Yet another "simple" headscratcher

[Josh English]

I am trying to whip up a quick matrix class that can handle multiplication.

Should be no problem, except when it fails.

--- Begin
#!/usr/bin/env python
# _*_ coding: utf-8

from operator import mul

class Matrix(object):
    """Matrix([data])
    Data should be a list of equal sized lists.
    Defaults to a 2d identity matrix
    """
    def __init__(self, data=None):
        if data is None:
            data = [[1,0], [0,1]]

        self.data = []
        if isinstance(data, (list, tuple)):
            ncols = len(data[0])
            for row in data:
                if len(row) != ncols:
                    raise ValueError("Rows are unequal lengths")
                self.data.append(row)
        self.size = (len(self.data), len(self.data[0]))

    def get_row(self, idx):
        if (0 <= idx < self.size[0]):
            return self.data[idx]
        else:
            raise ValueError("Bad row")

    def get_col(self, idx):
        if (0 <= idx < self.size[1]):
            return list(d[idx] for d in self.data)
        else:
            raise ValueError("Bad column index")

    def __mul__(self, other):
        if not isinstance(other, (Matrix,int, float)):
            raise ValueError("Cannot multiply by given value")
        if isinstance(other, (int, float)):
            res = []
            for row in self.data:
                res.append([d*other for d in row])
            return Matrix(res)
        # left with a matrix
        res = zero_matrix(self.size[0], other.size[1])
        for i in range(res.size[0]):
            for j in range(res.size[1]):
                print i, j, self.get_row(i), other.get_col(j),
                temp = map(mul, self.get_row(i), other.get_col(j))

                print temp,
                t = sum(temp)
                print t
                res.data[i][j] = t
                print res.data
        return res

    def as_string(self):
        # return a list of lines that look pretty
        stringed =[]
        for row in self.data:
            stringed.append(map(str, row))
        widths = []
        for col in range(self.size[1]):
            column = [s[col] for s in stringed]
            widths.append(max(map(len, column)))
        item_format = "{:>%s}"
        format_items = [item_format % w for w in widths]
        format_string = "  ".join(format_items)

        formatted = [format_string.format(*s) for s in stringed]
        return formatted

def zero_matrix(rows, cols):
    row = [0] * cols
    data = []
    for r in range(rows):
        data.append(row)

    return Matrix(data)

M = Matrix(([1, 0], [0, -1]))

N = M*4


print '\n'.join(M.as_string())
print '-'
print '\n'.join(N.as_string())


print '-'
S = N * M
print '\n'.join(S.as_string())
--- END

For some reason, my output from this is:

1   0
0  -1
-
4   0
0  -4
-
0 0 [4, 0] [1, 0] [4, 0] 4
[[4, 0], [4, 0]]
0 1 [4, 0] [0, -1] [0, 0] 0
[[4, 0], [4, 0]]
1 0 [0, -4] [1, 0] [0, 0] 0
[[0, 0], [0, 0]]
1 1 [0, -4] [0, -1] [0, 4] 4
[[0, 4], [0, 4]]
0  4
0  4
>>>

The print lines prove to me that the logic is working, but for some reason, assigning the sum to a particular item in a particular row is assigning the same row values to every row.

This should be one of those really simple Python things, but for the life of me I don't see it.

The first [[4, 0], [4, 0]] is clearly wrong. In each step, this algorithm is repeating the row.

Any ideas as to why this is happening?

Python 2.7.5, Windows 7, so nothing exotic.

Josh