages so we choose to append the error, like this:Įrr = open("Errors.log","a") err.write(msg) Even better, if the file doesn't exist it will open a new file just as if you'd specified "W".Īs an example, lets assume we have a log file that we use for capturing error messages. Fortunately there's another mode "a" that we can pass to open() which allows us to append directly to an existing file just by writing. If the file is short that's not a problem but if the file is very large, maybe over 100Mb, then you will simply run out of memory to hold the list. One way to do that would be to open the file for input, read the data into a list, append the data to the list and then write the whole list out to a new version of the old file. One final twist is that you might want to append data to the end of an existing file. # Now close the files inp.close() outp.close()ĭid you notice that I added a print statement just to reassure the user that something actually happened? This kind of user feedback is usually a good idea. # First open the files to read(r) and write(w) inp = open("menu.txt","r")įor line in inp.readlines(): outp.write(line) # Create the equivalent of: COPY MENU.TXT MENU.BAK To create a 'copy' command in Python, we simply open a new file in write mode and write the lines to that file instead of printing them. You open the file, read it in and manipulate it any way you want to. Request the user to press a key (enter say) before resetting the line_count to zero and continuing. You might then use a line_count variable which is incremented for each line then tested to see whether it is equal to 25 (for a 25 line screen). First of all you would need to read the file one line at a time (in Python by using readline() instead of readlines(). You can think of a file variable as being a reference to a module containing functions that operate on files and which we automatically import every time we create a file type variable.Ĭonsider how you could cope with long files. Don't worry about it for now, except to realize that it's related in some ways to modules. This notation is known as method invocation and is our first glimpse of Object Orientation. Note 2: We read and close the file using functions preceded by the file variable. The mode determines whether we are opening the file for reading(r) or writing(w), and also whether it's for ASCII text or binary usage - by adding a 'b' to the 'r' or 'w', as in: open(fn,"rb") The first is the filename (which may be passed as a variable or a literal string, as we did here). # First open the file to read(r) inp = open("menu.txt","r") Now we will write a program to read the file and display the output - like the 'cat' command in Unix or the 'type' command in DOS. We will assume that a file exists called menu.txt and that it holds a list of meals: One other thing you can do while processing a file is that you can go back to the beginning. You can write by creating a new file from scratch (or overwriting an existing one) or by appending to an existing one. The other difference is that you normally open the file as read only or write only. In practice the word processor often does the same, it just holds the entire file in memory while you work on it and then writes it all back out when you close it. The biggest differences are that in a program you access the file sequentially, that is, you read one line at a time starting at the beginning. Files in a programming sense are no different from files that you use in a word processor or other application: you open them, do some work and then close them again. Handling files often poses problems for beginners although the reason for this puzzles me. Controlling Appearance using Frames and the Packer.> bQuit = Button(F, text="Quit", command=F.quit).>lHello = Label(F, text="Hello world").Just don't panic if it doesn't make sense stright away. Occasionally, it is so useful that it is invaluable, so I present it here for your study. Note: This is a fairly advanced topic and for most applications you don't need to know anything about it.Looping - Or the art of repeating oneself!.More information on the Address example.