Understanding Python's Name Binding

I am trying to clarify for myself Python's rules for 'assigning' values to variables. Is the following comparison between Python and C++ valid? In C/C++ the statement int a=7 me

Solution 1:

Yes, you're basically correct. In Python, a variable name can be thought of as a reference to a value (not in terms of a C++ reference, though that works similarly, more just stating that it refers to something).

As an aside, the Python way is very similar to the C++ int &b = a, which just means a and b refer to the same value.

Or the C int *pb = &a, which means a and *pb refer to the same value, but with all the confusion that brings to people who haven't yet accepted the brain-bendedness of C :-)

Assigning to the variable name in Python makes the name refer to a different value, it never copies the value itself:

a = 7   # Create "7", make "a" refer to it.
b = a   # make "b" refer to  the "7"as well.
a = 42  # Create "42", make "a" refer to it, b still refers to the "7".

(I say "create" but that's not necessarily so - if a value already exists somewhere, it may re-use it).

In a C-like language, that second statement b = a creates a new value, copies the "7" into it and then names that b. In Python, it simply ends up with a and b referring to the same value.

Where the underlying data is immutable (cannot be changed), that usually makes Python look as if it's behaving identically to the way C does it.

But, for mutable data (same as using pointers in C or references in C++), people can sometimes be surprised because they don't realise that the value behind it may be shared:

>>>a = [1,2,3] ; print a
[1, 2, 3]

>>>b = a ; print b
[1, 2, 3]

>>>a[1] = 42 ; print a
[1, 42, 3]

>>>print b   #WTH?
[1, 42, 3]

There are ways to get independent copies of a value, with things such as:

b = a[:]
b = [item for item in a]

(which will work to one level, where b = a works to zero levels), or using deepcopy if you want if totally unique, to whatever level is necessary.

Solution 2:

In your example code, as "int" is a built-in type in C++, so the operator "=" could not be overloaded, but "=" doesn't always create new object, they could also reference to same object. The python object module is kind of like Java, most of the object is an reference but not a copy.

You can also try this:

a = 7
b = 7
printid(a), ' ' , id(b) 

it output the same result, as python will find both a and b point to same const variable