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?

• Martijn Pieters: You are passing in a list of two elements, not 3, to get that exception.

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.

• user3056783: Yes I've tried putting the code directly into interpreter. However how should I write the test so it passes?
• Martijn Pieters: @user3056783: You haven't shown us what sentence_list is in your test. It is not a list with enough elements.
• user3056783: I'm sorry I'm probably missing something here.. I'm still very new to programming, it's sentence_list = [('noun', 'I'), ('verb', 'kill'), ('noun', 'princess')] like I wrote before. My test file is something like this: from nose.tools import * import p sentence_list = [('noun', 'I'), ('verb', 'kill'), ('noun', 'princess')] def test_parse_subject_class(): test_obj = p.Sentence(*sentence_list) assert_is_instance(test_obj, p.Sentence) assert_equal(test_obj.subject, 'I') Where p is other module where the actual code is. I'm using nosetests to test it.
• Martijn Pieters: @user3056783: You'll want to print sentence_list then, because the only way you are getting your exception is if sentence_list contains two and not 3 elements. Add a print statement just before invoking Sentence(*sentence_list) (making sure you run nose -s so stdout is not being captured) and see what is being printed.
• Martijn Pieters: You have what is called a test isolation bug; the bug isn't there in isolation, but when you run multiple tests your test breaks. Don't reuse data between tests. ;-)