File modes

Learn modes 'r', 'w', 'a', 'b'.

All file modes

Basic modes: r read, w write (truncates), a append, x exclusive create (error if exists). Add + for read-write combinations: r+, w+, a+.

For binary data add b: rb, wb, ab. In binary mode data is read and written as bytes; no encoding parameter. Images, audio or compressed files are handled in binary mode.

Exercise Append with a mode

First create the file with with open("diary.txt", "w", encoding="utf-8") as f: f.write("First line\n"). Then append with with open("diary.txt", "a", encoding="utf-8") as f: f.write("Second line\n"). Finally print the line count with with open("diary.txt", "r", encoding="utf-8") as f: print(len(f.readlines())).

Show example solution

# w mode: write the first line
with open("diary.txt", "w", encoding="utf-8") as f:
    f.write("First line\n")
# a mode: append the second line
with open("diary.txt", "a", encoding="utf-8") as f:
    f.write("Second line\n")
# r mode: print the line count
with open("diary.txt", "r", encoding="utf-8") as f:
    print(len(f.readlines()))

When to use binary mode?

Binary mode is enabled with the b suffix: rb, wb, ab. This mode is for non-text files like images, audio, video or compressed data. Data is read and written as bytes.

In binary mode the encoding parameter is not used; encoding only applies to text. If you accidentally open a text file in binary mode you get bytes back and see prefixes like b"\n" instead of \n.

Test yourself

Which mode appends without truncating?
- w
- a
- r
- x
Show answer

a — The a mode (append) adds to the end without truncating.

Complete the binary read mode

f = open("image.png", "____")
data = f.read()
f.close()

Show answer

f = open("image.png", "rb")
data = f.read()
f.close()

Match the mode suffixes

b

Binary data suffix
+

Read-write suffix
x

Exclusive create
a

Append

Next lesson: Summary - File I/O

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