exec "x = 3; print x" in a - How does it work?

[Veek. M]

Steven D'Aprano wrote:

> On Wednesday 09 March 2016 16:27, Veek. M wrote:
> 
>> What is the return value of `exec`? Would that object be then used to
>> iterate the sequence in 'a'? I'm reading this:
>> https://www.python.org/download/releases/2.2.3/descrintro/
> 
> 
> exec is a statement, not a function, so it doesn't have a return
> value.
> 
> Are you referring to this line of code?
> 
> exec "x = 3; print x" in a
> 
> 
> That doesn't return a value. The "in a" part tells exec which
> namespace to execute the code in. It doesn't mean "test if the result
> is found inside sequence a".
> 
> py> namespace = {'__builtins__': None}
> py> exec "x = 3" in namespace
> py> namespace
> {'__builtins__': None, 'x': 3}
> 
> 
> If you leave the "in namespace" part out, then exec will use the
> current namespace, and x will become a local variable.
> 
> 
> What happens if you don't put the special __builtins__ key into the
> namespace? Python adds it for you:
> 
> 
> py> mydict = {}
> py> exec "foo = 99.99" in mydict
> py> mydict.keys()
> ['__builtins__', 'foo']
> 
> 
> 
> What's inside __builtins__? Every single built-in function and class:
> 
> py> mydict['__builtins__']
> {'bytearray': <type 'bytearray'>, 'IndexError': <type
> 'exceptions.IndexError'>, 'all': <built-in function all>,
> 
> ... dozens more entries ...
> 
> 'OverflowError': <type 'exceptions.OverflowError'>}
> 
> 
> 
> 
ah, okay - i'm familiar with the py3 syntax where you do:
eval('whatever stmt', globals={}, locals={})
I suppose this: exec " " in NS; syntax is strictly py2?

Many thanks :)