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

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  exec "x = 3; print x" in a - How does it work? "Veek. M" <vek.m1234@gmail.com> - 2016-03-09 10:57 +0530
    Re: exec "x = 3; print x" in a - How does it work? Ben Finney <ben+python@benfinney.id.au> - 2016-03-09 16:50 +1100
      Re: exec "x = 3; print x" in a - How does it work? "Veek. M" <vek.m1234@gmail.com> - 2016-03-09 11:51 +0530
    Re: exec "x = 3; print x" in a - How does it work? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-03-09 16:54 +1100
      Re: exec "x = 3; print x" in a - How does it work? "Veek. M" <vek.m1234@gmail.com> - 2016-03-09 11:55 +0530
        Re: exec "x = 3; print x" in a - How does it work? Chris Angelico <rosuav@gmail.com> - 2016-03-09 17:34 +1100

#104384 — exec "x = 3; print x" in a - How does it work?

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/ 

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#104385

"Veek. M" <vek.m1234@gmail.com> writes:

> What is the return value of `exec`?

You can refer to the documentation for questions like that.
<URL:https://docs.python.org/3/library/functions.html#exec>

> Would that object be then used to iterate the sequence in 'a'?

The ‘for’ or ‘while’ statements are correct for iteration.

> I'm reading this:
> https://www.python.org/download/releases/2.2.3/descrintro/

Why?

-- 
 \          “Generally speaking, the errors in religion are dangerous; |
  `\    those in philosophy only ridiculous.” —David Hume, _A Treatise |
_o__)                                           of Human Nature_, 1739 |
Ben Finney

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#104388

Ben Finney wrote:

> "Veek. M" <vek.m1234@gmail.com> writes:
> 
>> What is the return value of `exec`?
> 
> You can refer to the documentation for questions like that.
> <URL:https://docs.python.org/3/library/functions.html#exec>
> 
>> Would that object be then used to iterate the sequence in 'a'?
> 
> The ‘for’ or ‘while’ statements are correct for iteration.
> 
>> I'm reading this:
>> https://www.python.org/download/releases/2.2.3/descrintro/
> 
> Why?
> 
Well, i had a question on MRO and asked on freenode #python and they 
suggested i read that.

My question was:
class A(x, y, z):
 pass

class x(q,w,r)
 pass

I wanted to know how the __mro__ would be generated (c3 linearization)

I had assumed when using super() it would do:
x, q, w, r, y, z, object
basically hit a base class and finish with all it's parents before 
stepping to the next sibling

But then i read somewhere else that it's like this:
x, y, z, q, w, r, object
and of course if q has base-classes then:
x, y, z, q, w, r, e, f, g
which is utterly confusing because you can't tell by looking where e, f, 
g are coming from (q) in this case..

This doc: 
https://rhettinger.wordpress.com/2011/05/26/super-considered-super/
I was getting all cross-eyed reading this:
'The ancestor tree for our new class is: LoggingOD, LoggingDict, 
OrderedDict, dict, object. For our purposes, the important result is 
that OrderedDict was inserted after LoggingDict and before dict! This 
means that the super() call in LoggingDict.__setitem__ now dispatches 
the key/value update to OrderedDict instead of dict.'

I was wondering why they had chosen this notation over the other.

And if you're wondering why that paper - because i was reading Beazley 
super() pg 120 and the whole hard-coding avoided using super() bit.

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#104386

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

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#104389

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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#104390

On Wed, Mar 9, 2016 at 5:25 PM, Veek. M <vek.m1234@gmail.com> wrote:
> 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 :)

Yeah, that's one of the things that was cleaned up in Py3. Several
pieces of magic syntax were replaced with perfectly ordinary
functions. While there are plenty of people who complain about having
to put parentheses around their print calls, I'm firmly of the opinion
that print(..., file=somefile) is WAY better than the >> syntax that
Py2 used. I can never remember whether it has to go first or has to go
last, and whether it's >> or >>>, etc, etc. It's magic syntax. Py3?
It's a keyword argument. Easy!

ChrisA

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