Unexpected behavior using contextmanager on a class method

Peter Otten __peter__ at web.de
Tue Aug 7 18:04:29 CEST 2012

Thomas Draper wrote:

> I want to use with..as in a "reversible circuit generator". However, it
> seems that @contextmanager changes the expected nature of the class. I
> tried to distill the problem down to a simple example.
> import contextlib
> class SymList:

The problem you experience has nothing to do with context managers, you have 
a mutable default argument in your __init__(). 

>     def __init__(self, L=[]):

L is initialised with an empty list exactly once, when the method is 
defined; any changes you make to the list will be seen by all instances that 
use the default. The fix is

      def __init__(self, L=None):
          if L is None:
              L = []

>         self.L = L
>     @contextlib.contextmanager
>     def SymAdd(self, a):
>         self.L.append(a)
>         yield
>         self.L.append(a)
> SL = SymList()
> with SL.SymAdd(3):
>     SL.L.append(5)
> print(SL.L) # Expect and see [3, 5, 3]
> SL2 = SymList() # New object. Should have default values.
> print(SL2.L) # Expect [] and see [3, 5, 3]
> Why is the data member SL2.L referring to the data member SL.L? Has the
> @contextmanager somehow made all instantions of the class related?

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