In Python, a variable is a label that refers to an object rather than a container holding a value. Several labels can refer to the same object, and assignment changes which object a label refers to. section 4.3
Reference behavior becomes visible through assignment and mutation:
- Assigning a mutable object to another name creates another reference; mutating the object is visible through every name. section 4.3
- Assigning a new immutable value changes only the assigned name's reference; other names still refer to the original object. section 4.3
- A name can refer to objects of different types at different times, such as a string followed by an integer. section 4.3
Mutation versus reassignment
With a list, `a = [1, 2, 3]`, `b = a`, and `c = b` make all three names refer to one object; `b[1] = 5` therefore produces `[1, 5, 3]` through each name. With integers, `a = 1`, `b = a`, and `b = 5` leave `a` referring to `1` while `b` refers to `5`.
Function arguments are passed by object reference: a parameter becomes a new reference to the argument's object. Mutating a passed list affects the caller's object, but rebinding the parameter does not; immutable values likewise remain unchanged outside the function. section 9.3
