217

I split up my class constructor by letting it call multiple functions, like this:

class Wizard:
    def __init__(self, argv):
        self.parse_arguments(argv)
        self.wave_wand() # declaration omitted

    def parse_arguments(self, argv):
        if self.has_correct_argument_count(argv):
            self.name = argv[0]
            self.magic_ability = argv[1]
        else:
            raise InvalidArgumentsException() # declaration omitted

# ... irrelevant functions omitted

While my interpreter happily runs my code, Pylint has a complaint:

Instance attribute attribute_name defined outside __init__

A cursory Google search is currently fruitless. Keeping all constructor logic in __init__ seems unorganized, and turning off the Pylint warning also seems hack-ish.

What is a/the Pythonic way to resolve this problem?

Steven Liao
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    The warning just says what it says. I thinks it violates the [POLS](http://en.wikipedia.org/wiki/Principle_of_least_astonishment) if you initialize instance variables de-facto outside the constructor. Try to inline `parse_arguments` or use the return values of the function in `__init__` to initialize the variables and pylint will be happy, I guess. – miku Oct 10 '13 at 00:10

6 Answers6

238

The idea behind this message is for the sake of readability. We expect to find all the attributes an instance may have by reading its __init__ method.

You may still want to split initialization into other methods though. In such case, you can simply assign attributes to None (with a bit of documentation) in the __init__ then call the sub-initialization methods.

Sam R.
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sthenault
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    is there a reference to why "We expect to find all the attributes an instance may have by reading its __init__ method." It makes sense, but curious. – jouell Feb 19 '19 at 17:12
  • @MericOzcan either setting a bunch of attributes to None and then setting them later in a parse_args function OR returning a short tuple from parse_args is OK. ideally, parse_args should be testable without needing a wizard instance. – Erik Aronesty Sep 03 '19 at 18:26
  • Is this still relevant when the attributes are defined in a parent. It seems a big overhead for readability when super is called indicating members can be defined there? – Todd Cooper Mar 20 '20 at 01:51
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    But when I do that (initialize to None the attributes), mypy complains, any suggestions on how to fix both issues at the same time? – pablete Mar 20 '20 at 15:05
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    I would LOVE to keep all instance attributes inside of __init__, as this is what I'm used to with Java. However, I just discovered that when extending Python classes, the parent can see and operate on child instance variables EVEN when not passed to the parent through Super. This makes extending some classes very difficult, because the parent class will operate on the child specific class attributes in ways that are undesireable. This is why I have ignored this particular warning myself. I also like your idea about using `None` and initializing later! – rocksNwaves Jun 20 '20 at 18:45
40

Just return a tuple from parse_arguments() and unpack into attributes inside __init__ as needed.

Also, I would recommend that you use Exceptions in lieu of using exit(1). You get tracebacks, your code is reusable, etc.

class Wizard:
    def __init__(self, argv):
        self.name,self.magic_ability = self.parse_arguments(argv)

    def parse_arguments(self, argv):
        assert len(argv) == 2
        return argv[0],argv[1]
roippi
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    Since this is a relatively simple program and the main code will be constructing a single `Wizard` (the main class), I figured that exceptions were overkill. – Steven Liao Oct 10 '13 at 00:23
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    @StevenLiao that's all well and good, but it's still more readable, fewer lines of code, and develops good habits for the future. Up to you. – roippi Oct 10 '13 at 00:57
  • OK, but what if you only want to have those attributes added by calling parse_arguments to the object in special circumstances? In this case setting the attributes equal to None is a better solution. – Soldalma Dec 31 '16 at 17:37
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    also what if you have more than two members it gets uglier as that number grows – kroiz Apr 13 '20 at 05:37
4

The best practice to solve this question is you need to build the parameter in Init part first, Then adjust it in the Def

class MainApplication(tk.Frame):
    def __init__(self, master):
        self.master = master
        tk.Frame.__init__(self, self.master)
        self.settingsFrame = None
        self.create_widgets(master)

    def create_widgets(self, master):
        # frame Container
        self.settingsFrame = tk.Frame(self.master, width=500, height=30, bg='white')
oszkar
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W Kenny
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0

For each attribute you want to set via function, call the function from the init. For example, the following works for me to set the attribute ascii_txt...

def __init__(self, raw_file=None, fingerprint=None):
    self.raw_file = raw_file
    self.ascii_txt = self.convert_resume_to_ascii()

def convert_resume_to_ascii(self):
    ret_val = self.raw_file.upper()
    return ret_val
Ben
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0

Although the definition of instance variables outside init isn't recommended in general, there are rare cases in which it is natural. For example, when you have a parent class that defines several variables that its child classes won't use, and whose definition will make its child waste time or resources, or will be simply unaesthetic.

One possible solution to this is using an init-extention function, that each child class may override, and in this function use function setattr in order to define the class-unique instance variables. May be this is not too aesthetic as well, but it eliminates the here-discussed linting warning.

Ofer Elboher
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  • If child classes win't use some parent class members then this is OOP :). Or you could rewrite parent class constructor to get internal variable value as its argument. – Denis Barmenkov May 16 '22 at 08:42
-1

If you are using Python 3, you can try

class Wizard:
    def __init__(self, argv):
        self.name: str = str()
        self.magic_ability: str = str()
        self.parse_arguments(argv)
        self.wave_wand() # declaration omitted

    def parse_arguments(self, argv):
        if self.has_correct_argument_count(argv):
            self.name = argv[0]
            self.magic_ability = argv[1]
        else:
            raise InvalidArgumentsException() # declaration omitted

# ... irrelevant functions omitted

Although not as pythonic as the accepted answer, but it should get away the Pylint alert.

And if you don't concern about type and don't want to create a new object with object() use:

class Wizard:
    def __init__(self, argv):
        self.name = type(None)()
        # ...

As None will cause type not match error.

lowzhao
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    am using pyton3 and pycharm, and this solution has the merit to avoid another warning caused when you declare self.my_string:str = None => "Expected type 'str' got None instead. Am curious if str() takes more memory than None, and why this solution is less pythonic if it avoids an IDE warning... – Je Je Mar 07 '20 at 21:38
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    I checked onto the memory usage >>> import sys >>> print(sys.getsizeof(None)) 16 >>> print(sys.getsizeof(str())) 49 And the whether if my solution is pythonic I could not say. But I absolutely favour declaring variables on init as it makes the code easier to read. – lowzhao Mar 10 '20 at 07:27
  • why ` type(None)()` instead of `None`? – alper Feb 01 '21 at 16:39