Python: check if item or value exists in list or array

Introduction

Sometimes we need to know if an element or value is within a list or array in Python.

There may be a need to simply know if it exists, but it is also possible that we need to obtain the position of an element, that is, its index.

Today we will see how to do it in Python, to check if an element exists in array, as well as to obtain the index of a certain value.

Check if the element exists

We use the operator in, which returns a Boolean indicating the existence of the value within the array. This way:

lista = [1, 50, 30]
if 50 in lista: # Imprime lo de abajo
    print("El número 50 existe en la lista")

otra_lista = ["perro", "gato", "conejo"]
if "caballo" in otra_lista: #No imprime nada
    print("caballo existe dentro de otra_lista")

As we can see, it does not matter if our array or list is string or integer type. Of course, they must be primitive data.

Obtain element index

If we want to obtain the index or position, we use the index method of the lists. This way:

lista = [1, 50, 30]
indice = lista.index(50)
print("El número 50 está en la posición {} de la lista".format(indice))

When we run it, it prints position 1 (remember that the values in the list start at 0). It is important to remember that an error will be generated if the element does not exist in the list. To handle it, we can do something like this:

otra_lista = ["perro", "gato", "conejo"]
try:
    otro_indice = otra_lista.index("caballo")
    print("caballo está en la posición {} de otra_lista".format(otro_indice))
except:
    print("Lo siento, caballo no está en la lista")

In that case, an exception is generated; because the element does not exist in the list.