Python中使用enumerate存值入列表及获取对象长度等问题咨询

Hey there! Let's break down your questions one by one, with practical examples tailored to your code:

enumerate() is super handy when you need both the index and the value while iterating. Here are common ways to use it to populate a list:

• Scenario 1: Store values with conditional checks
  If you only want to keep certain values based on their index, enumerate lets you do that easily:

  fruits = ["apple", "banana", "cherry", "date"]
  filtered_fruits = []
  for idx, fruit in enumerate(fruits):
      # Keep fruits at odd indices
      if idx % 2 == 1:
          filtered_fruits.append(fruit)
  print(filtered_fruits)  # Output: ['banana', 'date']
  

• Scenario 2: Store index-value pairs
  You can directly convert the enumerate result to a list of tuples:

  fruits = ["apple", "banana", "cherry"]
  indexed_fruits = list(enumerate(fruits))
  print(indexed_fruits)  # Output: [(0, 'apple'), (1, 'banana'), (2, 'cherry')]
  

First, let's refactor your list comprehension into a reusable variable (since you don't want to just print it):

# Store the result instead of printing it immediately
detection_results = [category_index.get(value) for index,value in enumerate(classes[0]) if scores[0,index] > 0.5]

1. How to get the length of this object? #

Use Python's built-in len() function—it works for all iterable objects with a defined length (like lists, tuples, dictionaries):

result_length = len(detection_results)
print(result_length)  # Prints the number of items in the list

2. How to store only the item names in a new list? #

Assuming category_index.get(value) returns a dictionary (e.g., {"name": "cat", "id": 1}) or an object with a name attribute, you just need to extract that field in your list comprehension:

If it's a dictionary: #

item_names = [result["name"] for result in detection_results if result is not None]
# The "if result is not None" avoids errors if category_index.get(value) returns None

If it's a custom object with a name attribute: #

item_names = [result.name for result in detection_results if result is not None]

Alternatively, you can do it directly in one line (without the intermediate detection_results variable):

item_names = [
    category_index.get(value)["name"] 
    for index, value in enumerate(classes[0]) 
    if scores[0, index] > 0.5 
    and category_index.get(value) is not None
]

内容的提问来源于stack exchange，提问作者basviccc