Question

Hello i am trying to create a const in python using this example found from Creating constant in Python (in the first answer from the link) and use instance as module.

The first file const.py has

# Put in const.py...:
class _const:
    class ConstError(TypeError): pass
    def __setattr__(self,name,value):
        if self.__dict__ in (name):
            raise self.ConstError("Can't rebind const(%s)"%name)
        self.__dict__[name]=value
import sys
sys.modules[__name__]=_const()

And the rest goes to test.py for example.

# that's all -- now any client-code can
import const
# and bind an attribute ONCE:
const.magic = 23
# but NOT re-bind it:
const.magic = 88      # raises const.ConstError
# you may also want to add the obvious __delattr__

Although i have made 2 changes cause i am using python 3 i still get errors

Traceback (most recent call last):
  File "E:\Const_in_python\test.py", line 4, in <module>
    const.magic = 23
  File "E:\Const_in_python\const.py", line 5, in __setattr__
    if self.__dict__ in (name):
TypeError: 'in <string>' requires string as left operand, not dict

I dont understand what the line 5 error is. Can anyone explain? Correcting the example would also be nice. Thanks in advance.

Answer 1

This line:

   if self.__dict__ in (name):

should be

   if name in self.__dict__:

… you want to know if the attribute is in the dict, not if the dict is in the attribute name (which doesn’t work, because strings contain strings, not dictionaries).