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[python] functions help

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I'm learning python from a wiki-book http://en.wikibooks.org/wiki/Non-Programmer%27s_Tutorial_for_Python/Defining_Functions and having trouble completely figuring out functions an example code from the books is:
 
a_var = 10
b_var = 15
e_var = 25
 
def a_func(a_var):
    print "in a_func a_var = ", a_var
    b_var = 100 + a_var
    d_var = 2 * a_var
    print "in a_func b_var = ", b_var
    print "in a_func d_var = ", d_var
    print "in a_func e_var = ", e_var
    return b_var + 10
 
c_var = a_func(b_var)
 
print "a_var = ", a_var
print "b_var = ", b_var
print "c_var = ", c_var
print "d_var = ", d_var
and the output should be :
in a_func a_var =  15
in a_func b_var =  115
in a_func d_var =  30
in a_func e_var =  25
a_var =  10
b_var =  15
c_var =  125
d_var = 

Traceback (most recent call last):
 File "C:\Python24\def2", line 19, in -toplevel-
    print "d_var = ", d_var</source>

NameError: name 'd_var' is not defined
I don't get why a_var would equal 15 inside the function it's not changed from the global variable from what I see. the tutorial says: Inside of the function a_var is 15 since the function is called with a_func(b_var). Since at that point in time b_var is 15, the call to the function is a_func(15) This ends up setting a_var to 15 when it is inside of a_func. ---- but I don't understand this at all or where this "a_func(b_var)" is in the code Does anyone know any other good tuts about functions (or python in general) or have a brief explanation of functions/ def functions? Thanks-- Also is there a reason why sometimes the IDLE stops working after running a few things, I have to go into task manager and end the processes to be able to re-open it. I get a socket error

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Functions take parameters. For example the function:

def PrintXY(x,y):
print x
print y


takes two parameters, x and y. You call the function like this:

PrintXY(1,2)

so, inside the function, x=1 and y=2. So it will print '1' and '2'. Very important: x and y are variables *local* to the function PrintXY. The local x is not the same as a global 'x', if something like that exists. They happen to have the same name, but they're not the same variable.

Now, if you call with different values, PrintXY(42,24), then x=42 and y=24. And so on. Instead of contants, you can call it passing variables as parameters. So if you call it:

PrintXY(a,b)

so parameter x takes the value of a, and parameter y takes the value of b

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