scope question in a switch mixin

[John Machin]

browerg at verizon.net wrote:
> The code that follows is the result of noodling around with switches as a learning tool. I've played with python for a few years, but I'm self-taught, so . . .
> 
> Class Switch builds a set of functions. Method switch executes one of them given a value of the switch variable.
> 
> My question is, why are modules imported at the top of the program not visible to the functions Switch builds? There is
> no problem when the import is in the function, but I thought initially that imports at the top would be in its globals.

The global namespace of the to-be-exec'ed code is the dictionary that 
you supply. That dictionary doesn't contain "math". More comments below.

[snip]

> 
> #import math
> 
> class Switch(object):
> 
>    def __init__(self, keys, vals, base):
>        self.dictionary = {}
>        tmpd = {}

Uncomment the above import, remove the import from your constructed 
source code and try
     tmpd = {'math': math}

>        for i in range(len(vals)):
>            func = ''.join([base[0] % keys[i], vals[i], base[1]])
>            compile(func, '<stderr>', 'exec')
>            exec(func, tmpd)

compile returns a code object, which you are throwing away. Comment out 
that statement, and the behaviour of your code will not change. This is 
because if the first argument to exec is a string, it is compiled into a 
code object.

It's a long time since exec has been documented as a *function*. Why? 
Because it isn't:

 >>> exec "print 'xxx'"
xxx
 >>> exec("print 'xxx'")
xxx
 >>> foo = compile
 >>> foo
<built-in function compile>
 >>> foo = exec
   File "<stdin>", line 1
     foo = exec
              ^
SyntaxError: invalid syntax
 >>>

What book/tutorial did you get that from?

Cheers,
John