Beginning programmers can skip this section. This is a digression for experienced C and C++ programmers.

Most programming languages have a formal mechanism for determining if a parameter receives a copy of the argument (call by value) or a reference to the argument object (call by name or call by reference). The distinction is important because programming languages like C++ allow us to provide a reference to a global variable as input to a function and have that global variable updated by the function without an obvious assignment statement.

The following scenario is entirely hypothetical for Python programmers, but a very real problem for C and C++ programmers. Imagine we have a function to2, with this kind of definition in C.

int to2( int *a ) { 
    /* set parameter a's value to 2 */
    *a= 2;
    return 0;
}

This function changes the value of the variable a to 2. This would be termed a side-effect because it is in addition to any value the function might return normally.

When we do the following in C

int x= 27;
int z= to2( &x );
printf( "x=%i, z=%i", x, z );

We get the unpleasant side-effect that our function to2 has changed the argument variable, x, and the variable wasn't in an assignment statement! We merely called a function, using x as an argument. In C, the & operator is a hint that a variable might be changed. Further, the function definition should contain the keyword const when the reference is properly read-only. However, these are burdens placed on the programmer to assure that the program compiles correctly.

Python does not permit this kind of call-by-reference problem in the first place. The arguments to a function are always references to the original underlying object, not the variable. In the Python version of the above example, the variable x is a reference to an integer object with a value of 27. The parameter variable in the to2 function is a copy of the reference, and it is local to the function's scope. The original variable, x, cannot be changed by the function, and the original argument object, the integer 27, is immutable, and can't be changed either.

If an argument value is a mutable object, the parameter is a reference to that object, and the function has access to methods of that object. The methods of the object can be called, but the original object cannot be replaced with a new object.

Programmers intimate with the implementation details behind C know that manipulating a copy of a value directly can be far more efficient than working through references. This is true only when the language has some high-performance data types that would benefit from call by value processing. Unlike C or Java, Python has no data types that benefit from this kind of special case handling. All data elements are passed by reference in a simple, uniform manner.

It also means that, in general, all variable updates must be done explicitly via an assignment statement. This makes variable changes clear.