How to create class instance inside that class method?

Question

I want to create class instance inside itself. I tried to it by this way:

class matrix:
    (...)
    def det(self):
        (...)
        m = self(sz-1, sz-1)
        (...)
    (...)

but I got error:

m = self(sz-1, sz-1)
AttributeError: matrix instance has no __call__ method

So, I tried to do it by this way:

class matrix:
    (...)
    def det(self):
        (...)
        m = matrix(sz-1, sz-1)
        (...)
    (...)

and I got another error:

m = matrix(sz-1, sz-1)
NameError: global name 'matrix' is not defined

Of course matrix is not global class. I have no idea how to solve this problem.

As you have it right now, 'm' would be a local variable, not an instance variable, so it will disappear when the det() method returns. This might be appropriate for what you're doing, but when you say you want a "class instance inside itself" it sounds like you might want m to be an instance variable, in which case you need to refer to it as "self.m" — Tyler
– Tyler, Commented Jan 6, 2010 at 18:46
The last example works in my Python 2.6.4. I would not expect anything else… — Eric O. Lebigot
– Eric O. Lebigot, Commented Jan 6, 2010 at 22:32
the last example could work if matrix was global class, but it isn't. Maybe in python 2.6 it works, I don't know, I'm using 2.5.4. 'm' is local variable because I don't need it outside det() method - I calculate matrix determinant in recursion way. — pablo
– pablo, Commented Jan 9, 2010 at 15:56

Daniel Roseman · Accepted Answer · 2010-01-06 18:43:52Z

15

m = self.__class__(sz-1, sz-1)

or

m = type(self)(sz-1, sz-1)

answered Jan 6, 2010 at 18:43

Daniel Roseman

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4 Comments

Tyler Over a year ago

Okay, but type(self) is matrix, isn't it? So why doesn't pablo's second example work? Is it just a fact of life that you can't refer to a class from within itself in Python?

manifest Over a year ago

I'm deleting my hack of an answer in favor of this gem.

pablo Over a year ago

type(self)() doesn't work for me, but self.__class__() works properly and it's everything I needed. Thanks ;)

bukzor Over a year ago

@MatrixFrog: For "old-style" classes (python2 classes that don't inherit from object), type(self) is always just <type 'instance'>. The metaclass / new style classes was to enable users to make "real" types.

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