Stagnant Frame Data?

[Peter Otten]

Mike wrote:

> I'll apologize first for this somewhat lengthy example. It does
> however recreate the problem I've run into. This is stripped-down code
> from a much more meaningful system.
> 
> I have two example classes, "AutoChecker" and "Snapshot" that evaluate
> variables in their caller's namespace using the frame stack. As
> written, the output is not what is expected: the variables evaluate to
> "stagnant" values.
> 
> However, if the one indicated line is uncommented, then the result is
> as expected.
> 
> So my questions are: Is this a bug in Python? Is this an invalid use
> of frame data? Why does the single line "sys._getframe(1).f_locals"
> fix the behavior?

A simplified demonstration of your problem:

>>> def f(update):
...     a = locals()
...     x = 42
...     if update: locals()
...     print a
...
>>> f(False)
{'update': False}
>>> f(True)
{'a': {...}, 'x': 42, 'update': True}

The local namespace is not a dictionary, and the locals()/f_locals 
dictionary contains a snapshot of the local namespace. Accessing the 
f_locals attribute is one way to trigger an update of that snapshot. 

What's puzzling is that the same dictionary is reused.

Peter