Question

I’ve recently started to learn python , and I reached the with statement . I’ve tried to use it with a class instance , but I think I’m doing something wrong . Here is the code :

 from __future__ import with_statement import pdb  class Geo:    def __init__(self,text):     self.text = text    def __enter__(self):     print 'entering'    def __exit__(self,exception_type,exception_value,exception_traceback):     print 'exiting'    def ok(self):     print self.text    def __get(self):     return self.text   with Geo('line') as g :   g.ok() 

The thing is that when the interpreter reaches the ok method inside the with statement , the following exception is raised :

 Traceback (most recent call last):   File 'dec.py', line 23, in      g.ok() AttributeError: 'NoneType' object has no attribute 'ok' 

Why does the g object have the type NoneType ? How can I use an instance with the with statement ?

Answer 1

Your __enter__ method needs to return the object that should be used for the ‘as g‘ part of the with statement. See the documentation, where it states:

• If a target was included in the with statement, the return value from __enter__() is assigned to it.

Currently, it has no return statement, so g gets bound to None (the default return value)