Python密码重复元素检测代码疑问：空字符串与索引操作解析

First, here's the code we're analyzing, formatted for clarity:

def check_pass(): 
    password = raw_input('Enter a password:')
    unique_element = ''
    for i in range(len(password)):
        if len(unique_element) == 0:
            unique_element += password[i]
        else:
            not_unique = True
            for j in range(len(unique_element)):
                if unique_element[j] == password[i]:
                    not_unique = False
            if not_unique:
                unique_element += password[i]
    return unique_element == password

print check_pass()

Now let's walk through your questions one by one:

1. Why is unique_element initialized as an empty string? #

Think of unique_element as an empty "collection bucket" for characters we've confirmed are not repeated in the password. We start with an empty string because, at the very beginning, we haven't looked at any characters from the password yet—there's nothing to put in our bucket. As we iterate through the password, we'll add characters to this bucket only if they haven't been added before.

2. What does unique_element += password[i] do? #

This line appends the current character (password[i]) to the unique_element string. We only run this line when we've confirmed the character isn't already in unique_element. For example, if unique_element is "ab" and password[i] is "c", after this line runs, unique_element becomes "abc". It's how we build our list of all non-repeating characters from the password.

3. What does the [i] in password[i] mean? #

In Python, strings are sequences of characters, and [i] is how we access the i-th character in that sequence (note: Python starts counting from 0, not 1). For example, if the user enters "hello" as their password:

• password[0] is "h"
• password[1] is "e"
• password[2] is "l"
  And so on. The i here comes from the loop for i in range(len(password)), which runs once for each character in the password, giving us the position of each character to check.

A quick note on the code's overall purpose #

At the end, the code compares unique_element (the string of all non-repeating characters from the password) to the original password. If they're equal, that means every character in the password was unique—so it returns True. If there were duplicates, unique_element will be shorter than the original password, so it returns False.

内容的提问来源于stack exchange，提问作者Marshall Alife