Question

I am learning OOP python using the Markana tutorial and I’m having some trouble with Lab 16.8. (Among some other troubles) I can’t get the __iter__ method correct. My method looks like this:

def __iter__(self):
        for e in self.l_investments:
            yield e

Here is my code (and the result of running it): http://hastebin.com/wayuwakode.py

Here is the result that should appear:

>>> 'GOOG' in p
True

I get:

>>> 'GOOG' in p
False

p is an instance of the class Portfolio(). ‘GOOG’ is the .name property of an instance of the Investment() object. p contains a list and a dictionary of Investment() instances.

I can change my __iter__ method to yield the names:

def __iter__(self):
    for e in self.l_investments:
        yield e.name

This fixes it so that 'GOOG' in p == True, but it breaks another requirement:

Failed example:
    for stock in p:
        print stock
Expected:
    1000 shares of APPL worth 252730.00
    5000 shares of CSCO worth 118700.00
    500 shares of GOOG worth 245670.00
    2000 shares of MSFT worth 50880.00
Got:
    APPL
    CSCO
    GOOG
    MSFT

How do I change the __iter__ method (or any other part of the code) so that both of these requirements are met?

I more interested in learning how OOP works than solving this particular problem. Any help would be much appreciated!

Answer 1

To add more detail to BrenBarn’s answer — yes, you want to override __contains__. The x in object membership test only uses __iter__ if __contains__ isn’t defined.

It seems like you’ll want to make __contains__ work for both Investment instances and Investment name strings, since I’d assume you want both 'GOOG' in p and investment_object_with_GOOG_name in p to return True. If that’s the case, the code should look something like this:

def ___contains___(self, item):
    # checks investment name strings by comparing to dictionary keys
    if item in self.d_investments:
        return True
    # checks investment objects by comparing to dictionary values
    if item in self.d_investments.values():
        return True

Note that the Investment objects need to implement sensible comparison for this to work. If they don’t (which it looks like they don’t — object comparison uses IDs and not attribute contents), either give them a comparison operator, or replace the second half of the above with this version to just do the comparison on the name:

    # checks investment objects by comparing to dictionary keys
    if isinstance(item, Investment) and item.name in self.d_investments:
        return True

Of course, if you want the membership test to only work for name strings, just remove the second half altogether.

Also, after looking at your code — is there any reason why you keep a list and a dictionary of investments in Portfolio? Either should be sufficient as an internal representation. I used the dictionary for my code above, but it would be easy to re-implement with a list — I think you should just pick one, unless you really need both for performance or other reasons that I haven’t figured out from the code.