Object becomes None when using a context manager

Question

Why doesn`t this work:

class X:     var1 = 1     def __enter__(self): pass     def __exit__(self, type, value, traceback): pass  with X() as z:     print z.var1 

I get:

print z.var1 AttributeError: 'NoneType' object has no attribute 'var1' 

Answer 1

Change the definition of X to

class X(object):     var1 = 1     def __enter__(self):         return self     def __exit__(self, type, value, traceback):         pass 

with assigns the return value of the __enter__() method to the name after as. Your __enter__() returned None, which was assigned to z.

I also changed the class to a new-style class (which is not critical to make it work).

Answer 2

See the docs for context managers:

__enter__( ) Enter the runtime context and return either this object or another object related to the runtime context. The value returned by this method is bound to the identifier in the as clause of with statements using this context manager. An example of a context manager that returns itself is a file object. File objects return themselves from __enter__() to allow open() to be used as the context expression in a with statement.

An example of a context manager that returns a related object is the one returned by decimal.Context.get_manager(). These managers set the active decimal context to a copy of the original decimal context and then return the copy. This allows changes to be made to the current decimal context in the body of the with statement without affecting code outside the with statement.

Your __enter__ method doesn't return anything, which is the same as returning None.