TypeError: Argument Must Be str, Not bytes

Why This Happens

Python 3 strictly separates str (text) and bytes (binary data). File operations in binary mode ('rb', 'wb') work with bytes, while text mode uses str. Network sockets also use bytes. Mixing the two causes this error.

The Problem

with open('file.bin', 'rb') as f:
    content = f.read()
    if 'hello' in content:
        print('found')

The Fix

with open('file.bin', 'rb') as f:
    content = f.read()
    if b'hello' in content:
        print('found')

Step-by-Step Fix

1. 1

   Identify str vs bytes

   Use type() to confirm which is str and which is bytes.

2. 2

   Choose one type

   For text processing, decode bytes. For binary I/O, encode strings.

3. 3

   Use consistent file modes

   Open files in text mode ('r') for str or binary mode ('rb') for bytes.