Invalid syntax trying to create inline python object

Question

I'm trying to create a small inline object that uses a lambda function but I keep getting a SyntaxError for this line. The syntax as far as I can tell is correct

  mock_popen.return_value = type('obj', (object,), {'communicate' : lambda :'hello','world'})

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UPDATE:

In case anyone else is trying to create a module object like this ... don't . The code below is a much cleaner and more elegant way of solving the problem.

    mock_popen = MagicMock()
    mock_popen.return_value = mock_popen
    mock_popen.communicate.return_value = ('hello','world')

Answer 1

The problem is the two colons in the last expression; the Python parser doesn't parse the lambda colon followed by the comma-separated list. There's an visual ambiguity about whether that's a parameter-list comma or a dictionary comma.

Put the two parameters in parentheses, and you should be okay:

{'communicate' : lambda :('hello','world')}

Output (changing your assignment, since the ID isn't defined):

>>> type('obj', (object,), {'communicate' : lambda :('hello','world')})
<class '__main__.obj'>

Answer 2

Not quite.

>>> lambda: 'hello', 'world'
(<function <lambda> at 0x7fc13d86e758>, 'world')
>>> lambda: ('hello', 'world')
<function <lambda> at 0x7fc13d86e7d0>

Note the subtle difference in both the input and output.