How do I pass variables across functions?

You’re just missing one critical step. You have to explicitly pass the return value in to the second function.

def main():
    l = defineAList()
    useTheList(l)

Alternatively:

def main():
    useTheList(defineAList())

Or (though you shouldn’t do this! It might seem nice at first, but globals just cause you grief in the long run.):

l = []

def defineAList():
    global l
    l.extend(['1','2','3'])

def main():
    global l
    defineAList()
    useTheList(l)

The function returns a value, but it doesn’t create the symbol in any sort of global namespace as your code assumes. You have to actually capture the return value in the calling scope and then use it for subsequent operations.

Your return is useless if you don’t assign it

list=defineAList()

return returns a value. It doesn’t matter what name you gave to that value. Returning it just "passes it out" so that something else can use it. If you want to use it, you have to grab it from outside:

lst = defineAList()
useTheList(lst)

Returning list from inside defineAList doesn’t mean "make it so the whole rest of the program can use that variable". It means "pass the value of this variable out and give the rest of the program one chance to grab it and use it". You need to assign that value to something outside the function in order to make use of it. Also, because of this, there is no need to define your list ahead of time with list = []. Inside defineAList, you create a new list and return it; this list has no relationship to the one you defined with list = [] at the beginning.

Incidentally, I changed your variable name from list to lst. It’s not a good idea to use list as a variable name because that is already the name of a built-in Python type. If you make your own variable called list, you won’t be able to access the builtin one anymore.

Read up the concept of a name space. When you assign a variable in a function, you only assign it in the namespace of this function.
But clearly you want to use it between all functions.

def defineAList():
    #list = ['1','2','3'] this creates a new list, named list in the current namespace.
    #same name, different list!

    list.extend['1', '2', '3', '4'] #this uses a method of the existing list, which is in an outer namespace
    print "For checking purposes: in defineAList, list is",list
    return list

Alternatively, you can pass it around:

def main():
    new_list = defineAList()
    useTheList(new_list)

passing variable from one function as argument to other functions can be done like this

define functions like this

def function1():
    global a
    a=input("Enter any numbert")
 
def function2(argument):
    print ("this is the entered number - ",argument)

call the functions like this

function1()
function2(a)

Try do the following:

def funa():
    name=("Bob")
    age=("42")
    return name, age

def funb():
    name, age=funa()
    print(name)
    print(age)

funb()

Happens that you gotta call it via funa() instead of age.

Just return it.

Example:

def initiate_excel_corvert():
 
    df = pd.ExcelFile('empty_book.xlsx').parse('Sheet')
    x=[]
    x.append(df['Numbers'])
    x.clear()
    x.extend(df['Numbers'])
    print(len(x))
    print('n')
    len_of_column_0 =+1 + len(x)
    print("+ add-on ::")
    print(len_of_column_0)
    print('n')
    
    return render_template('index.html'), insert_into_excel(len_of_column_0)
    
def insert_into_excel(len_of_column_0):
    print ("len_of_column_0 = ", len_of_column_0)
    print('n')