Introduction to Files

1Why a program needs files at all

Think about a marks program. You run it, type in five subjects, and it prints the average. Run it again tomorrow and you type all five in again. The program did not remember anything — it never could.

Everything a running program holds — every variable, every list, every value input() collected — lives in RAM, the computer's working memory. RAM is fast, and it is temporary. When the program ends, the space is handed back and the values are wiped.

The two words worth keeping are named and permanent. Named, because a program has to be able to ask for it back by name. Permanent, because that is the entire point: this is data that outlives the program.

2Underneath, every file is a row of numbers

Here is the fact that makes the rest of this chapter make sense. A disk cannot store letters, colours or sound. It can only store bytes — whole numbers from 0 to 255. That is all.

So a file, any file, is one long row of numbers. A photo is a row of numbers. A song is a row of numbers. Your Python program is a row of numbers. What separates them is not the numbers — it is what those numbers are meant to stand for.

3Text files: the numbers are character codes

In a text file, every byte is a character code — the same codes you met with ord() and chr() in Class 11. Byte 72 is H, byte 101 is e, byte 10 is the newline you get by pressing Enter.

line = 'Hello'

for ch in line:
    print(ch, '->', ord(ch))

Those five numbers — 72, 101, 108, 108, 111 — are exactly what would be sitting on the disk if you saved Hello into a file called notes.txt. Nothing else goes in: no font, no size, no colour, no bold. A text file has no room for any of that, because it has nowhere to put it.

4Binary files: the numbers mean something else

In a binary file, the bytes are not character codes. They are whatever the program that created the file decided they should be — the red-green-blue values of a pixel, the loudness of a millisecond of sound, a compressed block of text, a machine instruction.

Try to read those bytes as characters anyway and you get nonsense, because you are asking the wrong question of them. Byte 137 is not a letter. Nobody typed it, and no letter will come out of it.

5Look inside five real files

You have almost certainly done this by accident: right-clicked a photo, chosen “Open with → Notepad”, and got a screenful of junk. Here is the same experiment, done on purpose. The left row is what is really on the disk; the right row is what a text editor makes of it.

6CSV files — which are text files

CSV stands for Comma Separated Values. It is how a table is stored when you want any program to be able to read it. One line per row, and the values in that row separated by commas:

name,marks
Asha,92
Rohit,78
Meera,85

The first line is usually the header — the column names. Every line after it is one record. Open this in Excel and you get a neat grid with three columns; open it in Notepad and you get exactly the four lines above, because that is all the file contains.

The syllabus lists three types of file: text, binary and CSV. That is worth reading carefully, because it is easy to come away thinking there are three separate things. There are two:

7Almost every file you meet is binary

This surprises people. Text files feel like the normal case, because they are the ones you can look at. In fact they are the minority. Nearly everything on a real computer — every document, every photo, every song, every installed program — is binary.

Count them: four text, eight binary — and that is a generous sample. On a real laptop the ratio is far more lopsided, because every single file that makes Windows work is binary too.

A Word document is the clearest case. It looks like text; it is text you typed. But the file has to store your text and the font, the size, the bold, the margins, the page breaks, the pictures you pasted in. None of that fits in a row of character codes, so .docx is binary — a zip folder, in fact, with your writing compressed inside it.

8Text vs binary, side by side

9Recap