[Tutor] calling global in funtions.

[Steven D'Aprano]

On Wed, Feb 26, 2014 at 12:42:00PM +0530, Santosh Kumar wrote:
> All,
> 
> Requirement : i want to call a variable assigned outside a function scope
> anytime within the function. I read "global" is a way.

You can *read* the value of a global from inside a function without 
needing to declare it at any time. In fact, this is how you can call 
other functions -- they are just globals that you read.

So this will work:

a = 10

def func():
    print("a equals %s" % a)

func()
=> prints "a equals 10"



But if you want to re-assign a global, you need to declare it global 
first. By default, if you say "x = ..." inside a function, Python will 
treat that variable x as a local variable. So here's an example:

a = 10

def func():
    global a
    print("Before, a equals %s" % a)
    a = 23
    print("After, a equals %s" % a)


func()
=> prints "Before, a equals 10" and "After, a equals 23")


> a) Case I looks fine.
> b) Case II is bombing out.

Actually no it isn't. You're just getting a warning, not an error. See 
below.


> Is this how it works or please correct me if i am wrong.
> 
> case I:
> 
> In [17]: a = 10
> 
> In [19]: def fun_local():
>    ....:     global a
>    ....:     print "the value of a is %d" %(a)
>    ....:     a = 5
>    ....:     print "the value of a is %d" %(a)
>    ....:
> 
> In [20]: fun_local()
> the value of a is 10
> the value of a is 5

This is completely fine.



> CASE II:
> 
> In [21]: a = 10
> 
> In [22]: def fun_local():
>    ....:     a = 5
>    ....:     print "the value of a is %d" %(a)
>    ....:     global a
> <input>:4: SyntaxWarning: name 'a' is assigned to before global declaration
> <input>:4: SyntaxWarning: name 'a' is assigned to before global declaration
> <input>:4: SyntaxWarning: name 'a' is assigned to before global declaration

This is just a warning. It is STRONGLY RECOMMENDED that you put the 
global declaration at the top of the function, but it is not compulsary. 
If you put it somewhere else, you will get a warning, but the function 
will still work.

For now. Some day Python may change that behaviour, so it is safest if 
you move the global to the top of the function.


-- 
Steven