Assignment operator

Python library reference says:

Assignment statements are used to (re)bind names to values and to modify attributes or items of mutable objects.

In short, it works as follows (simple assignment):

1. an expression on the right hand side is evaluated, the corresponding object is created/obtained
2. a name on the left hand side is assigned, or bound, to the r.h.s. object

Things to note:

• a single object can have several names bound to it:

  >>> a = [1, 2, 3]
  >>> b = a
  >>> a
  [1, 2, 3]
  >>> b
  [1, 2, 3]
  >>> a is b
  True
  >>> b[1] = 'hi!'
  >>> a
  [1, 'hi!', 3]
  

• By modifying b, we have also modified a! This is because a and b are just labels that point to the same object. If we want to make a copy, we can explicitly ask for one by using the list command:

  >>> a = [1, 2, 3]
  >>> b = list(a)
  >>> b
  [1, 2, 3]
  >>> a is b
  False
  >>> b[1] = 'hi!'
  >>> a
  [1, 2, 3]
  

• Note that taking slices of lists does perform a copy:

  >>> a = [1, 2, 3]
  >>> b = a[:2]
  >>> b
  [1, 2]
  >>> b[0] = 'hi!'
  >>> b
  ['hi!', 2]
  >>> a
  [1, 2, 3]
  

• The key concept here is mutable vs. immutable

  • mutable objects can be changed in place.
  • immutable objects cannot be modified once created.

A very good and detailed explanation of the above issues can be found in David M. Beazley’s article Types and Objects in Python.