Python program continuing to run after encountering an error - class

Question

I'm in the process of creating a program for my linear algebra class with vector and matrix classes, but I'm having trouble with stringifying my matrix class to print it. What's causing the problem is an if statement that adds a comma after an entry in the matrix if it's not the last entry in a row. What's curious about this is I've isolated the problem to the part of my program that assigns a variable to the index of the entry at hand, but when I added a line after that that printed that variable to try and figure out what was happening, running the program printed the variable AND THEN gave the error from the line before. Here's the code:
import copy
class vector:
def __init__(self, entries):
if type(entries) == list:
self.elements = []
self.dimensionality = len(entries)
for entry in entries:
self.elements.append(entry)
if type(entries) == vector:
self.elements = entries.elements
def __str__(self):
buff = "("
for e in self.elements:
buff += str(e)
if self.elements.index(e) < len(self.elements) - 1:
buff += ", "
buff += ")"
return buff
def __getitem__(self,index):
return self.elements[index]
def __len__(self):
return len(self.elements)
def __mul__(self, otherVector):
if self.dimensionality != otherVector.dimensionality:
raise RuntimeError("Cannot multiply vectors of different dimensions")
else:
product = 0
for e in self.elements:
product += e * otherVector.elements[self.elements.index(e)]
return product
def __eq__(self, otherVariable):
return size(self) == size(otherVariable)
def size(x):
return (x * x)**(1/2)
class matrix:
def __init__(self, entries):
for i in entries:
if len(entries[0]) != len(i):
raise RuntimeError("All rows of matrix must contain the same number of entries")
self.elements = []
for row in entries:
self.elements.append(vector(row))
def __str__(self):
buff = "("
for row in self.elements:
buff += str(row)
a = self.elements.index(row) #this is the line that prompts the error
b = len(self.elements) - 1
print (a) #but this line executes before the error cuts off the rest of the program
print(b)
print(a<b)
if a < b :
buff += ", "
buff += ")"
return buff
print(matrix([[1,2],[2,3]]))
and here's the error it gives me:
Traceback (most recent call last):
File "/Users/sebpole/Documents/vectors.py", line 127, in <module>
print(matrix([[1,2],[2,3]]))
File "/Users/sebpole/Documents/vectors.py", line 83, in __str__
a = self.elements.index(row)
File "/Users/sebpole/Documents/vectors.py", line 38, in __eq__
return size(self) == size(otherVariable)
NameError: name 'size' is not defined
I fixed that specific error by skipping a definition of the function 'size()' and just writing it in to the definition of vector equality. Since it was short that wasn't a problem and the program runs fine after that tweak, but I have the following two conceptual questions:
1) What's going on with the line after the error executing before the error did?
2)What was the problem exactly? Why did the program have a problem with calling a function I defined a little later? Why did taking the index of a row of a matrix call the definition of equality for that row (a vector)?

Answer 1

Use self.save to refer function defined in your class

Answer 2

The error is coming from the call to print in line 127, and the entire line is not being executed. Did you really see printed output in the console, other than the stack trace?
Conceptually, the line in question, print(matrix([[1,2],[2,3]])) does this:
the matrix instance is created successfully
print calls __str__ on that matrix instance
__str__ calls index on the list of vector instances
index needs to look through the list to find a matching value, and calls __eq__ on each member of the list to find the match
your original __eq__ code calls a missing function named size (which you noticed, and fixed)
I am surprised that this produced any output, other than the error.