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 = GetFunctionAsString()

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]

On Fri, 04 Feb 2005 12:01:35 +0100, Håkan Persson <ha***@zyberit.com>
wrote:

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 = GetFunctionAsString()

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.

Try this:

from sys import exit
globals() {'__builtins__': <module '__builtin__' (built-in)>, '__name__':
'__main__', 'exit': <built-in function exit>, '__doc__': None} afunc = globals()["exit"]
Do you really need to use the "from X import Y" style? Consider the
following alternative:
import sys
afunc = getattr(sys, "exit")

[Steve Holden]

Håkan Persson wrote:

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 = GetFunctionAsString()

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

Well, one really horrible way would be

getattr(sys.modules['__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 = GetFunctionAsString()
try:
f = funcMap(funcString)
except KeyError:
print "No such function"
raise SomethingElse
result = f(args)

regards
Steve
--
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Come to PyCon DC 2005 http://www.pycon.org/
Steve Holden http://www.holdenweb.com/

[Tim Roberts]

Håkan Persson <ha***@zyberit.com> wrote:

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 = GetFunctionAsString()

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.

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, ti**@probo.com
Providenza & Boekelheide, Inc.