how can I execute a function string

[Hans Nowak]

Rajarshi Guha wrote:

> Hi ,
>  I have some code that generates a function on the fly in a string. 
> At a later point in time I want to execute this function (which also
> requires a parameters to be passed to it). So the code is something like
> this:
> 
> def generate_func():
> 	s = """ def function(x):
> 	print x
> 	return 2
> """
> 	return s
> 
> funcstring = generate_func()
> retval = ....
> 
> That is, retval should have the value returned from the evaluation of the
> function in the string funcstring.
> 
> Is this possible by means of simple function calls or does this involve
> some sort of black magic?

It's possible:

 >>> def generate_func():
...     s = """def f(x):
...     print x
...     return 2"""
...     d = {}
...     exec s in d
...     return d['f']
...
 >>>
 >>> g = generate_func()
 >>> g(4)
4
2
 >>>

...but this way of doing it has several drawbacks.  For example, exec is picky 
about the strings it accepts; e.g. if you leave a trailing newline you get a 
syntax error.  Compare:

 >>> def generate_func():
...     s = """def f(x):
...     print x
...     return 2
...     """
...     d = {}
...     exec s in d
...     return d['f']
...
 >>> g = generate_func()
Traceback (most recent call last):
   File "<input>", line 1, in ?
   File "<input>", line 7, in generate_func
   File "<string>", line 4

     ^
SyntaxError: invalid syntax

Also, if the string containing the syntax comes from an "untrusted source", 
then it's a potential security hazard.

I'm not sure what you're trying to do here, but you might want to consider 
generating an actual function, rather than a string to be evaluated:

 >>> def generate_func():
...     def f(x):
...         print x
...         return 2
...     return f
...
 >>> h = generate_func()
 >>> h(5)
5
2

HTH,

-- 
Hans (hans at zephyrfalcon.org)
http://zephyrfalcon.org/