Customizing unittest.mock.mock_open for iteration

Question

How should I customize unittest.mock.mock_open to handle this code?

file: impexpdemo.py
def import_register(register_fn):
    with open(register_fn) as f:
           


        

Answer 1

As of Python 3.6, the mocked file-like object returned by the unittest.mock_open method doesn't support iteration. This bug was reported in 2014 and it is still open as of 2017.

Thus code like this silently yields zero iterations:

f_open = unittest.mock.mock_open(read_data='foo\nbar\n')
f = f_open('blah')
for line in f:
  print(line)

You can work around this limitation via adding a method to the mocked object that returns a proper line iterator:

def mock_open(*args, **kargs):
  f_open = unittest.mock.mock_open(*args, **kargs)
  f_open.return_value.__iter__ = lambda self : iter(self.readline, '')
  return f_open