Dynamically generate method from string?

Question

I have a dict of different types for which I want to add a simple getter based on the name of the actual parameter.

For example, for three storage parameters, let's say:

self.storage = {'total':100,'used':88,'free':1}

I am looking now for a way (if possible?) to generate a function on the fly with some meta-programming magic.

Instead of

class spaceObj(object):
    def getSize(what='total'):
      return storage[what]

or hard coding

@property
def getSizeTotal():
  return storage['total']

but

class spaceObj(object):
# manipulting the object's index and magic
@property
def getSize: 
    return ???

so that calling mySpaceObj.getSizeFree would be derived - with getSize only defined once in the object and related functions derived from it by manipulating the objects function list.

Is something like that possible?

I am not sure to understand the question, but you need either getattr, or __getitem__ — Paco
– Paco, Commented Oct 9, 2013 at 15:16
You could do that - but what do you hope to gain over your current approach? — Jon Clements
– Jon Clements, Commented Oct 9, 2013 at 15:18

Brian · Accepted Answer · 2013-10-09 15:28:42Z

5

While certainly possible to get an unknown attribute from a class as a property, this is not a pythonic approach (__getattr__ magic methods are rather rubyist)

class spaceObj(object):
    storage = None

    def __init__(self):  # this is for testing only
        self.storage = {'total':100,'used':88,'free':1}

    def __getattr__(self, item):
        if item[:7] == 'getSize':  # check if an undefined attribute starts with this
            return self.getSize(item[7:])

    def getSize(self, what='total'):
        return self.storage[what.lower()]

print (spaceObj().getSizeTotal)  # 100

edited Oct 9, 2013 at 15:28

answered Oct 9, 2013 at 15:19

Brian

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1 Comment

msw Over a year ago

@Thomas "While certainly possible to get an unknown attribute from a class as a property, this is not a pythonic approach" means "although you can make idiosyncratic language syntax, everyone else is going to have a hard time making sense of it". So "works like a charm" is probably more accurately "works like a charm for me this week".

Aaron Digulla · Accepted Answer · 2013-10-09 15:19:08Z

You can put the values into the object as properties:

class SpaceObj(object):
    def __init__(self, **kwargs):
        self.__dict__.update(kwargs)

storage = {'total':100,'used':88,'free':1}
o = SpaceObj(**storage)
print o.total

or

o = SpaceObj(total=100, used=88, free=1)
print o.total

or using __getattr__:

class SpaceObj(object):
    def __init__(self, **kwargs):
        self.storage = kwargs

    def __getattr__(self,name):
        return self.storage[name]

o = SpaceObj(total=100, used=88, free=1)
print o.total

The latter approach takes a bit more code but it's more safe; if you have a method foo and someone create the instance with SpaceObj(foo=1), then the method will be overwritten with the first approach.

maxywb · Accepted Answer · 2013-10-09 15:25:41Z

-2

>>> import new
>>> funcstr = "def wat(): print \"wat\";return;"
>>> funcbin = compile(funcstr,'','exec')
>>> ns = {}
>>> exec funcbin in ns
>>> watfunction = new.function(ns["wat"].func_code,globals(),"wat")
>>> globals()["wat"]=watfunction
>>> wat()
wat

answered Oct 9, 2013 at 15:25

maxywb

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1 Comment

Jon Clements Over a year ago

How does this help? (or even bear a resemblance to the question?)

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