list.index() / list.count() / in Operator

# Returns the index of the first element matching the value.
list.index(value, start=0, end=len(list))

# Returns the number of elements matching the value.
list.count(value)

# Checks whether the list contains the value (True / False).
value in list
value not in list

# Use index() to get the index of an element.
fruits = ["apple", "orange", "grape", "orange", "peach"]
print(fruits.index("orange"))  # 1 (index of the first match)

# Specify a search range (search from index 2 onward).
print(fruits.index("orange", 2))  # 3 (first match at index 2 or later)

# Searching for a value that doesn't exist raises a ValueError.
try:
    fruits.index("strawberry")
except ValueError:
    print("strawberry is not in the list.")

# It is safer to check with the in operator before calling index().
target = "grape"
if target in fruits:
    print(f"Index of {target}: {fruits.index(target)}")

# Use count() to find how many times a value appears.
nums = [1, 2, 2, 3, 2, 4, 5, 2]
print(nums.count(2))   # 4 (2 appears four times)
print(nums.count(9))   # 0 (9 is not present, so 0 is returned)

# A practical example of counting duplicates.
answers = ["A", "B", "A", "C", "B", "A"]
print(f"Chose A: {answers.count('A')}")
print(f"Chose B: {answers.count('B')}")
print(f"Chose C: {answers.count('C')}")

# Use the in operator to check whether an element exists.
shopping = ["milk", "eggs", "bread", "butter"]
print("eggs" in shopping)     # True
print("cheese" in shopping)   # False
print("cheese" not in shopping)  # True

# A typical use in a conditional statement.
if "milk" in shopping:
    print("Milk is in the list.")

# Check whether the list is empty.
if not shopping:
    print("The shopping list is empty.")
else:
    print(f"There are {len(shopping)} items.")

# Check whether all elements satisfy a condition (combined with all and any).
scores = [80, 90, 75, 88]
print(all(s >= 60 for s in scores))  # True (everyone scored 60 or above)
print(any(s >= 90 for s in scores))  # True (at least one person scored 90 or above)

index() returns the index of the first matching element. If the value does not exist, a ValueError is raised, so it is recommended to check for the value with the in operator first, or handle the exception with a try-except block.

The in operator performs a linear search (O(n)) by comparing each element in order. If you need to search a large dataset frequently, using a set instead of a list allows an average search time of O(1). For data whose primary purpose is lookup, a set or dictionary is more appropriate than a list.

When searching a sorted list, binary search (O(log n)) using Python's standard library bisect module is faster. For sorting and reversing elements, see list.sort() / list.reverse().