Python: finding an element in a list

When element is playing hide-and-seek (non-existent elements)

Sometimes, elements like to play hide-and-seek. index() throws a fit (ValueError) when it can't find the naughty element:

try:
    print(my_list.index('kiwi'))  # Who invited a kiwi to an apple-banana-cherry party? ValueError throws a party crasher out
except ValueError:
    print("Element not found!")
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In the realm of custom objects, redefine __eq__ method for index() to recognize them:

class Fruit:
    def __init__(self, name):
        self.name = name
    def __eq__(self, other):
        return self.name == other.name      # Now the objects can wear name tags

fruits = [Fruit('apple'), Fruit('banana')]
print(fruits.index(Fruit('banana')))  # Output: 1, our banana has been found at 1, yay!
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Quick search in a big library (finding elements with conditions)

List comprehension is like hiring a librarian who knows exactly where that specific book is:

even_numbers = [x for x in range(10) if x % 2 == 0]
print(even_numbers)  # Output: [0, 2, 4, 6, 8]
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Just ensure the library has only unique books to avoid the librarian bringing duplicates:

unique_elements = ['apple', 'banana', 'cherry', 'date']
indexes = [i for i, item in enumerate(unique_elements) if 'a' in item]
print(indexes)  # Output: [0, 1, 3] "a" has a lot of fans!
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Using the power of numpy

Your list is big and consists of numbers? Use numpy.where:

import numpy as np

numbers = np.array([1, 2, 3, 4, 5])
indexes = np.where(numbers > 3)
print(indexes)  # Output: (array([3, 4]),)
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You can specify custom comparison operators too. Now that's efficient hunting!

Searching the old school way (sorted lists)

Sorting lists: for a fast search, sorting helps:

import bisect

sorted_list = sorted(['cherry', 'apple', 'banana'])
index = bisect.bisect_left(sorted_list, 'banana')
print(index)  # Output: 1, banana slides into the middle as usual
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Notice, bisect_left is pretty advanced. It looks for the insertion place to keep everything tidy.

Good old for-loop with enumerate

When both the guest (element) and its seat number (index) are required, go hand-in-hand with enumerate and a loop:

for index, element in enumerate(my_list):
    print(f"Element: {element}, Index: {index}")
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It's efficient like having a list of seating arrangements, without bothering anyone!