Errors/Python

ValueError: Substring Not Found

ValueError: substring not found

Quick Answer

You called str.index() with a substring that does not exist. Use str.find() which returns -1 instead, or check with 'in' first.

Why This Happens

str.index() raises ValueError when not found, while str.find() returns -1. The same applies to list.index(). Use 'in' to check existence first.

The Problem

text = 'Hello, World!'
pos = text.index('Python')

The Fix

text = 'Hello, World!'
pos = text.find('Python')  # Returns -1 if not found
# Or: if 'Python' in text: pos = text.index('Python')

Step-by-Step Fix

  1. 1

    Use find() instead

    str.find() returns -1 when not found.

  2. 2

    Check with 'in'

    Use 'substring' in string to check existence first.

  3. 3

    Handle the exception

    Wrap in try/except ValueError.

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