Object destruction delayed on interactive prompt

[MRAB]

On 2015-01-25 18:23, Mario Figueiredo wrote:
> Consider the following class:
>
>      class A:
>          def __init__(self, value):
>              self.value = value
>
>          def __del__(self):
>              print("'A' instance being collected...")
>
>          def __repr__(self):
>              return str(self.value)
>
> If ran as a script, the destructor behavior works as expected:
>
>      if __name__ == '__main__':
>          x1 = A(12)
>          print(x1)                   # __repr__()
>          x1 = 2                      # __del__()
>          print(x1)
>
> But if I run it interactively, a weird behavior comes up:
>
>      >>> x1 = A(12)
>      >>> x1 = 'string'
>          A instance being collected...    # __del__() as expected
>      >>> x1 = A(12)				# Recreate x1
>      >>> x1					# __repr__()
>          12
>      >>> x1 = 'string'                    # __del__() isn't executed!
>      >>> x1                               # it is delayed until here
>          A instance being collected...
>          'string'
>
> This is reproducible in IDLE and at the system command prompt. Is this a
> REPL specific behavior?
>
In the REPL, the result of an expression is bound to '_':

 >>> x1 = A(12)
 >>> x1 # _ will be bound to the result.
12
 >>> x1 = 'string'
 >>> # At this point, _ is still bound to the object.
 >>> 0 # This will bind _ to another object, so the previous object can 
be collected.
'A' instance being collected...
0