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

[Steven D'Aprano]

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'>}




-- 
Steve