Generating a List Class in Python

Question

I am having some problems generating a list for a class in Python. I know there is something simple I'm overlooking, but I just can't figure it out.

My basic code so far:

class Test:
    def  __init__(self,test):
        self.__test = test

My problem is that if I enter

t = Test([1,3,5])

things will work just fine, but if I add

t = Test()

I get an error that I didn't enter enough parameters.

I've tried adding

def __init__(self,test=[])

as a default parameter, which sort of works, but then I don't have unique lists.

I've been looking all over and I can't quite figure out what I'm doing wrong. Any help would be greatly appreciated.

Since you use a double-leading underscore name for your attribute, it's probably worth pointing out stackoverflow.com/questions/165883/… — Thomas Wouters
– Thomas Wouters, Commented Apr 20, 2010 at 14:46

Fred Larson · Accepted Answer · 2010-04-20 14:53:42Z

4

I'm not exactly sure what you're looking for, but you probably want to use None as a default:

class Test:
    def  __init__(self,test=None):
        if test is None:
            self.__test = []
        else:
            self.__test = test

edited Apr 20, 2010 at 14:53

answered Apr 20, 2010 at 14:37

Fred Larson

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3 Comments

Thomas Wouters Over a year ago

Your two branches are a little lopsided. Either assign to self.__test in the is None branch as well, or just do the assignment after the if without an else.

Jef Over a year ago

This is where I was having trouble, I tried something similar, but wasn't using 'None' as the default for test. This fixed my program right up. Thanks a ton to everyone for the help and extra reading!

Xavier Ho Over a year ago

FYI: Personally, I prefer Thomas Wouters' answer.

SilentGhost · Accepted Answer · 2010-04-20 14:37:34Z

3

You could use the following idiom:

class Test:
    def  __init__(self,test=None):
        self.__test = test if test is not None else []

answered Apr 20, 2010 at 14:37

SilentGhost

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5 Comments

Fred Larson Over a year ago

@Thomas Wouters: I figured some people might feel that way. That's why I'm leaving my answer as-is.

Thomas Wouters Over a year ago

I'm not sure what's unclear about "yuck, ternary expression". I don't like the use of the ternary expression there. (Or, in fact, anywhere.)

SilentGhost Over a year ago

@Thomas, so you didn't use it in your answer. I find it entirely appropriate and readable.

Fred Larson Over a year ago

I agree that it's perfectly readable, and I think it's a poor reason for a down vote.

Thomas Wouters Over a year ago

Readability of code is important to me, so I did downvote -- I find the other answer (that isn't mine) to be better. Fortunately, differences of opinion are why we have sites like stackoverflow.

Thomas Wouters · Accepted Answer · 2010-04-20 14:38:03Z

3

Default arguments are evaluated once, when the function is defined, so when you do:

def __init__(self, test=[]):

the 'test' list is shared between all calls to __init__ that don't specify the test argument. What you want is commonly expressed so:

def __init__(self, test=None):
    if test is None:
        test = []

This creates a new list for each invocation where test is not passed an argument (or when it is passed None, obviously.)

answered Apr 20, 2010 at 14:38

Thomas Wouters

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1 Comment

Jef Over a year ago

Thanks for the explanation, I knew that's what was happening, I just didn't know the proper way to alleviate that problem. I appreciate it greatly!

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