How can I find the index for a given item in a list in Python?

To find the index of an item in a Python list, you can use the list.index() method or iterate with enumerate(). Here’s how to do it in different scenarios:

1. Using `list.index()` (First Occurrence)

my_list = ["apple", "banana", "cherry", "banana"]

# Find the first occurrence of "banana"
index = my_list.index("banana")
print(index)  # Output: 1

Key Notes:

Raises a ValueError if the item is not found.
Use in to check existence first:

  if "banana" in my_list:
      index = my_list.index("banana")

2. Get All Indices of an Item (Multiple Occurrences)

Use enumerate() in a list comprehension:

my_list = ["apple", "banana", "cherry", "banana"]

# Find all indices of "banana"
indices = [i for i, x in enumerate(my_list) if x == "banana"]
print(indices)  # Output: [1, 3]

3. Handle Missing Items Gracefully

Wrap in a try-except block to avoid ValueError:

try:
    index = my_list.index("mango")
except ValueError:
    print("Item not found.")

4. Custom Function (Return `-1` for Missing Items)

def find_index(lst, item):
    try:
        return lst.index(item)
    except ValueError:
        return -1

print(find_index(my_list, "mango"))  # Output: -1

5. Search with a Starting Point

Find the next occurrence after a specific index:

# Find "banana" starting from index 2
index = my_list.index("banana", 2)
print(index)  # Output: 3

When to Use Which Method

Method	Use Case
`list.index()`	Quick lookup for the first occurrence (error if not found).
`enumerate()` + loop	Find all indices or handle complex conditions.
`try-except`	Safely handle missing items.

Example Workflow

# Example list
fruits = ["apple", "banana", "cherry", "banana"]

# Find first occurrence
print(fruits.index("banana"))  # 1

# Find all occurrences
print([i for i, x in enumerate(fruits) if x == "banana"])  # [1, 3]

# Handle missing item
try:
    print(fruits.index("mango"))
except ValueError:
    print("Not in list")  # "Not in list"

Let me know if you need more specific examples!

1. Using list.index() (First Occurrence)