Clash Royale CLAN TAG

Better method to check all objects' parameters with a single call

I want to get all objects which age is less than 18. How can easily do that?
Should I had to pass all objects to get_nonage to get it?

class People:
def __init__(self, uid, age):
self.uid = uid
self.age = age

def get_nonage(self):
# print nonage here which age<18
# expected output: {1: 11, 3: 15}
pass

p1 = People(1, 11)
p2 = People(2, 20)
p3 = People(3, 15)


...

People.get_nonage()

People.get_nonage()

get_nonage

People

Person

people

[p for p in people if p.age < 18]

3 Answers
3

In order to search all of the People objects, you have to have all of the People objects.

People

People

But that implies you shouldn't have them in a bunch of separate variables in the first place, but in some collection, like a list.

Then, get_nonage can take that list.

get_nonage

While we're at it, there's no reason for get_nonage to be a method. It's not something that a People instance does, it's something you do to a bunch of People instances.

get_nonage

People

People

So:

class People:
# ...

def get_nonage(people):
nonage = {person.uid: person.age for person in people if person.age<18}
print(nonage)

people = [
People(1, 11),
People(2, 20),
People(3, 35)
]

get_nonage(people)


In a comment, you ask:

Can I just know the class is People, then use some method of People to directly get all data of its objects

What method would that be? You haven't written one. A class normally doesn't know all of its instances.

Of course if you really want it to, you can make a class record its instances in a class attribute. (A normal instance, like age, has a separate value for each instance. A class attribute has a single value shared by all instances.)

age

For example:

class People:
all_people =

def __init__(self, uid, age):
self.uid = uid
self.age = age
self.all_people.append(self)


If you do that, then you can make get_nonage into a class method—that is, a method meant to be called on the class itself, rather than on an instance. A class method gets the class, cls, as its first parameter, instead of an instance, self, and it can only access class attributes, like the all_people we just created above.

get_nonage

cls

self

all_people

@classmethod
def get_nonage(cls):
nonage = {person.uid: person.age for person in cls.all_people if person.age<18}
print(nonage)


And now, you can call it, without having to pass it anything, because it already knows everything:

people = [
People(1, 11),
People(2, 20),
People(3, 35)
]

People.get_nonage()


However, this is usually not a great design. For example, what if you wanted to delete a person? Previously, you'd just remove them from the list (or, in your original version, reassign p3 to something else). But then People.all_people won't know you did that; that person will still be there, and still be counted, even though you no longer have any way to access them.

p3

People.all_people

Try next:

class People:
def __init__(self, uid, age):
self.uid = uid
self.age = age

@classmethod
def get_nonage(cls, context):
print({obj.uid: obj.age for (obj_name, obj) in context.items() if type(obj) == cls and obj.age < 18})

p1 = People(1, 11)
p2 = People(2, 20)
p3 = People(3, 15)
People.get_nonage(globals())


Output:

{1: 11, 3: 15}


You can use global variable

non_age = {}


class People:
def __init__(self, uid, age):
self.uid = uid
self.age = age

if age < 18:
global non_age
non_age[uid] = age


if __name__ == '__main__':
p1 = People(1, 11)
p2 = People(2, 20)
p3 = People(3, 15)
print(non_age)


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