Creating objects in a loop Python -- puzzling behavior

Question

I have looked at solutions of questions (here, here and here) that look similar but aren't. I still can't make sense of what is happening here.

class Page:
    def __init__(self, l = []):
        self.lines = l

    def __repr__(self):
        return str(self.lines)

class Line:
    def __init__(self, string=None):
        self.str = string

    def __repr__(self):
        return str(self.str)


if __name__ == '__main__':
    data = [[1, 2, 3], [4, 5, 6]]
    pages = []
    for row in data:
        page = Page()
        #print(page)
        #print(id(page))
        for x in row:
            line = Line(str(x))
            page.lines.append(line)
        pages.append(page)
print('Pages: ', pages)

The answer I expect is

Pages: [1, 2, 3], [4, 5, 6]

What I get instead is

Pages: [[1, 2, 3, 4, 5, 6], [1, 2, 3, 4, 5, 6]]

I printed the page variable and see it is already populated when the outermost loop is in the second iteration. But how? Shouldn't I get a new empty object?

I am not looking for ways to fix the problem or getting my expected output, I know a few ways. I want to understand why I get this output.

Thanks!

score 2 · Accepted Answer · 2020-06-16 19:51:51Z

2

def __init__(self, l = []): kinda create global default value of l, which you change later (page.lines refer to this global array, it's not recreating on each call).

Slightly better implementation:

class Page:
    def __init__(self, l = None):
        self.lines = l if l else []

edited Jun 16, 2020 at 19:51

answered Jun 16, 2020 at 19:46

user3797387

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1 Comment

Aditya Bhagwat Over a year ago

Ahh! So self.lines is simply stores the reference to the global list l and creating an object of Page changes this global list each time.

Noctis Skytower · Accepted Answer · 2020-06-16 20:32:45Z

May I suggest something like the following instead?

def main():
    data = [[1, 2, 3], [4, 5, 6]]
    pages = []
    for row in data:
        page = Page()
        print(page)
        print(id(page))
        for x in row:
            line = Line(str(x))
            page.append(line)
        pages.append(page)
    print('Pages:', pages)


class Page:
    def __init__(self, lines=None):
        if lines is None:
            lines = []
        if not isinstance(lines, list):
            raise TypeError('argument must by of type list')
        if not all(isinstance(item, Line) for item in lines):
            raise TypeError('list must only contain items of type Line')
        self.__lines = lines

    def __repr__(self):
        return f'{type(self).__name__!s}({self.__lines!r})'

    def append(self, line):
        if not isinstance(line, Line):
            raise TypeError('argument must be of type Line')
        self.__lines.append(line)


class Line:
    def __init__(self, text=None):
        if text is None:
            text = ''
        if not isinstance(text, str):
            raise TypeError('argument must be of type str')
        self.__text = text

    def __repr__(self):
        return f'{type(self).__name__!s}({self.__text!r})'


if __name__ == '__main__':
    main()

The problem as Green_Wizard points out is that your initializer for your Page class is sharing the list between all instances it creates. One way you could fix that is by requiring the caller of the class to always pass in a list for storing your pages. Otherwise, you could do something similar to what is shown in the code up above.

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