Python importing class attributes into method loca

问题:

I have been wondering for a while if there is easier way to assign class attributes to method local namespace. For example, in dosomething method, I explicitly make references to self.a and self.b:

class test:
    def __init__(self):
        self.a = 10
        self.b = 20

    def dosomething(self):
        a = self.a
        b = self.b
        return(a + b)

But sometimes I have a lot of variables (more than 10) and it gets messy to type and look at - I would have bunch of var = self.var statements at the beginning of a method.

Is there any way to do this more compact way? (I know updating local() is not a good idea)

Edit: Ideally, what I want is:

def dosomething(self):
    populate_local_namespace('a', 'b')
    return(a + b)

回答1:

Q. Is there any way to do this more compact way?

1. If the variables are read-only, it would be reasonably Pythonic to factor-out a multi-variable accessor method:

class Test:

    def __init__(self):
        self.a = 10
        self.b = 20
        self.c = 30

    def _read_vars(self):
        return self.a, self.b, self.c

    def dosomething(self):
        a, b, c = self._read_vars()
        return a + b * c

    def dosomethingelse(self):
        a, b, c = self._read_vars()
        return a - b * c

If the variables aren't read-only, it is best to stick with self.inst_var = value. That is the normal way to write Python code and is usually what most people expect.

---

2. Once in a while you will see people abbreviate self with a shorter variable name. It is used when the readability benefits of decluttering outweigh the readability cost of using a non-standard variable name:

def updatesomethings(s):
    s.a, s.b, s.c = s.a + s.c, s.b - s.a, s.c * s.b

---

3. Another way to handle a very large number instance variable is to store them in a mutable container for ease of packing and unpacking:

class Test:

    def __init__(self, a, b, c, d, e, f, g, h, i):
        self._vars = [a, b, c, d, e, f, g, h, i]

    def fancy_stuff(self):
        a, b, c, d, e, f, g, h, i = self._vars
        a += d * h - g
        b -= e * f - c
        g = a + b - i
        self._vars[:] = a, b, c, d, e, f, g, h, i

---

4. There is also a dictionary manipulation approach that would work, but it has a code smell that most Pythonistas would avoid:

def updatesomethings(self):
    a = 100
    b = 200
    c = 300
    vars(self).update(locals())
    del self.self

回答2:

You can easily solve this problem with a tradeoff, by storing the variables in a dictionary.

data = {}
copy_to_local_variables = ["a", "b", "c", "d"]
for var_name in copy_to_local_variables:
    data[var_name] = getattr(self, var_name)

(Though I am unable to understand why you need to copy class attributes to method local namespace)

回答3:

I found another way:

def dosomething(self):
    for key in ['a', 'b']:
       exec('{} = self.{}'.format(key, key))

    return(a + b)

I don't know if this is dangerous or not. Would be great if someone can comment on this.