How do I have a value that can be both a floating decimal point or a string in python?

so, I need one of my variables to be A possibility. like, it is a user input var & it can be both a floating decimal point or a string. And, so I want to do stuff with it, like >/</= but if the user says "done" I exit. So, let me show you:

largest = None
smallest = None
while True:
    try:
        num = input("Enter a number: ")
        float(num)
    except:
        "invalid input"
    if num == "done":
        break
    if num > largest:
        largest = num
        
    if smallest == None:
        smallest= num
        
    if smallest > num:
        smallest = num
    print(num)

print("Maximum is", largest)
print("Minimum is", smallest)