About Modifying Globals

[shiyao.ma]

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