在Python中将对象属性传递给函数的最优选方法是什么？

Question

I am certain I am doing this "incorrectly" even though it works. Right now when I call a function I just pass the whole object ie

class my_file_obj:
    def __init__(self,filename):
        self.filename = filename
        self.owner = None
        self.file_type = None
        self.fileflag = 0
        self.md5 = None

The function call, where file_obj1 is an instance of my_file_obj :

some_function(file_obj1)

and then referencing the attributes I need as needed within the function.

What is the "python"/correct way of doing this?

• some_function(file_obj1)

• some_function(file_obj1.filename)

• the_filename = file_obj1.filename
some_function(the_filename)

Answer 1

Well, it's quite obvious. It depends on whether some_function needs the whole object or just the filename, in which case you pass the whole my_file_obj instance or just the filename.

Answer 2

I think some_function(file_obj1) is the most pythonic.

some_function(file_obj1.filename) doesn't pass the object to the function, it only passes the filename attribute as a string. It then would require a lot of fiddly code to get the rest of the object's attributes.

Answer 3

All the ways are acceptable and which you will choose will depend on your application (or design) choices.

# with this call the file_obj1 object reference is sent to some_function()
some_function(file_obj1)

# with this call there is no pass by reference. If filename is a 
# string it is copied to some_function
some_function(file_obj1.filename)

# same as before, but here you are allocating a new var and copying the content
# to some_function
the_filename = file_obj1.filename
some_function(the_filename)

Answer 4

Or you may add a def __str__(self): return self.filename method to your class (because, maybe, printing a file object should return its name in your design anyways) and accept any kind of object for your function. The function then, for example, looks like this:

def openFile(obj):
    return open(str(obj), 'rU')

Like this, the function accepts both strings and objects of your class. See http://en.wikipedia.org/wiki/Duck_typing .

Not sure if this design is recommended -- just want to point out something less obvious. Maybe this is even pythonic?

Answer 5

Well, it depends on what you want to do and what your goal is.

If you have a function that looks something like this:

def some_function(file_thingy):
   with open(file_thingy.filename, 'w') as f:
       f.write("Icky Icky Icky Patang NeeeeWom!")

Then it makes things much more generic - as long as you pass an object in that has a .filename attribute that is a string then your function will work. A more common example of this is when people talk about duck typing. If it looks like a duck, walks like a duck, and quacks like a duck, well then it's a duck!

So if you have the following function:

def do_duck_things(a_duck):
    print(a_duck.appearance)
    a_duck.waddle()
    a_duck.quack()
    print("It must be a duck!")

Then you could pass it an instance of:

class Duck:
    def __init__(self):
        self.appearance = "White, like the AFLAC duck"

    def quack(self):
        print("Quaaaaaack!")

    def waddle(self):
        print("The duck waddles.")

or an instance of either of these classes:

class UglyDuckling:
    def __init__(self):
        self.appearance = "Suspiciously like a baby goose"

    def waddle(self):
        print("The ugly duckling waddles a bit like a duck.")

    def quack(self):
        print("Hoooonk!")

class Human:
    def __init__(self):
        self.appearance = "Looks like a human in a duck costume"

    def waddle(self):
        print("Surprisingly, he waddles quite like a duck.")

    def quack(self):
        print("<A sound quite like Donald Duck would make>")

So in your case, it really depends on what your function should be doing. If all it's doing is reading the contents of a file, then you can (and probably should) just send it a filename. But if you want to do something like, say, check the file against your stored MD5, or set that MD5, then it's perfectly appropriate to pass in the object.

HTH