List of tuples unpacking does not work

Question

I am aware of * operator but this one does not seem to work. I basically want to unpack this list consisting of tuple pairs:

sentence_list = [('noun', 'I'), ('verb', 'kill'), ('noun', 'princess')]

Consider my class Sentence:

class Sentence(object):

    def __init__(self, subject, verb, object):
        self.subject = subject[1]
        self.verb = verb[1]
        self.object = object[1]

Now I create an object called test_obj and when I try to unpack sentence_list it does not seem to work:

test_obj = Sentence(*sentence_list)

When I test it with nose.tools using:

assert_is_instance(test_obj, Sentence)

I get this:

TypeError: __init__() takes exactly 4 arguments (3 given)

But when I change it to:

test_obj = Sentence(('noun', 'I'), ('verb', 'kill'), ('noun', 'princess'))

It passes the test. What am I doing wrong?

Answer 1

Your code works just fine, provided you actually pass in a list of 3 elements:

>>> class Sentence(object):
...     def __init__(self, subject, verb, object):
...         self.subject = subject[1]
...         self.verb = verb[1]
...         self.object = object[1]
... 
>>> sentence_list = [('noun', 'I'), ('verb', 'kill'), ('noun', 'princess')]
>>> Sentence(*sentence_list)
<__main__.Sentence object at 0x10043c0d0>
>>> del sentence_list[-1]
>>> Sentence(*sentence_list)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() takes exactly 4 arguments (3 given)

Note the error message here; __init__ takes four arguments, including self.

Ergo, it is your sentence_list value that is at fault here, not your technique.