Python-遍历传递给函数的参数

Question

Suppose I have the following example:

class foo:
   ...
   def bar(self, w, x, y, z, ...):
      self.w = w
      self.x = x
      self.y = y
      self.z = z
      ...

I wish to reduce the n-number of attribute assignment lines in bar() to one assignment line set using a setattr() cycle through the arguments. Is there a good way to cycle through said arguments for this purpose?

I wish to retain the defined parameter names so as to limit the number of parameters passed to the function as well as the order in which they are passed. I also understand that functions can be handled like objects ; so is it possible to obtain a list of the defined parameters as an attribute of the function and iterate through that?

Answer 1

Use locals() and you can get all the arguments (and any other local variables):

class foo:
    def bar(self, w, x, y, z):
        argdict = {arg: locals()[arg] for arg in ('w', 'x', 'y', 'z')}
        for key, value in argdict.iteritems():
            setattr(self, key, value)
        ...

Might be possible to do it more efficiently, and you could inline argdict if you prefer less lines to readability or find it more readable that way.

Answer 2

So you don't have to actually name the arguments explicitly use:

class foo:
    def __init__(self, w, x, y, z):
        args = locals()# gets a dictionary of all local parameters
        for argName in args:
            if argName!='self':
                setattr(self, argName, args[argName])

Answer 3

The __setattr__ attribute only assigns one attribute at a time, if you want to assign multiple attribute, you can use **kwargs in your function header and for limiting the number of arguments you can simply check the length of kwargs within your function. and call the __setattr__ for each each of the arguments one by one. One good reason for this recipe is that basically assigning attribute to an object without considering anything is not a correct and desirable job, due to a lot of reasons. Thus you have to assign each attribute one at a time by considering all the required conditions.

You can also do this manually by updating the instance dictionary but you should handle the exceptions too.

In [80]: class foo:                      
    def bar(self, **kwargs):
        if len(kwargs) != 4:
            raise Exception("Please enter 4 keyword argument")
        for k, v in kwargs.items():
            foo.__setattr__(self, k, v)
   ....:             

In [81]: f = foo()

In [82]: f.bar(w=1, x=2, y=3, z=4)

In [83]: f.w
Out[83]: 1

In [84]: f.bar(w=1, x=2, y=3, z=4, r=5)
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
<ipython-input-84-758f669d08e0> in <module>()
----> 1 f.bar(w=1, x=2, y=3, z=4, r=5)

<ipython-input-80-9e46a6a78787> in bar(self, **kwargs)
      2     def bar(self, **kwargs):
      3         if len(kwargs) != 4:
----> 4             raise Exception("Please enter 4 keyword argument")
      5         for k, v in kwargs.items():
      6             foo.__setattr__(self, k, v)

Exception: Please enter 4 keyword argument

By using __setatter__ it will take care of the exception automatically:

In [70]: f.bar(1, 2)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-70-07d1f3c9e27f> in <module>()
----> 1 f.bar(1, 2)

<ipython-input-65-1049e26120c1> in bar(self, *args)
      2     def bar(self, *args):
      3         for item in args:
----> 4             foo.__setattr__(self, item, item)
      5 

TypeError: attribute name must be string, not 'int'