New submission from Peter Mawhorter <pmawhor...@gmail.com>: The behavior of the built-in exec() function differs in Python3.1 from the behavior in Python 2.6 when only a single argument is passed. Additionally, the documentation for the function does not suggest the behavior that it has. In Python2.6, an exec statement could both access and change the values of local variables. In Python3.1, it can read values from the local scope, but can't change them. Run the attached script to see examples of this behavior. Note especially that in the first function, the exec'd code changes the value of a variable called 'value'. However, the change isn't visible after the call to exec(), because the value changed was a local variable and presumably, the update is dropped after the exec() finishes. To me, this conflicts with the statement in the documentation that "In all cases, if the optional parts are omitted, the code is executed in the current scope." At the very least, a sentence or two should be added to the documentation explaining that exec() can't affect the value of local variables. In addition, it's strange that, for example, in a function declaring "global value", an exec() statement that modifies 'value' doesn't affect the value of the global variable. In this sense, the code being exec'd doesn't seem to be running in a perfect copy of the function's scope.
If it were up to me, I'd allow code being executed by an exec() statement to modify the value of local variables, because that keeps with the paradigm of the exec'd code being simply dynamic code that behaves as if it were written in-line. It also means that programmers who need to change things with an exec statement aren't limited to pushing their changes through global variables, which can lead to conflicts with name sharing (for example, what if my exec() statement is in a recursive function?). Of course, implementing this may be difficult... so it may be better just to add to the documentation to explain this behavior of exec(). ---------- assignee: georg.brandl components: Documentation, Interpreter Core files: test.py messages: 95036 nosy: georg.brandl, pmawhorter severity: normal status: open title: odd exec() behavior or documentation type: behavior versions: Python 3.1 Added file: http://bugs.python.org/file15289/test.py _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue7286> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
