Python: Unexpected behavior using contextmanager on class method

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“Least Astonishment” and the Mutable Default Argument 30 answers

Python constructor and default value [duplicate] 4 answers

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This behavior is due to how mutable default arguments work in Python.

Try changing SymList.__init__() to the following:

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

As you modify self.L in one instance you are also modifying the L that is passed into SymList.__init__() , so the result with your code is that all instances would share the same L attribute when the instance is first initialized.

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