Question

I was trying to make the connection between python string and C character array assignment to string literals.
For example:

char* word="Hello";
        word="Now";

assigns to the character String “Hello” placed in read only memory location. And now a reassignment of word to “Now” means a now the character array is assigned a memory location corresponding to “Now”.

In python even numbers (and obviously strings) seem to work like that with a being assigned a memory location with value 2 and then reassigned a memory location with value 3.

 a=2
 a=3

This contrasts with C where for almost all variable assignments the variable contains the value it is assigned. Am I making a good comparison here?

Answer 1

In C, char * is the type “pointer to character.” A pointer is like a “handle” that gives you access to the value itself. When you assign word = "Now";, you are changing the handle, not the characters of the string. Both "Hello" and "Now" still exist as groups of bytes in constant storage.

Python hides more of what it’s doing, but internally its built-in string references essentially act like pointers. So your observation is correct up to a point.

The big difference in strings of these languages is that in Python all strings are immutable. C lets you manipulate (by assignment) characters within a (non-constant) string. For example:

char word [] = "Hello";
// word is now the name of a 6-character (including final null) mutable string
word[0] = 'h';
// Now word has the value "hello".

In Python, you would have to create a completely new string:

word = 'h' + word[1:]

Here the bytes that make up the new string are different than those of word.

You are right that Python has “reference semantics”. Everything behaves as though you are manipulating a handle rather than a value.

>>> a=[1]
>>> b=a
>>> a
[1]
>>> b
[1]
>>> a[0]=5
>>> a
[5]
>>> b
[5]

Here the assignment b = a copied a handle or reference to the list. The list itself is not copied. This is evident because changing the first element of a also changed b.