[Tutor] Why thus?
Wesley J. Chun
Sun, 12 Mar 2000 13:51:44 -0800 (PST)
> Date: Sun, 12 Mar 2000 22:29:57 +0100
> From: Christian Tismer <email@example.com>
> The difference is: When compiling your code, the compiler
> tries to figure out what is global and what not. Since
> counter is assigned to, it becomes a local. But the object
> does not yet exist when you request it:
> wants to retrieve the current value, and it isn't set yet.
> You are accessing a local variable which already has its
> slot and is ready to receive a value, but cannot until
> it is initialized.
chris is absolutely right. when doing an assignment in a
local scope, the compiler will search only for the local
name, and since you haven't done a "counter = XXX" within
__init__(), it will complain.
here is an easier example of the exact same problem outside
the class realm (since it really doesn't have anything to
do with classes):
# this works (no assignment)
i = 4
print i # print global 'i'
# this fails (references local var that hasn't been
# assigned/doesn't exist)
i = 4
print i # "print global 'i'"
i = 6 # set local 'i'
the result is a "NameError: i". upon seeing the presence
of a local 'i' makes any previous code *think* it should
access a local 'i' vs. the global one.
i asked Guido to confirm this or to check if it was his
way to punish the programmer for writing such horrid code? ;-)
here was his reply:
The compiler does an analysis of the whole func-
tion body and decides that i is a local variable.
Then all references to i are made into local vari-
able references. Local refs are much faster than
global variable references: locals are indexed in
an array, globals are a dict lookup. So the
``print i'' accesses the local variable i but it
isn't defined yet. In Python 1.6, this will
raise UnboundLocalError (a subclass of NameError):
>>> def f(): print i; i = 1
Traceback (innermost last):
File "<stdin>", line 1, in ?
File "<stdin>", line 1, in f
hope this helps!!
"Core Python Programming", Prentice-Hall, TBP Summer 2000
(will have an example like this in here too!)
wesley.j.chun :: firstname.lastname@example.org
cyberweb.consulting :: silicon.valley, ca