Variables python

Simple question, I like to declare my variables at the top of my programs below my function declarations so that it is a little easier to read. I run into problems when my variables are based on user input. I know I could do something like user_input = float and then when it was time to get the input later in the program I could redeclare it as user_input = float(input("")). But this runs into problems if I have variables declared up top that call a function using user_input as an argument, since user_input didn't actually have a value at run time. So how do I get the best of both worlds? I want to be able to declare all my variables up top and it not throw an error, then whenever the program actually needs the input I would reassign the value to the variable. Maybe the question wasn't so simple after all? Thank you guys