Nick Coghlan wrote:
> Ben Wing wrote: > >> apologies if this has been brought up on python-dev already. >> >> a suggestion i have, perhaps for python 3.0 since it may break some >> code (but imo it could go into 2.6 or 2.7 because the likely breakage >> would be very small, see below), is the elimination of the misfeature >> whereby the iteration variable used in for-loops, list >> comprehensions, etc. bleeds out into the surrounding scope. >> >> [i'm aware that there is a similar proposal for python 3.0 for list >> comprehensions specifically, but that's not enough.] > > > List comprehensions will be fixed in Py3k. However, the scoping of for > loop variables won't change, as the current behaviour is essential for > search loops that use a break statement to terminate the loop when the > item is found. Accordingly, there is plenty of code in the wild that > *would* break if the for loop variables were constrained to the for > loop, even if your own code wouldn't have such a problem. > > Outside pure scripts, significant control flow logic (like for loops) > should be avoided at module level. You are typically much better off > moving the logic inside a _main() function and invoking it at the end > of the module. This avoids the 'accidental global' problem for all of > the script-only variables, not only the ones that happen to be used as > for loop variables. i did in fact end up doing that. however, in the process i ran into another python annoyance i've tripped over repeatedly: you can't assign to a global variable without explicitly declaring it as `global'. instead, you "magically" get a shadowing local variable. this behavior is extremely hostile to newcomers: e.g. foo = 1 def set_foo(): foo = 2 print foo --> 1 the worst part is, not a single warning from Python about this. in a large program, such a bug can be very tricky to track down. now i can see how an argument against changing this behavior might hinge upon global names like `hash' and `list'; you certainly wouldn't want an intended local variable called `hash' or `list' to trounce upon these. but this argument confuses lexical and dynamic scope: global variables declared inside a module are (or can be viewed as) globally lexically scoped in the module, whereas `hash' and `list' are dynamically scoped. so i'd suggest: [1] ideally, change this behavior, either for 2.6 or 3.0. maybe have a `local' keyword if you really want a new scope. [2] until this change, python should always print a warning in this situation. [3] the current 'UnboundLocal' exception should probably be more helpful, e.g. suggesting that you might need to use a `global foo' declaration. ben _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
