Does python have 'private' variables in classes?

Question

I'm coming from the JAVA world and reading bruce eckels' python 3 patterns idioms.

While reading about classes...it goes on to say that in python there is no need to declare class variables. You just use them in the constructor...and boom..they are there.

So for example:

class Simple:
        def __init__(self1, str):
                print("inside the simple constructor")
                self1.s = str
        def show(self1):
                print(self1.s)
        def showMsg (self, msg):
                print (msg + ':', self.show())

My question is, if above is the case then any object of class Simple can just change the value of variable 's' outside of the class. For example:

if __name__ == "__main__":
        x = Simple("constructor argument")
        x.s = "test15" #this changes the value
        x.show()
        x.showMsg("A message")

In java, we have been taught about public/private/protected variables. Those keywords make sense because at times you want variables in a class to which no one outside the class has access to.

Why is that not required in python?

Answer 1

Python has limited support for private identifiers, through a feature that automatically prepends the class name to any identifiers starting with two underscores. This is transparent to the programmer, for the most part, but the net effect is that any variables named this way can be used as private variables.

See here for more on that.

In general, Python's implementation of object orientation is a bit primitive compared to other languages. But I enjoy this, actually. It's a very conceptually simple implementation and fits well with the dynamic style of the language.

Answer 2

It's cultural. In Python, you don't write to other classes' instance or class variables. In Java, nothing prevents you from doing the same if you really want to - after all, you can always edit the source of the class itself to achieve the same effect. Python drops that pretense of security and encourages programmers to be responsible. In practice, this works very nicely.

If you want to emulate private variables for some reason, you can always use the "__" prefix from PEP 8. Python mangles the names of variables like __foo so that they're not easily visible to code outside the class that contains them (although you can get around it if you're determined enough, just like you can get around Java's protections if you work at it).

By the same convention, the "_" prefix means "stay away even if you're not technically prevented from doing so." You don't play around with another class's variables that look like __foo or _bar.

Answer 3

"In java, we have been taught about public/private/protected variables"

"Why is that not required in python?"

For the same reason it's not required in Java.

You're free to use -- or not use private and protected.

As a Python and Java programmer, I've found that private and protected are very, very important design concepts. But as a practical matter, in tens of thousands of lines of Java and Python, I've never actually used private or protected.

Why not?

Here's my question "protected from whom?"

Other programmers on my team? They have the source. What does protected mean when they can change it?

Other programmers on other teams? They work for the same company. They can -- with a phone call -- get the source.

Clients? It's work-for-hire programming (generally). The clients (generally) own the code.

So, who -- precisely -- am I protecting it from?

Right. The schizophrenic sociopath who refused to read the API comment blocks.

Answer 4

My method is to use 512 bit variable name generate by Whirlpool and a random number generator. When variables names look like this "_P_3CCF8252D8BBB258460D9AA999C06EE38E67CB546CFFCF48E91F700F6FC7C183AC8CC3D3096DD30A35B01F4620A1E3A20D79CD5168544D9E1B7CDF49970E87F1". And this variable names change from one release version to another by automated script. It will not actually prevent or make the variables privat but it will make life difficult for anyone not using the provided API.

Answer 5

Python does not need to protect anyone from assigning a value to a member variable, because: A) In Python you cannot accidentally make a typo such that a comparison turns into an assignment. B) Python developers do not make mistakes. C) All of the above. D) What was the question again?