(Iron)Python inheritance constructor issue

Question

So I'm trying to make a subclass of DataGridViewColumn, which has both a no-argument constructor, and a constructor that takes one argument, which expects type DataGridViewCell. This is my class:

class TableColumn(DataGridViewColumn):
    def __init__(self, string):
        super(TableColumn, self)
        self.Text = string
        self.CellTemplate = DataGridViewTextBoxCell()
        self.ReadOnly = True

Whenever I try to pass in a string as the argument like such:

foo = TableColumn('Name')

It always gives me this:

TypeError: expected DataGridViewCell, got str

So it seems that it's always passing 'string' to the single-argument constructor of the superclass. I've tried replacing super(TableColumn, self) with super(TableColumn,self).__init__() to be explicitly sure that I want to call the no-argument constructor, but nothing seems to work.

I'm not super familiar with IronPython, but the __init__() seems like it should be fine. Maybe the whole traceback would be helpful. — mayhewr
– mayhewr, Commented Jun 14, 2012 at 22:25
The only thing is the traceback doesn't show me what happens past calling the constructor, so there's no useful information. — jtcramer
– jtcramer, Commented Jun 14, 2012 at 23:04
Hmm so it seems that it's an error with Python 2.7 (which the latest version of IronPython uses), which doesn't allow this. — jtcramer
– jtcramer, Commented Jun 14, 2012 at 23:28
Python 2.7 definitely allows this. Unless there's something weird about the parent class. But did you mean to call super(TableColumn, self) rather than, say, super(TableColumn, self).__init__()? — abarnert
– abarnert, Commented Jun 15, 2012 at 0:08
Also, is that TypeError being raised by TableColumn's init method, or from somewhere else? — abarnert
– abarnert, Commented Jun 15, 2012 at 0:09

Jeff Hardy · Accepted Answer · 2012-06-15 15:25:43Z

1

You actually don't want to implement __init__ when deriving from a .NET class; you need to implement __new__ instead.

class TableColumn(DataGridViewColumn):
    def __new__(cls, string):
        DataGridViewColumn.__new__(cls)
        self.Text = string
        self.CellTemplate = DataGridViewTextBoxCell()
        self.ReadOnly = True

Basically, the constructors of .NET base classes need to be called before the Python subclass' __init__ (but after __new__), which is why you were getting the wrong DataGridViewColumn constructor called.

answered Jun 15, 2012 at 15:25

Jeff Hardy

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