BMI Calculator Not letting a converter pick one or the other
<p>I am making a BMI calculator and I am trying to add a converter into the code for people who don't know their SI proportions. The problem is the way I want to write this is if you know your SI you just press enter and it will ask you for the SI preferences.</p>

<pre><code>print("\n")
print("     -------------------------------          -------------------------------     ")
print("                                 BMI Calculator                                 ")
print("This calculator uses SI unites to determine BMI, use the converters below for pounds and feet.")
print("               If you know the SI units for just press enter.              ")
print("     -------------------------------          -------------------------------     ")

none=()
c1=input("Please state your Weight in Pounds: ")
if c1 is none: 
    f1=float(c1)
    w1=f1*.45659237

c2=input("Please state your Height in Feet: ")
if c2 is none:
    f2=float(c2)
    h1=f2*.3048

if c1 is not none and c2 is not none:
    B=(w1/h1**2)
    print(B)

else:
    print("\n")
    print("Great you know your SI Preferances: ")
    W=input(("Please state your Weight in Kg: "))
    H=input(("Please state your Height in m: "))
    h=float(H)
    w=float(W)
    B=(w/h**2)
    print(B)
</code></pre>

<p>I set none = () so that if you enter nothing it goes onto the next question. Essentially this works if you know the SI proportions to your body. If you press enter and enter the SI units it works. If you enter your pounds and feet it ignores it and moves on. I am wondering where I am going wrong with the loop.</p>

<p>Here is the error I get when I try to run this</p>

<pre><code>B=(w1/h1**2)
NameError: name 'w1' is not defined
</code></pre>