How can I join one function to another

I tries to write one function. is_substr is not needed, all information in is_substr should put inside of long_substr without function. Now in this code, is_substr used in inside of main function. So instead of using it as function, the codes should be inside of is_substr. I want to do it without using lamda

def long_substr(data):
    substr = ''
    if len(data) > 1 and len(data[0]) > 0:
        for i in range(len(data[0])):
            for j in range(len(data[0])-i+1):
                if j > len(substr) and is_substr(data[0][i:i+j], data):
                    substr = data[0][i:i+j]
    return substr

def is_substr(find, data):
    if len(data) < 1 and len(find) < 1:
        return False
    for i in range(len(data)):
        if find not in data[i]:
            return False
    return True