Different ways to convert Binary to Decimal in Python 3.x -

i have write small python program converts 4 number binary input decimal equivalent of input. it's school assignment.

so stumbled upon while searching web help:

num_digits = 4  value = 0  err = false   in range(num_digits):      if i==0:          n = int(input("enter leftmost digit: "))      else:          n = int(input("enter next digit: "))      if n==0:          pass      elif n==1:          power = (num_digits-1)-i          value += (int) (pow(2, power))      else:          err = true          break   if err:      print ("error! incorrect binary number.") else:      print ("the value is",value)  

the above code works, don't understand why yet. part of assignment figure out how write such program ourselves.

so went ahead , wrote this:

value = 0  = int(input('geef eerste getal: ')) b = int(input('geef het volgende getal: ')) c = int(input('geef het volgende getal: ')) d = int(input('geef het volgende getal: '))  v = a+b+c+d  print(int(v, 2)'het getal is: ') 

i know there's wrong, not exactly. error when using int() string (like 'v' or 'value')

could explain me why happening? , maybe give me little breakdown of first program. understand what's behind larger code , how works.

thanks!