Peter Otten wrote:
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?
No. The existence and use of sys._getframe -- notice the leading
underscore -- is a CPython implementation artifact. Use at your own risk.
>> Is this an invalid use of frame data?
Up to you to decide.
>> Why does the single line "sys._getframe(1).f_locals"
fix the behavior?
It updates the dictionary.
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.
Efficiency? Consistency? The doc for locals says "locals()
Update and return a dictionary representing the current local symbol
table." In class statements, where (currently) the local namespace *is*
a dict, no update is needed and locals() simply returns the dict, the
same one each time.
Terry Jan Reedy
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