Converting a string to a function pointer

[Håkan Persson]

Hi.

I am trying to "convert" a string into a function pointer.
Suppose I have the following:

from a import a
from b import b
from c import c

funcString = GetFunctionAsSt ring()

and funcString is a string that contains either "a", "b" or "c".
How can I simply call the correct function?
I have tried using getattr() but I don't know what the first (object)
argument should be in this case.

Thanks,
Håkan Persson

[John Machin]

Re: Converting a string to a function pointer

On Fri, 04 Feb 2005 12:01:35 +0100, Håkan Persson <hakan@zyberit. com>
wrote:
[color=blue]
>Hi.
>
>I am trying to "convert" a string into a function pointer.
>Suppose I have the following:
>
>from a import a
>from b import b
>from c import c
>
>funcString = GetFunctionAsSt ring()
>
>and funcString is a string that contains either "a", "b" or "c".
>How can I simply call the correct function?
>I have tried using getattr() but I don't know what the first (object)
>argument should be in this case.[/color]

Try this:
[color=blue][color=green][color=darkred]
>>> from sys import exit
>>> globals()[/color][/color][/color]
{'__builtins__' : <module '__builtin__' (built-in)>, '__name__':
'__main__', 'exit': <built-in function exit>, '__doc__': None}[color=blue][color=green][color=darkred]
>>> afunc = globals()["exit"][/color][/color][/color]

Do you really need to use the "from X import Y" style? Consider the
following alternative:
[color=blue][color=green][color=darkred]
>>> import sys
>>> afunc = getattr(sys, "exit")[/color][/color][/color]

[Steve Holden]

Re: Converting a string to a function pointer

Håkan Persson wrote:
[color=blue]
> Hi.
>
> I am trying to "convert" a string into a function pointer.
> Suppose I have the following:
>
> from a import a
> from b import b
> from c import c
>
> funcString = GetFunctionAsSt ring()
>
> and funcString is a string that contains either "a", "b" or "c".
> How can I simply call the correct function?
> I have tried using getattr() but I don't know what the first (object)
> argument should be in this case.
>
> Thanks,
> Håkan Persson
>
>[/color]
Well, one really horrible way would be

getattr(sys.mod ules['__main__'], funcString)

and, of course, if you were going to use it a lot you could optimize
slightly with

this = sys.modules['__main__']

followed later by

this = getattr(this, funcString)

Overall, however, it might be better to set uip a mapping for the
functions you need to be accessible. This has the further merits that

a) You can heve different, and/or multiple names for a function
b) You can trap errors more easily
c) erm, I forgot the third advantage, consider this the Spanish
Inquisition in reverse :-)

So:

funcMap = {
"a": a,
"A": a,
"b": b,
"exoticName ": c }

...

funcString = GetFunctionAsSt ring()
try:
f = funcMap(funcStr ing)
except KeyError:
print "No such function"
raise SomethingElse
result = f(args)

regards
Steve
--
Meet the Python developers and your c.l.py favorites March 23-25
Come to PyCon DC 2005 http://www.pycon.org/
Steve Holden http://www.holdenweb.com/

[Tim Roberts]

Re: Converting a string to a function pointer

Håkan Persson <hakan@zyberit. com> wrote:[color=blue]
>
>I am trying to "convert" a string into a function pointer.
>Suppose I have the following:
>
>from a import a
>from b import b
>from c import c
>
>funcString = GetFunctionAsSt ring()
>
>and funcString is a string that contains either "a", "b" or "c".
>How can I simply call the correct function?
>I have tried using getattr() but I don't know what the first (object)
>argument should be in this case.[/color]

You've received a lot of hi-tech answers, but don't overlook the simple
(and possibly more secure) answer:

lookup = {
'a': a,
'b': b,
'c': c,
}

if lookup.has_key( myFunction):
return lookup[myFunction]()
else:
print "Couldn't find", myFunction
--
- Tim Roberts, timr@probo.com
Providenza & Boekelheide, Inc.