a^2 + b^2 = c^2

import math #Import library that is able to perform math.sqrt()

def isFloating(x):
    try:
        float(x) #If this can be executed, the number must be floating.
        return True #Therefore, return True.
    except:
        return False #If float(x) doesn't work, False is returned

def calcC(a, b):
    c = math.sqrt(a**2 + b**2) #Mathematical implementation of core function a**2 + b**2 = c**2
    return c

def calcAB(x, c):
    if c < x: #Test if the side is shorter than the inserted length.
        return "Erroneous input." #The side cannot be shorter than either length in a triangle.
    else:
        y = math.sqrt(c**2 - x**2) # Mathematical implementation of core function a**2 + b**2 = c**2
        return y

def getValue(letter, unknown):
    while True: #Loop forever until "return" is called.
        value = input("What is the value of " + letter + "?: ") #User input
        if isFloating(value): #Test if the user inserted a correct value
            return float(value), unknown #Forward value
        elif value == "": #If nothing is inserted (""), it checks if it is the first unknown variable
            if unknown == 0:
                unknown = letter #If it's the first unknown variable, it assigns the variable "unknown" as the letter for later use.
                return None, unknown
            else:
                print("You have to insert a numeric value.") #If "unknown" already has a value, it tells the user that a numeric value MUST be inserted.
        elif unknown == 0:
            print("Leave blank if you don't have a value.") #If a non-blank non-floating entry was added, it'll display this so long as there's room for a blank entry.
        else:
            print("You have to insert a numeric value.") #If nonsense is entered and there's no room for an unknown variable, this is given.

def inputLetters():
    unknown = 0 #changes to a letter string so soon as one input is blank
    a, unknown = getValue("A", unknown)
    b, unknown = getValue("B", unknown)
    if unknown == 0:
        print(calcC(a, b))
    else:
        c, unknown = getValue("C", unknown)
        if unknown == "A":
            print(calcAB(b, c))
        else: #Or: if unknown == "B"
            print(calcAB(a, c))

inputLetters()