Setting properties programmatically

Question

Question

I want to accomplish the following:

tmp = do_stuff(tmp2)
if tmp != None:
    num = tmp

However the variables to be assigned in my problem are variables (better: properties) of an object instance:

class mycls(object):
    def __init__(self):
        self.__a, self._b, self.c = 2, 3, 5
    def set_a(self, val): self.__a = val
    def set_b(self, val): self._b = val
    def set_c(self, val): self.c = val
    A = property(fset = set_a)
    B = property(fset = set_b)
    C = property(fset = set_c)
tmp2 = [7, 11, 13]
inst = mycls()
tmp = do_stuff(tmp2[0])
if tmp != None: inst.A = tmp
tmp = do_stuff(tmp2[1])
if tmp != None: inst.B = tmp
tmp = do_stuff(tmp2[2])
if tmp != None: inst.C = tmp

Obviously the last part looks highly repetitive and in my case needs to be applied to over 10 variables. My question now is: how can those last 6 lines be compressed?

Own attempts at solving the problem

Ideally I would prefer a solution like:

for i in [[inst.A, 0], [inst.B, 1], [inst.C, 2]]:
    tmp = do_stuff(tmp2[i[1]])
    if tmp != None: i[0] = tmp

However this doesn't work (because the inst.X are evaluated and you can't store primitive datatypes by reference/pointer). Another approach I came up with was to use the variable names as strings and fiddle with inst.__dict__, e.g. like this:

for i in [["A", 0], ["B", 1], ["C", 2]]:
    tmp = do_stuff(tmp2[i[1]])
    if tmp != None: inst.__dict__[i[0]] = tmp

However this plan was foiled by the fact that A, B and C are properties.

LISP-like macros would be an enormous help here, unfortunately Python doesn't seem to support macros and I don't want to add dependencies to one of the macro libraries for Python I found on-line (which I didn't test since I won't be able to use them anyway).

I also want to avoid using anonymous functions in a list since this will likely result in the same amount of code as in the second code listing.

My last resort to solve this problem would currently be evals, which I wanted to avoid at all cost.

@Peter Kirby: my goal is to reduce the amount of code, not just transform it into a different form. In the case I misunderstood, please clarify. — user1624579
– user1624579, Commented Aug 25, 2012 at 14:33
I did misunderstand, sorry. However, Martjin's answer is correct, and should work. — Peter Kirby
– Peter Kirby, Commented Aug 25, 2012 at 14:34

Martijn Pieters · Accepted Answer · 2012-08-25 14:37:03Z

5

You are looking for setattr():

for i in [["A", 0], ["B", 1], ["C", 2]]:
    tmp = do_stuff(tmp2[i[1]])
    if tmp != None: setattr(inst, i[0], tmp)

To quote the documentation:

This is the counterpart of getattr(). The arguments are an object, a string and an arbitrary value. The string may name an existing attribute or a new attribute. The function assigns the value to the attribute, provided the object allows it. For example, setattr(x, 'foobar', 123) is equivalent to x.foobar = 123.

Note that since None is a singleton in python, usually one tests for it using the is or is not operators:

for i in [["A", 0], ["B", 1], ["C", 2]]:
    tmp = do_stuff(tmp2[i[1]])
    if tmp is not None:
        setattr(inst, i[0], tmp)

and the PEP 8 python styleguide discourages compound statements; keep the if suite on it's own line, even if it is just one line.

edited Aug 25, 2012 at 14:37

answered Aug 25, 2012 at 14:30

Martijn Pieters

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2 Comments

user1624579 Over a year ago

Considering the styleguide comments regarding indentation: When writing example code, I try to keep it as short as possible. While the statements in PEP 8 are most certainly helpful in programs and projects, I don't think the guidelines hold for minimal example code.

Martijn Pieters Over a year ago

Well, in a 3 or a 4 line example isn't going to make much odds; I find the clear indentation more readable, and with example code you want to go for maximum readability. :-)

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