On Tue, Feb 26, 2019 at 6:58 AM DL Neil <pythonl...@danceswithmice.info> wrote: > > On 26/02/19 5:25 AM, ast wrote: > > I noticed a quirk difference between classes and functions > > >>> x=0 > > >>> class Test: > > x = x+1 > > print(x) > > x = x+1 > > print(x) > ... > > > Previous code doesn't generate any errors. > > x at the right of = in first "x = x+1" line is > > the global one (x=0), then x becomes local > > within a function, this is not allowed > > >>> x = 0 > > >>> def f(): > > x = x+1 > > >>> f() > > UnboundLocalError: local variable 'x' referenced before assignment > > Since x is written inside the function, it is considered as a local > > variable and x in x+1 is undefined so this throw an exception > > Any comment ? > > > At first I misunderstood the question, and even now I'm slightly mystified:- > > Is the observation to do with the principles of "closure" (being applied > to the function) compared with the differences between class and > instance variables?
Classes and functions behave differently. Inside a function, a name is local if it's ever assigned to; but in a class, this is not the case. (Honestly, I'm not sure why this is, but it's hardly ever significant; metaprogramming seldom runs into this kind of thing.) > A question from me: (I'm not an O-O 'native' having taken to it long > after first learning 'programming') I've used class attributes to hold > 'constants', eg > > MAXIMUM_WEIGHT = 100 > > Thus only an assignment to which reference/assertions will be made 'later'. > > I have initialised a counter to zero and then incremented within the > instantiated object's __init__(), ie keeping track of how many of 'these > objects' have been instantiated. Once you're inside a method, you would reference it with a dot - either "self.MAXIMUM_WEIGHT" or "cls.MAXIMUM_WEIGHT". So it's now an attribute, not a local name. > So, I can imagine taking a value from outside the class' namespace, > modifying it in some way (per the first "x = x+1") and then retaining > the result as a class attribute (a calculation performed once - when the > class code is compiled). Yes, this is perfectly legitimate. Not common but legit. > However, (after that first calculation/definition of the class > attribute) why would one (normally) want to redefine the same class > attribute (the second x = x+1), at the 'class level'? > (perhaps just done for its amusement value?) That would not be common either. If you're redefining a variable multiple times within a class block, you probably don't also have it at top level. (For instance, you can put a "for" loop inside a class statement to create attributes/methods, but the loop variable is unlikely to also be a global.) But hey! It's fun! :) ChrisA -- https://mail.python.org/mailman/listinfo/python-list
