Summary

Python object references

• Think of Python working in terms of named references to objects rather
  than variables and values.

• Assignment doesn't put a value in a box. It attaches a name tag to an
  object.

• Assigning from one reference to another puts two name tags on the same object.

• The Python garbage collector will reclaim unreachable objects - those
  objects with no name tag.

Object identity and equivalence

• The id() function returns a unique and constant identifier but should rarely, if ever, be used in production.

• The is operator determines equality of identity. That is, whether two
  names refer to the same object.

• We can test for equivalence using the double-equals operator.

Function arguments and return values

• Function arguments are passed by object-reference, so functions can modify their arguments if they are mutable objects.

• If a formal...