Check if an object (with certain properties values) not in list

Question

I am new in Python. I am using Python v2.7.

I have defined a simple class Product:

class Product: def __init__(self, price, height, width): self.price = price self.height = height self.width = width

Then, I created a list, which is then appended with a Product object:

# empty list prod_list = [] # append a product to the list, all properties have value 3 prod1 = Product(3,3,3) prod_list.append(prod1)

Then, I created another Product object which is set the same initialize values (all 3):

prod2 = Product(3,3,3)

Then, I want to check if prod_list doesn't contain a Product object that has price=3, width=3 & height=3, by:

if prod2 not in prod_list: print("no product in list has price=3, width=3 & height=3")

I expect there is no print out message, but it is printed out. In Python, how can I check if a list doesn't have an object with certain property values then?

Kasravnd · Accepted Answer · 2016-02-29 08:46:12Z

You need to add an equality attribute to your object. For getting the objects attributes you can pass the attribute names to operator.attrgetter which returns a tuple of gotten attributes then you can compare the tuples. Also you can use __dict__ attribute which will gives you the module’s namespace as a dictionary object. Then you can get the attributes names which you want to compare the objects based on them.

from operator import attrgetter class Product: def __init__(self, price, height, width): self.price = price self.height = height self.width = width def __eq__(self, val): attrs = ('width', 'price', 'height') return attrgetter(*attrs)(self) == attrgetter(*attrs)(val) def __ne__(self, val): attrs = ('width', 'price', 'height') return attrgetter(*attrs)(self) != attrgetter(*attrs)(val)

Edit:

As @Ashwini mentioned in comment based on python wiki:

There are no implied relationships among the comparison operators. The truth of x==y does not imply that x!=y is false. Accordingly, when defining __eq__(), one should also define __ne__() so that the operators will behave as expected.

So as a more comprehensive way I also added the __ne__ attribute to the object. Which will return True if one of the attributes is not equal to it's relative one in other object.

Demo:

prod_list = [] prod1 = Product(3, 3, 3) prod_list.append(prod1) prod2 = Product(3, 3, 2) prod_list.append(prod2) prod3 = Product(3, 3, 3) print prod3 in prod_list True prod3 = Product(3, 3, 5) print prod3 in prod_list False

Note: The truth of x==y does not imply that x!=y is false. Accordingly, when defining __eq__(), one should also define __ne__() so that the operators will behave as expected.
@AshwiniChaudhary, thanks for the great comment, but could you please show me how to define __ne__() ? I have no idea about it though I have read your link.
@Kasramvd , thanks! Though I have accpeted your answer, but could you please also explain a bit what does the __ne__() part code do?
@Kasramvd , thanks! Looks like there is a typo in __ne__() code, you missed the . between attrs and getter , besides you should change attr to attrs

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