Creating Behave (Cucumber for Python) Matrix assignments

Update - for now, I've just opted to change the syntax of the .feature files, so the Scenario Gherkin text becomes:
Scenario: Constructing and inspecting a 4x4 matrix
Given the following 4x4 matrix M: [[1, 2, 3, 4], [5.5, 6.5, 7.5, 8.5], [9, 10, 11, 12], [13.5, 14.5, 15.5, 16.5]]
Then M[0,0] = 1
And M[0,3] = 4
And M[1,0] = 5.5
And M[1,2] = 7.5
And M[2,2] = 11
And M[3,0] = 13.5
And M[3,2] = 15.5

I'm then just parsing the string with eval() in the matrix_steps.py file:

from behave import *
from hamcrest import assert_that, equal_to
from parse_type import TypeBuilder
import numpy as np


# create the "type" representing the various matrix names
valid_test_matrices = ["M", "A", "B", "C", "inv", "half_quarter"]
parse_test_matrix = TypeBuilder.make_choice(valid_test_matrices)
register_type(TestMatrix=parse_test_matrix)


# "matrix" is left as an untyped string to be parsed by "eval()"
@given("the following {height:g}x{width:g} matrix {item:TestMatrix}: {matrix}")
def step_impl_matrix(context, height, width, item, matrix):
try:
if (context.dict is None):
context.dict = {}
except:
context.dict = {}
new_matrix_string = "np.array(" + matrix + ", dtype=float)"
new_matrix = eval(new_matrix_string)
assert (new_matrix.shape == (height, width))
context.dict[item] = new_matrix