Hi there,
I have a function name stored in a variable like this:
myvar = 'mypackage.mymodule.myfunction'
and I now want to call myfunction like this
myvar(parameter1, parameter2)
What's the easiest way to achieve this?
views:
776answers:
6
+1
A:
Easiest
eval(myvar)(parameter1, parameter2)
You don't have a function "pointer". You have a function "name".
While this works well, you will have a large number of folks telling you it's "insecure" or a "security risk".
S.Lott
2010-02-17 18:10:16
"insecure": If myvar comes from user input, yes :)
Federico Ramponi
2010-02-17 18:12:12
They'd be "right".
Derrick Turk
2010-02-17 18:12:34
...and those large number of folks are right.
Triptych
2010-02-17 18:12:44
Aside from security issues, I get a "NameError: name 'mypackage' is not defined", while I can import modules from there going the "normal" way. And: of course, the string that shall be evaluated is fixed and not changed by the user.
schneck
2010-02-17 18:13:41
@schneck, then why would it possibly have to be a string?
Mike Graham
2010-02-17 18:17:59
@schneck: If eval('the string') does not produce the correct function, then your question is incomplete. You've omitted something crucial. You might try posting something that *does* work along with the detailed error message of what doesn't work.
S.Lott
2010-02-17 18:19:26
@Derrick Turn, @Truptych: They'd be right only if the string came from a malicious sociopath. User input from unauthenticated people on the internet is likely to involve malicious sociopaths. Most everything else does not generally involve malicious sociopaths, reducing the security risk to exactly the same risk as someone deleting all the source code for the application.
S.Lott
2010-02-17 18:20:54
It's only insecure if the data comes from external sources. If myvar is defined in your app, it's no less secure that directly calling the function.
Bryan Oakley
2010-02-17 18:22:22
@Mike Graham: It must be a string because at the place where it is defined, the application does not know the desired function, since it's a generic app.
schneck
2010-02-17 18:26:34
@schneck, I don't understand what you mean by "since it's a generic app" could mean here. If you have defined this as a string literal, you already know enough about it that you don't need to do so.
Mike Graham
2010-02-17 18:31:25
Given the ability to run any functions given in `myvar`, why worry about `eval`?
KennyTM
2010-02-17 19:32:32
@Mike Graham: I thought about that and I think you're right; I should reference a real function name rather than by a string.
schneck
2010-02-17 20:29:54
+5
A:
funcdict = {
'mypackage.mymodule.myfunction': mypackage.mymodule.myfunction,
....
}
funcdict[myvar](parameter1, parameter2)
Ignacio Vazquez-Abrams
2010-02-17 18:11:21
+3
A:
def f(a,b):
return a+b
xx = 'f'
print eval('%s(%s,%s)'%(xx,2,3))
OUTPUT
5
TheMachineCharmer
2010-02-17 18:15:22
I'll give an upvote to counteract the downvote. It may not be the best solution, I do think it's a helpful answer since it shows a complete, working example.
Bryan Oakley
2010-02-17 18:23:39
+1
A:
modname, funcname = myvar.rsplit('.', 1)
getattr(sys.modules[modname], funcname)(parameter1, parameter2)
Matt Anderson
2010-02-17 18:18:42
+1
A:
Why not store the function itself? myvar = mypackage.mymodule.myfunction is much cleaner.
ironfroggy
2010-02-17 18:55:15
+7
A:
It's much nicer to be able to just store the function itself, since they're first-class objects in python.
import mypackage
myfunc = mypackage.mymodule.myfunction
myfunc(parameter1, parameter2)
But, if you have to import the package dynamically, then you can achieve this through:
mypackage = __import__('mypackage')
mymodule = getattr(mypackage, 'mymodule')
myfunction = getattr(mymodule, 'myfunction')
myfunction(parameter1, parameter2)
Bear in mind however, that all of that work applies to whatever scope you're currently in. If you don't persist them somehow, you can't count on them staying around if you leave the local scope.
AndrewBC
2010-02-17 19:03:41