On 28/08/11 05:45, John O'Hagan wrote: > Somewhat apropos of the recent "function principle" thread, I was recently > surprised by this: > > funcs=[] > for n in range(3): > def f(): > return n > funcs.append(f) > > [i() for i in funcs] > > The last expression, IMO surprisingly, is [2,2,2], not [0,1,2]. Google tells > me I'm not the only one surprised, but explains that it's because "n" in the > function "f" refers to whatever "n" is currently bound to, not what it was > bound to at definition time (if I've got that right), and that there are at > least two ways around it: either make a factory function:
This does not do what you'd like it to do. But let's assume that, it did, that Python, when encountering a function definition inside a function, "froze" the values of nonlocal variables used in the new function, from the point of view of that function — that *might* be more intuitive, at least in certain situations. However, what if you wanted a closure to access a nonlocal variable that changes - acting differently depending on what has since happened in the parent function. That may not be as common, but it is a perfectly plausible situation, and the hack required to support that behaviour in a Python that acts as you had expected it to, a surrogate namespace using a class, list, or dict, is infinitely more cryptic and ugly than the default-parametre hack. Thomas -- http://mail.python.org/mailman/listinfo/python-list
