exec "x = 3; print x" in a - How does it work?
Veek. M
vek.m1234 at gmail.com
Wed Mar 9 01:25:52 EST 2016
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 :)
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