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Python: how to utilize instances of a class

New to OOP and python, I am struggling enormously to grasp what good classes actually are for. I tried to ask help from a lecturer who said "oh, then you should read about general methods to classes". Been putting in a days work but get no where.
I get it that a class allow you to collect an instance structure and methods to it, like this:
class Items:
def __init__(self, item_id, item_name):
self.item_id = item_id
self.item_name = item_name
def show_list(self):
print(self.item_id, self.item_name)
idA = Items("idA", "A")
idA.show_list()
But what is even the point of a class if there were not MANY instances you would classify? If I have a method within the class, I must hard code the actual instance to call the class for. What if you want a user to search and select an instance, to then do operations to (e.g. print, compute or whatever)??
I thought of doing it like this:
class Items:
def __init__(self, item_id, item_name):
self.item_id = item_id
self.item_name = item_name
def show_list(self):
print(self.item_id, self.item_name)
idA = Items("idA", "A")
idB = Items("idB", "B")
select_item = input("enter item id")
select_item.show_list()
Replacing hard coded variable with input variable doesn't work, probably logically. I then played with the idea of doing it like this:
class Items:
def __init__(self, item_id, item_name):
self.item_id = item_id
self.item_name = item_name
iL = [Items('idA', 'A'), Items('idB', 'B')]
selected_item = input("enter item id")
for selected_item in iL:
print(f'{selected_item.item_id} {selected_item.item_name}')
Now all are called thanks to making it a list instead of separate instances, but how do I actually apply code to filter and only use one instance in the list (dynamically, based on input)?
I would love the one who brought me sense to classes. You guys who work interactively with large data sets must do something what I today believe exist in another dimension.
See examples above^^

It seems you want to find all the instances of a certain element within a class.
This is as simple as:
print([x for x in iL if x.item_id == selected_item])
Now, you may ask why you can't just store the elements of iL as tuples instead of classes. The answer is, you can, but
("idA", "A")
is much less descriptive than:
item_id = "idA"
item_name = "A"
Any code you write with classes, you should in theory be able to write without classes. Classes are for the benefit of the coder, not the end-user of the program. They serve to make the program more readable, which I'm sure you'll find is a desirable property.

Your point here is to lookup for Items instances based on their item_id attribute.
That's a thing to create instances of a class.
It's a completely different thing to search for items objects stored in memory - that is not directly linked to the concept of OOP, classes and instances.
You could use dictionary to store references of your objects and then lookup in your dictionary.
class Items:
def __init__(self, item_id, item_name):
self.item_id = item_id
self.item_name = item_name
def show_list(self):
print(self.item_id, self.item_name)
idA = Items("idA", "A")
idB = Items("idB", "B")
lookup_dict = {"idA": idA, "idB": idB}
select_item = input("enter item id")
found_item = lookup_dict.get(select_item)
if found_item:
found_item.show_list()
else:
print(f"item {select_item} not found")

Classes, self, input variables in self and self default values

I wrote variables in init's brackets, which should be default if I don’t enter anything, but I get an error if I don’t enter anything and python don’t see the default values ​​​​for the variables. I am using there input to understand the user what to enter and I would like to leave it.
class News:
def __init__(self, content='Test NEWS name', city='None', news_date_and_time='Not defined'):
self.content = str(input('Write down news content:'))
self.city = str(input('Write down news CITY:'))
self.news_date_and_time = datetime.now().strftime("%d/%m/%Y %H:%M")
pub = News()
I can achieve the input by passing it to the function, as I wrote below, but I want the function use an input and default values ​​which are written
class News:
def __init__(self, content='Test NEWS name', city='None', news_date_and_time='Not defined'):
self.content = content
self.city = city
self.news_date_and_time = datetime.now().strftime("%d/%m/%Y %H:%M")
pub = News('dog ate potatoes','New York')
pub = News(str(input('Input the content:')), str(input('Input the city:')))
How can I implement in the function an input of the variables and the default values ​​are which are written in the function if the wasn't written anything?

but I want the function use an input and default values ​​which are written
This violates the Single Responsibility Principle. Typically we design classes to take arguments similar to how you show in the second code example. This allows the flexibility of reuse because the class doesn't care where the value comes from. In your current situation, you get the values from user input, but you can easily create News objects from database values instead, for example.
To implement default values, I suggest building a separate class or function that decides whether to get input or to use default values. This will make your code in line with the Seperation of Concerns principle.

This function will work the way you want it to work if you test the value of input() and assign the parameter if it's empty:
class News:
def __init__(self, content='Test NEWS name', city='None', news_date_and_time='Not defined'):
self.content = input('Write down news content:') or content
self.city = input('Write down news CITY:') or city
self.news_date_and_time = datetime.now().strftime("%d/%m/%Y %H:%M")
As noted, though, this is bad design, because now it's impossible to create a News object without having it stop your script to ask for input. You should instead do this outside of the constructor:
class News:
def __init__(self, content, city):
self.content = content
self.city = city
self.news_date_and_time = datetime.now().strftime("%d/%m/%Y %H:%M")
pub1 = News("Dog ate potatoes", "New York")
pub2 = News(input("Enter content: "), input("Enter city: "))
Note that there's no reason to have a news_date_and_time parameter since you always use datetime.now in your constructor.

how to make the method run once for specific record on odoo?

i'm trying to do a method with a button the add the total value of a Bill to the value of another object it working well but every time I press the button it adds the total value of the bill to the other object value, I need anything make scenes to prevent adding the bill value multiple times , here is the code
class acc_inv_prop(models.Model):
_inherit = 'account.invoice'
property_id = fields.Many2one(comodel_name="account.asset.asset", string="Property", required=False, )
def prop_up_value(self):
self.property_id.value += self.amount_total
pass
what I can do on the object account.asset.asset to check the current bill was added before and raise error and if the bill is a new one add it's value normally but also for one time only
any help will be appreciated

Besides adding something to the value of an account.asset.asset making me uncomfortable (Shouldn't you make a logic that determines the full value of the asset from the invoice?)
You would need an additional field to remember that you added the amount already:
class acc_inv_prop(models.Model):
_inherit = 'account.invoice'
property_id = fields.Many2one(comodel_name="account.asset.asset", string="Property", required=False, )
property_last_added_value = fields.Float(string='Last Value Added to Property', readonly=True, digits=0)
def prop_up_value(self):
if self.property_last_added_value != self.amount_total:
self.property_id.value -= self.property_last_added_value
self.property_id.value += self.amount_total
self.property_last_added_value = self.amount_total

Python issues with objects inside of dictionary? Cant get all data back out

Sorry this is the second post in two days.. I am pulling my hair out with this. I am attempting to take data from reddit and put it into an array in a way I can pull the data out later for tensorflow to parse it. Now the issue is my second object inside of the other object is not giving me whats inside it... "<main.Submission" why am I getting this back?
Goals of this post:
1: Why am I getting <main.Submission> and how should I be doing this.
File "C:/automation/git/tensorflow/untitled0.py", line 35, in <module>
submissions[sm.id].addSubSubmission(Submission.addComment(cmt.id, cmt.author.name, cmt.body))
TypeError: addComment() missing 1 required positional argument: 'body'
Sorry for the long winded and most likely basic questions. Going from powershell to python was not as straight forward as I thought..
Thanks
Cody
import praw
# sets log in data for session
reddit = praw.Reddit(client_id='bY',
client_secret='v9',
user_agent='android:com.example.myredditapp:'
'v1.2.3 (by /u/r)')
class Submission(object):
def __init__(self, id, title, author):
self.id = id
self.title = title
self.subSubmission = {}
self.author = author
def addComment(self, id, author, body):
self.id = id
self.author = author
self.body = body
def addSubSubmission(self,submission):
self.subSubmission[submission,id] = submission
def getSubSubmission(self,id):
return self.subSubmission[id]
submissions = {}
for sm in reddit.subreddit('redditdev').hot(limit=2):
# pulls the ID and makes that the head of each
submissions[sm.id] = Submission(sm.id, sm.title, sm.author.name)
mySubmission = reddit.submission(id=sm.id)
mySubmission.comments.replace_more(limit=0)
# Get all the comments and first post and list their id author and body(comment)
for cmt in mySubmission.comments.list():
submissions[sm.id].addSubSubmission(Submission.addComment(cmt.id, cmt.author.name, cmt.body))
# My trying to read what all there??!? ##
for key in submissions.keys():
value = submissions[key]
print(key, "=", value)
for key, value in submissions.items():
print(key, "=", value)
expecting to see:
{Title = test {comment.id = 1111 {Comment = 'blah', Author = 'Bob'}}
{comment.id = 1112 {Comment = 'blah2', Author = 'Bob2'}}
}

It is giving you back the entire Submission object - but then you're printing it. How should a submission object look on screen when printed? This is something you can define in the Submission class - check out the first answer in this post: Difference between __str__ and __repr__ in Python
To explain this further: python doesn't know how to represent a class on screen. Sure, the class has attributes that are strings, lists, dicts etc, but python knows how to print those. Your class you just created? What's important? What should be printed? python doesn't know this, and is smart enough not to make any assumptions.
If you add a __repr__ function to your class, python will call it and print whatever that function returns.

How to create dynamic methods with python?

For my project I need to dynamically create custom (Class) methods.
I found out it is not so easy in Python:
class UserFilter(django_filters.FilterSet):
'''
This filter is used in the API
'''
# legacy below, this has to be added dynamically
#is_field_type1 = MethodFilter(action='filter_field_type1')
#def filter_field_type1(self, queryset, value):
# return queryset.filter(related_field__field_type1=value)
class Meta:
model = get_user_model()
fields = []
But it is giving me errors (and headaches...). Is this even possible?
I try to make the code between #legacy dynamic
One option to do this I found was to create the class dynamically
def create_filter_dict():
new_dict = {}
for field in list_of_fields:
def func(queryset, value):
_filter = {'stableuser__'+field:value}
return queryset.filter(**_filter)
new_dict.update({'filter_'+field: func})
new_dict.update({'is_'+field: MethodFilter(action='filter_'+field)})
return new_dict
meta_model_dict = {'model': get_user_model(), 'fields':[]}
meta_type = type('Meta',(), meta_model_dict)
filter_dict = create_filter_dict()
filter_dict['Meta'] = meta_type
UserFilter = type('UserFilter', (django_filters.FilterSet,), filter_dict)
However, this is giving me
TypeError at /api/v2/users/
func() takes 2 positional arguments but 3 were given
Does anyone know how to solve this dilemma?

Exception Value: 'UserFilter' object has no attribute 'is_bound'
You are getting this error because the class methods you are generating, are not bound to any class. To bound them to the class, you need to use setattr()
Try this on a console:
class MyClass(object):
pass
#classmethod
def unbound(cls):
print "Now I'm bound to ", cls
print unbound
setattr(MyClass, "bound", unbound)
print MyClass.bound
print MyClass.bound()
Traceback:
UserFilter = type('Foo', (django_filters.FilterSet, ), create_filter_dict().update({'Meta':type('Meta',(), {'model':
get_user_model(), 'fields':[]} )})) TypeError: type() argument 3 must
be dict, not None
Now, this is failing because dict.update() doesn't return the same instance, returns None. That can be fixed easily
class_dict = create_filter_dict()
class_dict.update({'Meta':type('Meta',(), {'model': get_user_model(), 'fields':[]})}
UserFilter = type('Foo', (django_filters.FilterSet, ), class_dict))
However, just look how messy that code looks. I recommend to you to try to be
clearer with the code you write even if it requires to write a few extra lines. In the long run, the code will be easier to maintain for you and your team.
meta_model_dict = {'model': get_user_model(), 'fields':[]}
meta_type = type('Meta',(), meta_model_dict)
filter_dict = create_filter_dict()
filter_dict['Meta'] = meta_type
UserFilter = type('Foo', (django_filters.FilterSet,), filter_dict)
This code might not be perfect but it is more readable than the original line of code you posted:
UserFilter = type('Foo', (django_filters.FilterSet, ), create_filter_dict().update({'Meta':type('Meta',(), {'model': get_user_model(), 'fields':[]})}))
And removes a complication on an already kinda difficult concept to grasp.
You might want to learn about metaclasses. Maybe you can overwrite the new method of a class. I can recommend you 1 or 2 posts about that.
Another option is that maybe you are not adding the filters correctly or in a way django doesn't expect? That would explain why you get no errors but none of your functions gets called.

You can use classmethod. Here is example how you can use it:
class UserFilter:
#classmethod
def filter_field(cls, queryset, value, field = None):
# do somthing
return "{0} ==> {1} {2}".format(field, queryset, value)
#classmethod
def init(cls,list_of_fields ):
for field in list_of_fields:
ff = lambda cls, queryset, value, field=field: cls.filter_field(queryset, value, field )
setattr(cls, 'filter_'+field, classmethod( ff ))
UserFilter.init( ['a','b'] )
print(UserFilter.filter_a(1,2)) # a ==> 1 2
print(UserFilter.filter_b(3,4)) # b ==> 3 4

You are asking for:
custom (Class) methods.
So we take an existing class and derive a subclass where you can add new methods or overwrite the methods of the original existing class (look into the code of the original class for the methods you need) like this:
from universe import World
class NewEarth(World.Earth):
def newDirectionUpsideDown(self,direction):
self.rotationDirection = direction
All the other Methods and features of World.Earth apply to NewEarth only you can now change the direction to make the world turn your new way.
To overwrite an existing method of a class is as as easy as this.
class NewEarth(World.Earth):
def leIitRain(self,amount): # let's assume leIitRain() is a standard-function of our world
return self.asteroidStorm(amount) #let's assume this is possible Method of World.Earth
So if someone likes a cool shower on earth he/she/it or whatever makes room for new development on the toy marble the burning way.
So have fun in your way learning python - and don't start with complicated things.
If I got you completely wrong - you might explain your problem in more detail - so more wise people than me can share their wisdom.