On 12/04, LJ wrote:
> Hi All,
>
> I have a quick question regarding the modification of global variables within
> functions. To illustrate, consider the following toy example:
>
> a={"1": set()}
> b=9
>
> def gt(l):
> a["1"] = a["1"] | set([l])
>
> When calling this last function and checking the a dictionary, I get:
>
> >>> gt(5)
> >>> a
> {"1": set([5])}
>
>
> The set in the dictionary was modified. The question is, why isn't it
> necessary to declare a as global within the gt function, as apposed to a case
> like
>
> def gt2(l):
> b=b+l
>
> where I need to declare b as global within the function to avoid:
>
> UnboundLocalError: local variable 'b' referenced before assignment.
Well. To understand the difference, one first has to know the mechanism of
CPython.
When you have a script, CPython will first parse it, and then generate bytecode
representation, and then finally execute it.
If you take a look at the bytecode of both your source code, you will notice,
that.
In the first snippet, there is something like LOAD_NAME, which loads the global
name a.
However, in the second snippet, there would be a LOAD_FAST, which loads the
local name b (which is, technically speaking, stored in the PyFrameObj).
The reason Python treats it differently, is, IIUC, for better semantic meaning,
and also, for huge performance improvement.
Hope that explains.
Regards.
--
Shiyao Ma
http://introo.me
--
https://mail.python.org/mailman/listinfo/python-list
