Python函数superindex代码解析：列表推导式工作原理及疑问解答

Hey there! Let's break down this code step by step—since you're a beginner, I'll keep things straightforward and skip overly technical jargon.

First, What Does the Whole Function Do? #

The function superindex(string, word) returns a list of all indexes where a specific character (quick note: word is a misleading name here—it should be a single character like "a", not a full word like "apple") appears in the input string.

How Does the Code Create the List? #

The core of this function is a list comprehension—Python's concise way to build lists without writing clunky for loops with append() calls. Let's break down the line piece by piece:

[i for i, ltr in enumerate(string) if ltr == word]

Here's what each part does:

• enumerate(string): This built-in function pairs every character in the string with its position (index). For example, if string is "hello", enumerate generates:

  (0, 'h'), (1, 'e'), (2, 'l'), (3, 'l'), (4, 'o')
  

• i, ltr in enumerate(string): This "unpacks" each tuple from enumerate—i gets the index number, ltr gets the corresponding character.
• if ltr == word: This is a filter condition. We only keep the index i if the current character ltr matches the input word.
• The outer [ ... ]: This wraps the whole expression to tell Python we want to collect all matching i values into a new list.

What's the Role of "i for i"? #

You might notice the repetition of i at the start—let's clarify that.

The full structure of a list comprehension is:

[output_expression for variable in iterable if condition]

In your code, the output expression is just i. The for i, ltr in ... part is where we define the variables we're working with. So putting it all together, it's saying:

"For each index i and character ltr in the enumerated string, if ltr equals word, add i to the list."

The repetition of i is just because we want the index (not the character) to be our output. If we wanted to collect the matching characters instead, we'd write [ltr for i, ltr in ... if ltr == word].

Example to Make It Concrete #

Let's say you call:

superindex("banana", "a")

Here's what happens:

1. enumerate("banana") gives (0,'b'), (1,'a'), (2,'n'), (3,'a'), (4,'n'), (5,'a')
2. We check each ltr:
   • 'b' != 'a' → skip
   • 'a' == 'a' → add 1 to the list
   • 'n' != 'a' → skip
   • 'a' == 'a' → add 3 to the list
   • 'n' != 'a' → skip
   • 'a' == 'a' → add 5 to the list
3. The function returns [1, 3, 5]—all the indexes of 'a' in "banana".

内容的提问来源于stack exchange，提问作者Sebastian Obermeyer