请求解析Codecademy Python题目提示中find()返回-1的含义

Hey there! Let's clear up this confusion between str.find() returning -1 and Python's negative indices—they're totally separate things, so it's totally normal to mix them up at first.

First: What does str.find() actually return? #

The find() method on strings is designed to look for the first occurrence of a substring (in your case, a single character like start or end) and return its 0-based index. If the substring doesn't exist anywhere in the string, the method returns -1 as a standard signal that the search failed—this is a predefined return value for "not found", not a reference to a position in the string.

This is different from Python's negative indices (like word[-1] to get the last character), which are used to count positions from the end of the string. The -1 from find() has nothing to do with that end-of-string position.

How this applies to your Codecademy problem #

Your function substring_between_letters needs to return the original word if either start or end isn't present. Using -1 from find() is the perfect way to check for that:

• If word.find(start) == -1, that means start isn't in the string at all.
• If word.find(end) == -1, same for end.

For example:

• If you called substring_between_letters("apple", "x", "e"), word.find("x") would return -1—so you'd just return "apple" as required.
• When both characters exist (like the sample input "apple", "p", "e"), find() returns 1 for "p" and 4 for "e". Then you slice word[start_idx + 1 : end_idx] (since we want the part between the two characters, not including them) to get "pl".

Quick recap to avoid mix-ups #

• Negative indices (e.g., word[-2]): Refer to positions counting backward from the end of the string.
• find() returning -1: A special value that means "the substring you searched for doesn't exist in this string".

内容的提问来源于stack exchange，提问作者Ayush Roy