[Tutor] exec('a=1') in functions

[Peter Otten]

On 17/08/2021 18:37, Jan Kenin wrote:
> Hello,
> 
> in python 2.7 the following code works:
>  >>> def f( b ):
> ...     exec( 'a=%s'%b )
> ...     print 'a=', a
> ...
>  >>> f( 1 )
> a= 1
> 
> in python3.6 this works in the interpreter:
>  >>> b=1
>  >>> exec('a=%s'%b)
>  >>> a
> 1
> 
> but not in a function:
>  >>> def f( b ):
> ...     exec( 'a=%s'%b )
> ...     print( 'a=', a )
> ...
>  >>> f( 1 )
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
>    File "<stdin>", line 3, in f
> NameError: name 'a' is not defined
> 
> How can I get the latter working?

You can't. In Python 2 the exec statement modified the byte code, with 
sometimes surprising consequences.

In Python 3 exec() is an ordinary function; you have to copy data from 
its namespace into the function explicitly. Example:

 >>> def f(b):
	ns = dict(b=b)
	exec("a = b", ns)
	a = ns["a"]
	print(f"{a=}")

	
 >>> f(42)
a=42

Less magic, but much cleaner ;)