On Tue, May 3, 2011 at 9:31 AM, Dun Peal <dunpea...@gmail.com> wrote: > Hi! > > Here's the demonstrating code: > > # module foo.py > var = 0 > > def set(): > global var > var = 1 > > Script using this module: > > import foo > from foo import * > > print var, foo.var > set() > print var, foo.var > > Script output: > > 0 0 > 0 1 > > Apparently, the `var` we imported from `foo` never got set, but > `foo.var` on the imported `foo` - did. Why?
Because imports (and assignments generally) bind names to values, they don't alias names to other names. from foo import * can be thought of as essentially doing: import foo set = foo.set var = foo.var del foo So the new, separate name __main__.var gets the current value of foo.var at import-time, which is the integer 0. You then call foo.set(), which re-binds foo.var to a new value (i.e. 1) rather than mutating the existing value (which would be impossible anyway since integers are immutable). This has absolutely no effect on __main__.var, which is an entirely separate binding. The behavior is comparable to that of function arguments. Values can be mutated, but re-binding names has only local effect: >>> a = 0 >>> def incr(b): ... b = 1 # rebinds local name b ... >>> incr(a) >>> a # outside name unaffected, just like in your example 0 >>> c = [7] >>> def mutate_then_rebind(b): ... b.append(99) # mutates passed-in value ... b = [42] # rebinds local name; has no outside effect ... >>> mutate_then_rebind(c) >>> c # name still references same obj, but that obj has been mutated [7, 99] Cheers, Chris -- http://rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
