Proper way in Python to raise errors while setting variables

Question

What is the proper way to do error-checking in a class? Raising exceptions? Setting an instance variable dictionary "errors" that contains all the errors and returning it?

Is it bad to print errors from a class? Do I have to return False if I'm raising an exception?

Just want to make sure that I'm doing things right. Below is some sample code:

@property def password(self):     return self._password  @password.setter def password(self,password):     # Check that password has been completed     try:         # Check that password has a length of 6 characters         if (len(password) < 6):             raise NameError('Your password must be greater \                              than 6 characters')      except NameError:         print 'Please choose a password'         return False      except TypeError:         print 'Please choose a password'         return False                                                                                                                                      #Set the password     self._password = password      #Encrypt the password     password_md5 = md5.new()     password_md5.update(password)     self._password_md5 = password_md5.hexdigest() 

Answer 1

Your code is out of a context so is not obvious the right choice. Following some tips:

• Don't use NameError exception, it is only used when a name, as the exception itself said, is not found in the local or global scope, use ValueError or TypeError if the exception concerns the value or the type of the parameter;

• Don't print error messages. Raise meaningful exceptions with a meaningful error message:

  raise ValueError("password must be longer than 6 characters") 

• Returning a value from a setter is meaningless while assignment is not an expression, i.e. you cannot check the value of an assignment:

  if (user.password = 'short'): ... 

• Just raise an exception in the setter and let the code that set the property handle it.

Example:

class Test:      minlen = 6      @property     def password(self):         return self._password      @password.setter     def password(self, value):         if not isinstance(value, basestring):             raise TypeError("password must be a string")         if len(value) < self.minlen:             raise ValueError("password must be at least %d character len" % \                                  self.minlen)         self._password = value 

Look also at this forms handling library, there the validators , here an example, are entities in their own: they can be set dynamically with higher control and less coupled code, but maybe this is much more than you need.