Unexpected behavior using contextmanager on a class method

[Thomas Draper]

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:
    def __init__(self, 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?