为何Python中string.find()方法会返回-1？

Great question! Let’s break down why Python’s str.find() behaves this way.

First, let’s recap what find() is supposed to do: its job is to return the starting index of the first occurrence of the substring you’re looking for in the original string. Since string indices in Python start at 0, all valid position values are non-negative integers (0, 1, 2, etc.).

So when the substring isn’t found, the method needs a clear, unambiguous way to signal that. Here’s why -1 is the chosen value:

• No conflict with valid indices: Since all real positions are ≥0, -1 can never be confused with an actual character position in the string. It’s a universal "not found" flag that leaves zero room for misinterpretation.
• Industry convention: This isn’t unique to Python! Many other languages (like C, JavaScript, and Java) use -1 to indicate a missing substring in their string search functions. It’s a familiar pattern that developers pick up quickly across languages.
• Gentle error handling: Compare this to Python’s str.index() method, which throws a ValueError if the substring isn’t found. find() returns -1 instead, letting you handle the "not found" case with a simple conditional check (like if result == -1:) without needing to write try/except blocks.

To tie this back to your example:

theString = 'something whatsoever'
print(theString.find('anything'))  # Returns -1

Since 'anything' doesn’t appear anywhere in theString, find() uses -1 to tell you the substring couldn’t be located.

内容的提问来源于stack exchange，提问作者PHD