which a is used?

[Steven D'Aprano]

On Mon, 24 Sep 2012 16:43:24 -0700, Jayden wrote:

> Dear All,
> 
> I have a simple code as follows:
> 
> # Begin
> a = 1
> 
> def f():
>     print a
> 
> def g():
>     a = 20
>     f()
> 
> g()
> #End
> 
> I think the results should be 20, but it is 1. Would you please tell me
> why?

You are expecting "dynamic scoping", Python uses "static scoping" (or 
lexical scoping). With lexical scoping, you can reason about the 
behaviour of a function by knowing only how and where it is defined. The 
caller is irrelevant.

Since function f is defined globally, and does not have its own local 
variable a, it will always see the global variable a no matter where it 
is called. So when you call f() from inside g(), f prints 1, the global 
a, not 20, g's local a.

While dynamic scoping has its uses, it is more tricky to use correctly. 
One can no longer understand the behaviour of a function just by reading 
the function's own code, knowing where and how it is defined. You also 
need to know where it is called. A function f that works perfectly when 
you call it from functions g, h, k, ... will suddenly misbehave (crash, 
or worse, behave wrongly) when called from function v because v 
accidentally changes some global variable that f relies on.

This is especially a danger for Python, because built-in functions like 
len, chr, ord, print (version 3 only), and many others are all global 
variables.

(Technically, they are in a third scope, the builtins, but that's 
equivalent to being global.)



-- 
Steven