Writing Files

It is just about as easy to write to a file as to read from one.

Here we go. As a reminder, function open takes the

name of a file, either a full path or just the name of the file is in the same

directory as the program and the mode, one of a read, write, and depend. Much like

files have read and read line and read lines methods, they have a write method if

we've opened them for writing. Method write works much like Python's

print function except that it doesn't add a new line character.

We're going to write a short bit of code that makes a copy of a file, but puts the

word copy as the first line in the new copied file.

We will start by showing you how to prompt a user for a file.

Module tkinter has a sub-module called file dialog.

We import it like this. Inside that module, is a function

askopenfilename. This will pop open a dialogue and allow us

to select a file using your operating system's file chooser.

When we call askopenfilename, we'll need the switch to Python in order to select

the file. We will go find, In Flanders Fields txt..

When we select open, we see that the, the file dialogue disappears.

But we are left with this window with title tk.

This window does no harm, and we will ignore it, and switch back to the Python

shell. Askopenfilename has returned a string

containing the full path to the file that we chose so that we can use this when we

call function open. We will call this again, and remember, the

string that was returned. We need to switch back to Python in order

to select our file. Now, from file name, is the path to that

file. We will use a new function in the same

file dialogue module in order to choose a file that we would like to write to.

Instead of askopenfilename, we will asksaveasfilename.

Asksaveasfilename allows us to choose a new file name to save to, but if we choose

a file that already exists, we will be prompted to confirm that we really do want

to replace that file. We'll switch back to Python.

And you'll notice that it says, save as, with a place to type the name of the new

file. Well, I don't want to save it in

6readfiles because this lecture is 6writefiles.

We'll call this one, Flanders copy.txt. And here is what we chose.

We will now open from<u>filename in read mode.</u>

Read the contents, Close the file,

And check to make sure that we got what we expected and indeed, contents is a string

containing what was in that file including all of the new line characters.

Next, we will open the file that we want to write to write the word copy and the

new line. Remember, that we have to add any new

lines that we want because unlike print, the write method does not add any extra

characters. Then, we'll write the contents,

And finally, close the file. Notice that when we opened the to<u>file,</u>

the file that we're copying to, we used the character w when we opened to file.

And the write method returned information, It returned an int.

In particular, it returned the number of characters that were written.

Notice that this shows that the backslash then is treated as a single character,

because c, o, p, y, and the new line make five characters.

Write returns this information so you can check to make sure that the length of your

string is the number of characters that were successfully written.

We will now verify that, that file was written successfully.

Here's our Flanders copy txt.. When I double click on it, it opens and we

can see that the word copy and the new line character appear at the top, just as

we had written.

Learn to Program: The Fundamentals

Skills You'll Learn

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