[New-bugs-announce] [issue16619] LOAD_GLOBAL used to load `None` under certain circumstances

[Bruno Dupuis]

New submission from Bruno Dupuis:

We found some strange behaviour of the compiler in this discussion on python-list: http://mail.python.org/pipermail/python-list/2012-December/636104.html

The fact is, `return` and `return None` result in inconsistent bytecode depending on the context.

Consider :

>>> import dis
>>> def f(x):
...     return None
... 
>>> dis.dis(f)
  2           0 LOAD_CONST               0 (None) 
              3 RETURN_VALUE         
>>> def g(x):
...     return None
...     print(x)
... 
>>> dis.dis(g)
  2           0 LOAD_GLOBAL              0 (None) 
              3 RETURN_VALUE         

  3           4 LOAD_GLOBAL              1 (print) 
              7 LOAD_FAST                0 (x) 
             10 CALL_FUNCTION            1 (1 positional, 0 keyword pair) 
             13 POP_TOP              

`return None` statement results in LOAD_GLOBAL 0 if there is some unreachable code after it. I first saw that as an optimization issue, but Ian Kelly's message http://mail.python.org/pipermail/python-list/2012-December/636117.html gives an extensive analysis and some examples:

"""
  I think this should even be considered a bug, not just a missing
  optimization.  Consider:

  >>> globals()['None'] = 42
  >>> def f(x):
  ...     return None
  ...     print(x)
  ...
  >>> f('test')
  42

  The use of the LOAD_GLOBAL allows None to effectively be reassigned.

"""

Ian also points out in this message that `return` and `return None` don't result in the same bytecode when followed by trash code.

----------
components: Interpreter Core
messages: 176999
nosy: Horpner, bruno.dupuis, ikelly, python-dev, stevenjd
priority: normal
severity: normal
status: open
title: LOAD_GLOBAL used to load `None` under certain circumstances
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4

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<http://bugs.python.org/issue16619>
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