Computer Science · Class 12 · Text Files

Text filesModes⏱️ 16 min read

The Open Modes

The second argument to open() is one or two characters long, and it decides more than any other line in the program. One of these six letters empties the file before you have written anything to it — and there is no way to get the old contents back.

1The mode is a promise

When you open a file you tell Python what you intend to do with it. That is the mode. Python then holds you to it: open for reading and a write() is refused, open for writing and a read() is refused.

held_to_it.py

f = open('notes.txt', 'r')
f.write('one more line')
f.close()

Output

Traceback (most recent call last):
  File "held_to_it.py", line 2, in <module>
    f.write('one more line')
io.UnsupportedOperation: not writable

Why not just allow everything?

Because a program that says what it will do can be trusted with a file. Opening in 'r' is a guarantee, checked by Python, that this program cannot damage the file — not by accident, not by a typo in a line you never tested.

2Three letters: r, w, a

Everything starts with three. Learn what these do to a file that already has something in it and the rest of the lesson is detail.

'r'

read

Look at the file. You may not change it. If it is not there, the program stops with FileNotFoundError.

'w'

write

Write into the file — after emptying it completely. If it is not there, it is created.

'a'

append

Add to the end of the file. What was there stays. If it is not there, it is created.

Two programs, one letter apart. Run them and read the file after each:

w_erases.py

f = open('demo.txt', 'w')
f.write('first line\n')
f.close()

f = open('demo.txt', 'w')
f.write('second line\n')
f.close()

f = open('demo.txt', 'r')
print(f.read())
f.close()

Output

second line

a_adds.py

f = open('demo.txt', 'a')
f.write('added at the end\n')
f.close()

f = open('demo.txt', 'r')
print(f.read())
f.close()

Output

second line
added at the end

'first line' is gone, and nothing can bring it back

The second open('demo.txt', 'w') emptied the file at the moment it was opened — before write() ran, and whether or not it ran at all. A program that opens your notes in 'w' and then crashes on the next line has still destroyed them. This is the single most expensive mistake in the chapter.

3And then the + versions

Each of the three has a twin with a + after it. The + means “and the other job too”: a reading mode that may also write, or a writing mode that may also read.

What the + does not change is what happens to the file when it is opened. 'w+' empties the file exactly as 'w' does. 'a+' appends exactly as 'a' does. Read the letter first and the + second.

r_plus.py

f = open('demo.txt', 'w')
f.write('Python is easy\n')
f.close()

f = open('demo.txt', 'r+')
f.write('JAVA')
f.close()

f = open('demo.txt', 'r')
print(f.read())
f.close()

Output

JAVAon is easy

r+ writes ON TOP, it does not push anything along

'r+' opens with the pointer at the start, so those four characters landed on the first four: Pyth became JAVA and on is easy was never touched. Writing into the middle of a file overwrites. Nothing is ever inserted.

4Six modes, one file

The same file and the same one-line program, opened six ways. The right-hand pane is the file afterwards — every one of them was run for real.

🎛️ The mode lab

The same file and the same one-line program, opened six different ways. Watch the right-hand pane.

f = open('demo.txt', 'r')
f.write('JAVA') # io.UnsupportedOperation: not writable
f.close()

demo.txt before

Python is easy

demo.txt after

Python is easy

Can it read?yes

Can it write?no

File not there?FileNotFoundError

What is already inside?kept — you cannot write at all

Pointer starts atstart of the file

Short forread

The safe one. You may look and nothing else. Writing raises io.UnsupportedOperation: not writable.

5All six, side by side

Let's Recap!

Mode	Read?	Write?	If the file is missing	What happens to the contents
'r'	yes	no	FileNotFoundError	kept
'r+'	yes	yes	FileNotFoundError	kept — writes overwrite
'w'	no	yes	created	ERASED
'w+'	yes	yes	created	ERASED
'a'	no	yes	created	kept — writes go to the end
'a+'	yes	yes	created	kept — writes go to the end

Two questions, six answers

Every row above is the same two questions. Which letter? r keeps the file and reads it, w empties it, a adds to the end. Is there a +? If so, the other job is allowed as well. Learn it that way and there are three things to remember, not six.

6Where each mode leaves the pointer

An open file has a pointer — the place the next read or write will happen. The mode decides where it starts, and that is the difference between 'r+' and 'a+', which otherwise look alike.

'r', 'r+', 'w', 'w+' — pointer at the start

Position 0. For 'w' and 'w+' the file is empty by then anyway, so the start is also the end.

'a', 'a+' — pointer at the end

Straight past everything already in the file, which is exactly why an append can never overwrite anything.

That has one consequence worth seeing, because it catches everyone: open a file in 'a+' and read it immediately, and you get an empty string. There is nothing after the pointer.

a_plus_reads_nothing.py

f = open('demo.txt', 'w')
f.write('line one\n')
f.close()

f = open('demo.txt', 'a+')
print('read gives:', repr(f.read()))
f.close()

f = open('demo.txt', 'r')
print('but the file holds:', repr(f.read()))
f.close()

Output

read gives: ''
but the file holds: 'line one\n'

Tip

The file is not empty — the second handle proves it. The 'a+' handle simply had nothing in front of it to read. Sending the pointer back to the start without opening the file again is what seek() is for, and it has a lesson later in this chapter.

7The letter you never see: t

There is a second half to every mode. 'r' is really 'rt' — read, in text mode. Text mode is the default, so nobody writes the t.

Swap it for 'b' and you are in binary mode: 'rb', 'wb', 'ab'. That is how a program opens a photo or a .docx — the files the last chapter called binary. In binary mode you read and write bytes rather than strings.

Text mode vs binary mode

	Text mode ('t', the default)	Binary mode ('b')
You read and write	str — ordinary strings	bytes
Line endings	Translated for your OS	Left exactly as they are
Used for	.txt .py .csv .html	.png .pdf .docx .mp3
Written as	'r', 'w', 'a' (the t is assumed)	'rb', 'wb', 'ab'

Note

This whole chapter is text mode. Binary files are the next chapter's business — but if you meet 'rb' in a question paper, now you know it is the same six modes with one letter added.

8Choosing, in one question

Ask what should happen to what is already in the file. The answer names the mode:

'r'

I only want to look at it.

The safest choice, and it is the default for a reason.

'w'

I want a fresh file — throw away anything old.

A report generated afresh every run. Nothing worth keeping is in there.

'a'

I want to add today's records to the ones already saved.

Logs, attendance, anything that grows. This is the one real programs use most.

'r+'

I want to read it, then write back to the same handle.

Rare, and easy to get wrong — writes overwrite from wherever the pointer is.

Try it yourself. Change the mode on the first line, run it, and watch what the file ends up holding:

try_a_mode.py

📄 demo.txt

📁 demo.txt (virtual file)

Python is easy

Accessible via standard python read calls.

# change 'a' to 'w', then to 'r+', and run it again
f = open('demo.txt', 'a')
f.write('JAVA')
f.close()

f = open('demo.txt', 'r')
print(f.read())
f.close()

# change 'a' to 'w', then to 'r+', and run it again
f = open('demo.txt', 'a')
f.write('JAVA')
f.close()

f = open('demo.txt', 'r')
print(f.read())
f.close()

9Recap

The mode is the second argument

A string. Leave it out and Python uses 'r'.

r keeps, w erases, a adds

Three letters, and the whole table follows from them.

+ adds the other job

Reading to a writing mode, writing to a reading mode. It never changes what happens to the file.

w and a create a missing file

r and r+ do not — they raise FileNotFoundError instead.

Writing overwrites, never inserts

In 'r+' the new characters land on top of the old ones from wherever the pointer is.

'b' means binary

'rb', 'wb', 'ab' read and write bytes. Without it you are in text mode, which is all of this chapter.

✍️ Now write these yourself

1
Write three lines into diary.txt with 'w'. Run the program a second time. How many lines are in the file?
Hint · Three. Not six — and that is the point of the exercise.
2
Change the mode to 'a' and run it twice more.
Hint · Now it grows by three lines every run.
3
Open a file that does not exist in 'w', then open another that does not exist in 'r'.
Hint · One creates a file. The other raises FileNotFoundError.
4
Put Hello world in a file, open it in 'r+', write HELP, and read the file.
Hint · HELPo world. Four characters replaced, none inserted.

Quick Check

A file holds three lines. You open it with mode 'w' and the program crashes before write() runs. What is in the file?

Quick Check

Which mode adds to a file, and creates it if it is not there?

Quick Check

demo.txt holds 'Python is easy'. You open it in 'r+' and write 'JAVA' straight away. What does the file hold now?

Quick Check

What does the + in 'w+' change?

Quick Check

You open a file in 'a+' and call read() immediately. What comes back?

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Closing a Text File 🔒