Python memory model – I

Coming from a C/C++ background, Python looked deceptively familiar for me – and I made a big mistake here. I assumed that python uses the same memory model as C. This is first of a series of posts to try and summarize in broad terms how python’s memory model works.

Btw, if you are a newbie, you should definitely check ipython out. It’s an awesome interpreter with a lot of useful features, like say tabbing for autocompletion etc. Fun thing to learn python with.

The very first thing is to NOT compare C with Python. They both are completely different languages with different idioms, so its best not to compare them.

Everything in python is an object – yes, everything. Numbers are objects. Strings are objects. Lists are objects. Classes are objects. And objects have an identity. Lets find out more about identity of objects.

In python, the id() function can be used for checking identity of an object.

In [1]: id(5)
Out[1]: 151765128

In [2]: id('string')
Out[2]: 3075578752L

In [3]: id([1,2,3])
Out[3]: 156305932

Objects have values too. The values of the object may or may not change ( more on that later ) but the identity of an object cannot change.

That wasn’t so interesting… I know, I know. Well, my point is, in Python everything is an object and has an identity. Please re-read that last line – because this is important.

Lets move onto variables [ in python, it is referred to as names ]. In python, we’d do something like x = 5. What happens here is x gets mapped to the object which has the value 5.
Or, you could also say, x is bound to object having value 5.

x —-> [ object with value 5 ]

Everything’s good till now? awesome. Lets talk more about values now.

In Python, objects can be of two types with respect to values. Mutable and immutable. If you can modify the object and make it have a new value, then the object is mutable and otherwise immutable (Note: There are some caveats – an immutable container may contain a mutable object, but the content of container itself cannot be modified )

Integers, Floats, strings, tuples etc are immutable in python – that is you cannot modify the value of the object.

Wait, what the heck, then how am I able to do the following?

In [1]: name = 5

In [2]: name = 6

In [3]: name
Out[3]: 6

I hear you ask, didn’t I change the value of ‘name’ now?
Nope, you didn’t.

lets view the operation in terms of names and objects.

name = 5
name ---------> [ Object with value 5 ]
============================================================================================
name = 6
name ----x [ Object with value 5 ]
|
|------------> [ Object with value 6 ]


Okay, I’m not so good with asciiart now. Well, the point is, ‘name’ gets rebound to object with value 6. So what you are doing is not changing the value of object, but constructing a new object with value 6.

Whee! I hope that helped. If you like to read long technical documents then this link from docs.python will be very useful for you. I’ll be back with mutable objects in my next post.

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