Human: Write me a Python program to return list index if string is string. Assistant: 
Here is a Python program to return list index if string is string

import pandas as pd

def return_list_index(string):
    """Return the index of a string in a list."""
    for i in range(len(string)):
        if string[i] =='':
            return i
    return -1

# Enter the list
string = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l','m', 'n', 'o', 'p', 'q', 'r','s', 't', 'u', 'v', 'w', 'x', 'y', 'z']

# Calling the function
print((return_list_index(string)))

#Example Output
# -1

#Remember to inspect code for bugs.