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How do getters and setters work in Python?

I am learning getters and setters , what I understand is that they are used so that no one could change the object's attributes directly. In the example
class Person:
def __init__(self, name, age):
self._name = name
self._age = age
def get_age(self):
return self._age
def set_age(self, new_age):
if isinstance(new_age, int) & new_age>0 & new_age<120:
self._age = new_age
def get_name(self):
return self._name
def __str__(self):
return 'Person[' + self._name + '] is ' + str(self._age)
p1 = Person("Sandeep", 49)
I created an object p1 where I set the age 49. As I have made a set_age function so I expect we can change the age of p1 through set_age only, not through the routine way. But it is not happening, I am able to change the age of p1 through , for example, p1._age = 35 as well. Then, what is the advantage to make set_age function, if I am still able to access the attributes directly?
I think, I am missing something, please help.

You need to tell python how to associate the getter and setter with the actual variable name. To do this you can use the builtin property function like so:
class Person
def __init__(self, name, age):
self._name = name
self._age = age
def get_age(self):
return self._age
def set_age(self, new_age):
if isinstance(new_age, int) & new_age>0 & new_age<120:
self._age = new_age
def get_name(self):
return self._name
name = property(get_name)
age = property(get_age, set_age)
def __str__(self):
return 'Person[' + self.name + '] is ' + str(self.age)
p1 = Person("Sandeep", 49)
Then instead of referring to _name and _age use name and age

The reason to use a getter and setter, is if you want to do something more complex than just set and attribute with foo.bar. In your case, set_age has an
isinstance(new_age, int) & new_age>0 & new_age<120
check, which is not possible to do with a raw attribute. (Side-note: you should use and instead of &.)
Yes, someone can still do p1._age = -1, and then their code won't work, but why would they? It just makes their code not work.
Your get_name function is less useful than the age one. It basically makes name read-only, which might or might not be useful.
When creating setters and getters in Python, it is usual to use the #property decorator. This means the functions can be called as if they were attributes, so instead of p1.get_name() you can just do p1.name. Similarly p1.set_age(3) becomes p1.age = 3.
You probably want to use the age setter in __init__, because then the age of the Person is validated when it is created.
Here is a version that makes these changes (and a couple of other readability improvements).
class Person:
def __init__(self, name, age):
self._name = name
self.age = age
#property
def age(self):
return self._age
#age.setter
def age(self, new_age):
if isinstance(new_age, int) and 0 < new_age < 120:
self._age = new_age
#property
def name(self):
return self._name
def __str__(self):
return f"Person[{self.name}] is {self.age}"
p1 = Person("Sandeep", 49)

what I understand is that they are used so that no one could change the object's attributes directly.
Your understanding is wrong. There is no magic rule that tells the python interpreter oh, this class has a setter, so direct access to the fields aren't allowed any more.
In other words: this is merely "works by convention". When you see a python class that has setter methods, then you know you should not access the fields directly, although you still can do that.
And note: python is mostly rather lenient on such things. A lot of things are "by convention" only. It is just "the nature" of python: you can do a lot of things that other languages, especially statically typed ones like Java do not allow.

Now I realise that I asked a silly question. The reason is obvious of using setters, creating functions for setting a value is a very convenient way to apply the constraints on it. In the example above, the constraint is that the age of the person should be positive and less than 120. Implementation of such constraints is not possible without setters.

Generally speaking you do not code getters and setters in python unless you specifically need them, right now. (But what if you need them later?? I'll explain that in a moment.)
Most of the time you should just define the instance variables, and then in the code that needs to use those variables, just access them directly, i.e.:
p1 = Person("Sandeep",49)
print("%s is %d years old." % (p1._name, p1._age))
# prints "Sandeep is 49 years old."
p1._age = 50
print("Now %s is %d years old." % (p1._name, p1._age))
# prints "Now Sandeep is 50 years old."
You only add getter/setter methods later, when you actually need them.
When you add the getter/setter methods later, you do not need to change any of the existing code that directly accesses the instance variables. Instead, use the #property decorator, shown in LeopardShark's answer above (https://stackoverflow.com/a/71080050/1813403).
The #property decorator does some magic code generation that makes your new getter/setter methods intercept when the existing code tries to directly access the instance variables, and send the request through your getter/setter methods instead. So the existing code will keep working but will call your new getter/setter methods instead of just messing directly with the attribute.

when to use getter and setter with property?

When should you use a property with getters/setters? It is not pythonic or wrong to not use a property with getters and setters? Should or shouldn't I write it with a property?
Examples:
class Person:
def __init__(self, firstname, lastname, age):
self.firstname = firstname
self.lastname = lastname
self.age = age
def say_hi(self):
print(f"""Hi i'm {self.firstname} {self.lastname} and i'm {self.age}""")
#property
def age(self):
return self._age
#age.setter
def age(self, newage):
if not isinstance(newage, int):
raise TypeError("Expect an Integer")
self._age = newage
versus
class Person2:
def __init__(self, firstname, lastname, age):
self.firstname = firstname
self.lastname = lastname
self.age = age
def say_hi(self):
print(f"""Hi i'm {self.firstname} {self.lastname} and i'm {self.age}""")
def get_age(self):
return self.age
def set_age(self, newage):
if not isinstance(newage, int):
raise TypeError("Expect an Integer")
self.age = newage

You should generally prefer to use "protected" variables (such as those starting with _) with properties (not separate functions that users need to call, that's just clunky), as it confers some advantages. This encapsulation is very handy as it:
lets you control the internal data completely, such as preventing people entering ages like -42 (which they will do if they can); and
lets you change the underlying implementation in any manner you want, without affecting clients.
For example on that last point, you may want to maintain a separate structure of all names and simply store references to those names in your Person class. This can allow you to store many more names, as the surname "Von Grimmelshausen" would be stored once (in the separate structure) and as much smaller indexes in all the Person objects that use it.
You can then totally change the naive getter from:
#property
def surname(self):
return self._surname
to:
#property
def surname(self):
return self._surname_db[self._surname_index]
without any changes to clients.

The pythonic way would be not to use setters and getters at all; just have an attribute:
class Person:
def __init__(self, firstname, lastname, age):
self.firstname = firstname
self.lastname = lastname
self.age = age
def say_hi(self):
print(f"Hi i'm {self.firstname} {self.lastname} and i'm {self.age}")
If you want to check types, use type annotations and a checker like mypy:
class Person:
def __init__(self, firstname, lastname, age):
self.firstname: str = firstname
self.lastname: str = lastname
self.age: int = age
def say_hi(self):
print(f"Hi i'm {self.firstname} {self.lastname} and i'm {self.age}")
If it later turns out that you do need to do something more complex, you can always turn it into a property later with no change of interface.

"Pythonic" is a holy struggle.
I personally prefer the Class under full control.
In your case:
class Person:
def __init__(self, firstname, lastname, age):
self.firstname = firstname
self.lastname = lastname
self.age = age
def say_hi(self):
print(f"Hi i'm {self.firstname} {self.lastname} and i'm {self.age}")
def test_str(self, cosi):
return self.test(cosi, str)
#staticmethod
def test(cosi, neco):
assert isinstance(cosi, neco), f"Bad value! {cosi} is not instance" \
f" from {neco.__name__}"
return cosi
#staticmethod
def test_positiv_int(num):
assert 0 < int(num), f"Expect an positiv integer" # negative value protect
return int(num) # if int is like string this returned int
def __setattr__(self, key, value):
# important!!!:
whitedict = dict(firstname=self.test_str,
lastname=self.test_str,
age=self.test_positiv_int
)
# Call fn from whitedict with parameter
self.__dict__[key] = whitedict[key](value)

The second version of your code (referring to class2) utilizes two instance methods i.e get_age and
set_age which are not serving much of a purpose because you can retrieve the age attribute of an instance without calling the get_age method, also you can set the age attribute to literally anything without invoking your set_age method. Also if you want user to retrieve or set the age attribute using your given instance methods, the user who was using your class previously will have to make changes in their code which is something we do not want.
Now, the first version of your code (referring to class1) is very helpful because you can pose restrictions on the age attribute by using property decorators. You can make the age attribute read only or both read and write and you can just retrieve or set the age attribute of an instance normally without having to call any methods.
Also, as you explicitly need to call the set_age method on an instance in second version of your code, for this
piece of logic :
if not isinstance(newage, int):
raise TypeError("Expect an Integer")
self._age = newage
to execute so the user cannot put any arbitrary value into the age attribute, on the other hand it happens implicitly whenever you try to set the age attribute when you use properties.

How can I make the OOP code print my classes objects name?

I've tried to make an OOP based program in python. I gave it an object to work with and tried to make it print the name, but its not working.
class human:
def __init__(self, name):
print("this is a human")
def name(self, name):
print("this is {}".format(bob.name))
bob = human("bob")
Anyone know what the problem could be?

Beyond the answers you already received (which solve your problem), I'd suggest not having a method that prints the name. Rather, you should have a __str___ dunder method that defines the object's behavior when an instance is printed.
class human:
def __init__(self, name):
self.name = name
def __str__(self):
return self.name
person = human("bob")
print(person)
'bob'
You can also define the object's behavior when the instance name is entered in the console, for instance just running the line
>>> person
You can do it with __repr__:
def __repr__(self):
return f'when entering the instance name in the console: {self.name}'
This will print:
when entering the instance name in the console: bob
This appears more pythonic to me than having a method that simply prints the name.

You're never storing the name on the instance, where would it get the name from? Your __init__ needs to do something along the lines of self.name = name
the name method and attribute are going to conflict, the latter will shadow (hide) the former, and it should look up whatever attribute its using on self

You never assigned the passed name to the object. Try:
class human:
def __init__(self, name):
print("this is a human")
self.name = name
def print_name(self):
print("this is {}".format(self.name))
bob = human("bob")
bob.print_name()

there are couple of things to update in the code:
bob is an instance which is not defined at human class
notice that init, name functions expect external param but you never use it in the function. (in self. = name)
in order to use it:
define a var in the class named 'name' and update you function to:
class human:
_name = ""
def __init__(self, name):
print("this is a human")
self._name = name
def name(self):
print("this is "+ self._name)
bob = human("bob")
bob.name()
bob = human("bob") only init function and you should call bob.name() in order to call the print-name function

Is this code correct

I have a question to ask, please. Given the code below, can you please let me know why in manager (or in the worker) class why
self.FirstName
gives the same result as
self._firstName
I would have thought that self._firstName would not be accessible in either of the classes (Manager/Worker) since it local to the Employee class and should not be accessible outside it, no ?
Please suggest.
import gc
class Employee(object):
"""Employee Base Class"""
def __init__(self, FirstName, LastName,Age, Role):
super(Employee, self).__init__()
self._firstName = FirstName
self._lastName = LastName
self._age = Age
self._role = Role
#property
def FirstName(self):
return self._firstName
#property
def Age(self):
return self._age
#property
def Role(self):
return self._role
#FirstName.setter
def FirstName(self, value):
self._firstName = value;
pass
#Role.setter
def Role(self, value):
self._role = value;
pass
class Manager(Employee):
"""Manager class"""
def __init__(self, FirstName,LastName,Age):
Employee.__init__(self,FirstName, LastName, Age, 'Manager')
# super(Manager, self).__init__()
def getParents(self):
"""Get parents of the class"""
print(gc.get_referrers(self))
pass
def ManagerInfo(self):
print("FirstName : " + self.FirstName)
print("Role : " + self.Role)
print("Age : " + str(self.Age))
class Worker(Employee):
"""docstring for Worker"""
def __init__(self, FirstName, LastName, Age):
Employee.__init__(self,FirstName, LastName, Age, 'employee')
def getParents(self):
"""Get parents of the class"""
print(gc.get_referrers(self))
pass
def WorkerInfo(self):
print("FirstName : " + self.FirstName)
print("Role : " + self.Role)
print("Age : " + str(self.Age))
pass
# manager = Employee('John','Doe' , 40, 'Manager')
# print("{0}'s age is {1} years.".format(manager.FirstName, manager.Age))
anEmp = Worker('WorkerName', 'LastName', 20)
aManager = Manager('John', 'Doe', 40)
print(anEmp.WorkerInfo())
print(anEmp.getParents())
print("----------------------------")
print(aManager.ManagerInfo())
print(aManager.getParents())
Thanks

why self.FirstName gives the same result as self._firstName
Because you defined FirstName as a property returning self._firstname. What did you expect actually ?
I would have thought that self._firstName would not be accessible in either of the classes (Manager/Worker) since it local to the Employee class
It's not 'local to the Employee class', it's an attribute of Employee instances (it doesn't exist in the Employee class itself).
and should not be accessible outside it, no ?
While prefixing a name with a single underscore denotes an implementation attribute (IOW something that is NOT part of the public API - the equivalent of 'protected' in most mainstream languages), it doesn't prevent access to the attribute. Actually there's absolutely NO enforcement of access restriction in Python, it's all convention (and eventually name mangling for __pseudoprivates names).
Python's philosophy is that we are all consenting adults and are wise enough to not do stupid things like messing with what is clearly labelled as an implementation attribute without accepting full responsability for breaking encapsulation.
can you please let me know what I should be doing in order to make sure that the user can only set the value using the setters and not by doing self._firstName
Nothing more than you already did actually. Re-read the above paragraphs, I already mentionned that Python did NOT enforced access restriction of any kind. self._firstname is prefixed with a single leading underscore, which is the way to tell "this is an implemention detail and not part of the API, you should not be messing with this attribute, you should not even know it exists, so if you break something by messing with it well too bad for you dude, but you're on your own".
so if in case, I have some arbitrary logic that manipulates the value in the setter before setting it, the updated value will not be available if the user just does self._firstName instead of self.FirstName
The chances this would happen are rather low actually (and that's an understatement) but theoritically yes this could happen. But this is totally unrelated since you'd have the very same problem if the user used self.FirstName instead since it would still return the stale value...

How should I handle Python list properties with respect to getters/setters?

It is my understanding that one aspect of the Pythonic way is to use direct member variable access of classes until 'getters/setters' are needed. Code is available on Ideone
class Person():
def __init__(self, name, age=None, friends=None):
self.name = name
self.age = age
if friends is None:
self.friends = []
else:
self.friends = friends
me = Person('Mr. Me')
you = Person('Ms. You')
me.age = 42
me.friends.append(you)
Because of the #property decorator approach this access to member variable age can be 'converted' to a 'getter/setter' interface in the future without rewriting the line that sets it to 42. But what about the last line where I add you to my friends list? Is it possible for the Person class to intercept the append() call and take other actions? Perhaps in the future I would decide to add the feature that you would get notified that they have been added to me's friends list.

Of course, once I ask the question my brain turns on and comes up with a solution. Let me know if this is good or not. Instead of intercepting the .append() call in Person, create a class PersonFriendList(List) and override append() with the desired functionality. Then, instead of assigning [] to self.friends assign PersonFriendList(). The .friend value should probably also get decorated as a #property so that assignment can be intercepted by Person to avoid .friend being written to the wrong kind of list.
Code available on Ideone.
class Person():
def __init__(self, name, age=None, friends=None):
self.name = name
self.age = age
if friends is None:
friends = []
self.friends = PersonFriendList(friends)
class PersonFriendList(list):
def __init__(self, *args):
super(PersonFriendList, self).__init__(*args)
self.DebugPrint('constructed with {}'.format(str(*args)))
def DebugPrint(self, string):
print('{}(): {}'.format(self.__class__.__name__, string))
def append(self, *args):
super(PersonFriendList, self).append(*args)
self.DebugPrint('appending {}'.format(str(*args)))
me = Person('Mr. Me')
you = Person('Ms. You')
me.age = 42
me.friends.append(you)