Call methods by string [duplicate]

Question

I have the following class.

func_list= ["function1", "function2", "function3"]

class doit(object):
    def __init__(self):
        for item in func_list:
            if item == "function1":
                self.function1()
            elif item == "function2":
                self.function2()
            elif item == "function3":
                self.function3()

    def function1(self):
        #do this
        pass
    def function2(self):
        #do that
        pass
    def function3(self):
        pass

If an instance of this class is created, it iterates over a list of strings and calls methods depending on the actual string. The strings in the list have the names of the corresponding methods.

How can I do this in a more elegant way? I don't want to add another elif-path for every "function" I add to the list.

Drollery

as a note, don't call your variable list, as it is the name of a python built-in function. Something like funclist or flist would be fine ;) – Ellieellinger

You should put function objects in the list, rather than strs, and just call each one. – Projective

Answer 1

func_list= ["function1", "function2", "function3"]

class doit(object):
    def __init__(self):
        for item in func_list:
            getattr(self, item)()
    def function1(self):
        print "f1"
    def function2(self):
        print "f2"
    def function3(self):
        print "f3"



>>> doit()
f1
f2
f3

For also private functions:

for item in func_list:
     if item.startswith('__'):
         getattr(self, '_' + self.__class__.__name__+ item)()
     else:
         getattr(self, item)()

.

getattr(object, name[, default])

Return the value of the named attribute of object. name must be a string. If the string is the name of one of the object’s attributes, the result is the value of that attribute. For example, getattr(x, 'foobar') is equivalent to x.foobar. If the named attribute does not exist, default is returned if provided, otherwise AttributeError is raised.

http://docs.python.org/library/functions.html#getattr

Parotitis

I think this answer would be slightly better if you mentioned that the magic was done in the getattr function and provided a link to the documentation. – Elli

It would also be good to get rid of the list variable, since it is not used. – Loathe

@black_dragon: Great, this is what I wanted to do. But how do I access private-methods like def __function4(self): print "private"? – Drollery

The private function __function4 would be accessed as _doit__function4, as that is how the name gets mangled to make it appear to be private. – Suziesuzuki

@JonathanCallen: Thanks for this advice. I already though of this. But how can we make this more elegant? The problem is, if I rename the class, I've to make the change in every method I call a private-function with getattr. How do I get the class´ name? – Drollery

@Drollery You should avoid __names. See #166383 for details. Just use _names. – Projective

Answer 2

This is usually solved with a dictionary lookup because functions are first-class datatypes in Python. For example:

# map string names to callable functions
dispatch = {'func1': func1, 'func2': func2, ...}

want_to_call = 'func1'
dispatch[want_to_call](args)

Duluth

Answer 3

Why don't you use lambdas?

Such as say,

def func1(a):
    return a ** 2

def func2(a, b):
    return a ** 3 + b

dic = {'Hello':lambda x: func1(x), 'World':lambda x,y: func2(x, y)} #Map functions to lambdas
print dic['Hello'](3)
print dic['World'](2,3)

Output, 9 11

OR, you can do this...

class funs():
    def func1(self, a):
        return a ** 2

    def func2(self, a, b):
        return a ** 3 + b

f = funs()
dic = {'Hello': lambda x: f.func1(x), 'World': lambda x,y: f.func2(x,y)}
print dic['Hello'](3)
print dic['World'](5,4)

Will give you, 9 129

Mosa

Answer 4

class doit(object):
    def __init__(self, func_list):
        for func in func_list:
            func(self)
    #insert other function definitions here

func_list = [doit.function1, doit.function2, doit.function3]
foo = doit(func_list)

Zannini

Answer 5

You can use eval which make's your program more simple and short.

list= ["function1", "function2", "function3"]

class doit(object):
    def __init__(self):
        for item in list:
            eval(item)()
    def function1(self):
        print "f1"
    def function2(self):
        print "f2"
    def function3(self):
            print "f3"

Harass

To use exec would be another possibility. – Drollery

-1, eval and exec are things that should never appear in Python code. They're security vulnerabilities and completely overkill here. – Projective