How to convert Textinput text to a matrix - python-2.7

Question

I am new to kivy/python. I wanted to test my programming skills, so I modified this code off a post. It take texts as numbers from the textinput and converts them into a matrix and performs an operation. It works perfectly for positive numbers, however I want to capture negative numbers as well. But that always gives an error.
from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.uix.textinput import TextInput
class Screen(GridLayout):
def __init__(self, **kwargs):
super(Screen, self).__init__(**kwargs)
self.input = TextInput(multiline=False, size_hint = (None, None))
self.add_widget(self.input)
self.input.bind(on_text_validate=self.print_input)
def print_input(self, value):
"""function to convert Textinput text into a matrix"""
ans1 = [i for i in list(value.text) if i != ',']
new = []
for i in range(len(ans1)):
if ans1[i] == "-":
n = ans1[i] + ans1[i + 1] # concatenate "-" and the next item after it
ans1.remove(ans1[i]) # delete "-"
ans1.remove(ans1[i+1]) # delete the next element after "-"
new.append(n)
new.append(ans1[i])
# the list new can now be converted into a matrix of which dimension depends on the list
class MyApp(App):
def build(self):
return Screen()
if __name__ == '__main__':
MyApp().run()
This is the error I keep getting:
if ans1[i] == "-":
IndexError: list index out of range

Answer 1

Hurray! I have seen the magic. The secret was string.split()
This is the modified code. Works perfectly for both positive and
negative entries.
from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.uix.textinput import TextInput
from sympy import *
class Screen(GridLayout):
def __init__(self, **kwargs):
super(Screen, self).__init__(**kwargs)
self.input = TextInput(multiline=False, size_hint = (None, None))
self.add_widget(self.input)
self.input.bind(on_text_validate=self.print_input)
def print_input(self, value):
"""function to convert Textinput text into a matrix. works
for only square matrices"""
m = 3; n = 3 # rows and columns
list = [float(el) for el in value.text.split(',')] # entries must be separated with ','
mat = Matrix(list)
mat1 = mat.reshape(3, 3) # can be made general
pprint(mat1)
class MyApp(App):
def build(self):
return Screen()
if __name__ == '__main__':
MyApp().run()