python constructing functions on the fly

Question

I have several little functions f1, f2, f3 and a function f.

I want f to be a "container" to f1, f2, f3: to do the some of operations f1, f2, f3, depending on the program configuration (for example f1 and f2 or f1 and f3 or all the three) and nothing more.

I see two simple solutions: first to add some if's in the function f:

if configuration_f1_f2:
    f1()
    f2()

second, I can add a list of operations in f:

for op in operations:
    op()

and add f1, f2, f3 in operations or remove them, depending on configuration.

But can I somehow construct dynamically code of 'f' adding to it calls of f1, f2 and f3 exact what I need without any if's or list's or for's? I mean something like on the fly code manipulation. So if my configuration is "f1 and f3", I set code of f so that it is

f1()
f3()

and when my configuration changes to "f2 and f3" I modify code of f to

f2()
f3()

Can I manipulate the code of the function that way?

Greg Hewgill · Accepted Answer · 2009-10-20 10:24:06Z

3

If you're brave, you can construct a function definition as a string and pass it to the exec statement. For example:

func = "def f():\n"
if config_f1:
    func += " f1()\n"
if config_f2:
    func += " f2()\n"
exec func in globals()

At this point, you should have a new global f() that executes the appropriate bits of code.

answered Oct 20, 2009 at 10:24

Greg Hewgill

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1 Comment

netimen Over a year ago

Thank you! this seems to be the best solution!

Stephan202 · Accepted Answer · 2009-10-20 10:05:18Z

3

If f1, f2 etc. are functions with side effects, than you should use an explicit for loop (no fancy map solution). Perhaps you want something like this?

configurations = {
  'config_1': (f1, f2, f3),
  'config_2': (f1, f2),
}

def f(config='config_1'):
    for op in configurations[config]:
        op()

If f1, f2 etc. receive arguments, then perhaps this is a more suitable definition of f:

def f(config, *args, **kwargs):
    for op in configurations[config]:
        op(*args, **kwargs)

answered Oct 20, 2009 at 10:05

Stephan202

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Comments

Michał Niklas · Accepted Answer · 2009-10-20 10:03:15Z

1

Create dictionary

fun_dict = {'f1': f1, 'f2': f2}

then parse your config and run function from dict:

fun_dict[parsed_name]()

answered Oct 20, 2009 at 10:03

Michał Niklas

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Comments

Anurag Uniyal · Accepted Answer · 2009-10-20 10:16:41Z

1

There can be various ways to achieve what you want, but for that you need to fully define the problem and context

a way could be like this, and most of the other will be variant of this, using a dict to lookup the function we need

def f1(): print "f1"
def f2(): print "f2"
def f3(): print "f3"

def f(fList):
    for f in fList: globals()[f]()

f(["f1", "f2"])
f("f1 f3 f1 f3 f2 f1 f3 f2".split())

answered Oct 20, 2009 at 10:16

Anurag Uniyal

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2 Comments

Martin DeMello Over a year ago

why do you need a list of strings? this would work as well: def f(*flist): [x() for x in flist] f(f1, f2)

Anurag Uniyal Over a year ago

because OP was talking abt config, where values will be strings

S.Lott · Accepted Answer · 2009-10-20 10:24:55Z

1

Use objects and the Command design pattern.

class Function( object ):
    pass

class F1( Function ):
    def __call__( self ):
        whatever `f1` used to do

 class F2( Function ):
    def __call__( self ):
        whatever `f1` used to do

 class Sequence( Function ):
     def __init__( self, *someList ):
         self.sequence= someList
     def __call__( self ):
         for f in self.sequence:
             f()

f= myDynamicOperation( F1(), F2(), F3() )
f()

That's how it's done. No "constructing a function on the fly"

answered Oct 20, 2009 at 10:24

S.Lott

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1 Comment

Anurag Uniyal Over a year ago

but how is it different from calling f1(), f2(), f3(); directly

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