Testing a Generator in Python

Question

I programmed a very simple generator in Python 3.4:

class Gen:
  def __init__(self, xml_lines, attribs):
    #...

  def _get_values(self, xml_line):
    # get only specific "attribs" from a line in a "xml"

  def values(self):
    for line in self.xml_lines:
      yield self._get_values(line)

The code works when I use a for loop to consume the generated values:

reader = Gen(...):
for v in reader.values():
   print(v)

But I'm now trying to create a unit test, and for that, I'd need to get each value at a time (outside a loop). I'm trying something like this:

import unittest
#...
reader = Gen(...):
v = reader.values()
self.assertIsNone(v)

When I try that, I always get a

AssertionError: <generator object next_values at 0x7fe2a5f9e3f0> is not None

So, when I call the values myself, it return something (is it a pointer?) instead of the generated value.

As I'm considering I'm using a basic generator pattern, my question is a little broader then my own code: what's the proper way to test a Python Generator?

Answer 1

A generator is meant to be iterated through. You can use the next() function to get the next value of the generator. Be aware that if your generator has exhausted it's values, it will raise a StopIteration exception.

reader = Gen(...):
values = reader.values()
v = next(values)
self.assertIsNone(v)