why when I use while loop , the variable in it can't be 0?

When you give Python a Non-Boolean (string, int, etc) and it tries to convert it to a Boolean (True or False) it does so by checking if the value is on a list of values it considers to be a "Falsey" value. If that is the case it converts it to False, if the value is not on that list then it is treated as "Truethy" by default and converted to True.

Here is the list of values that Python considers "Falsey":

• None
• False
• Zero of any numeric type, for example, 0, 0L, 0.0, 0j.
• Any empty sequence, for example, '', (), [].
• Any empty mapping, for example, {}.
• Instances of user-defined classes, if the class defines a nonzero() or len() method, when that method returns the integer zero or bool value False.

So When start is any number other than 0 it is treated as True since that is not found on the above list. Once it becomes 0 it is considers False since 0 is on the above list.