Chapter 2

Objects

• Variables are names for objects.
• Objects represent values
  • All values (integers, strings, functions, lists, etc.)
  • Think of an object as a chunk of memory (RAM) that represents something
• Variables are bound to objects.
  • This binding can change.
  • x = 2
  • x = 4
• Objects are automatically garbage collected when no variable names them.
• Objects can have more than one name. (That will become relevant later.)
• All objects have
  • Value (‘20’, ‘bob’, 3.14159, etc.)
  • Type (integer, string, float)
  • Identity
• You can use type() to determine the type of the object referenced by a variable.
• The identity is a unique identifier for each object. (This identifier is not a variable)
  • use id() to get the identity of an object.
• Why do we need to keep track of the type of a variable?
  • Tells computer how to interpret the bits.
  • Also specifies what the legal operations are on those bits.
    • x/y is legal if x and y are integers, but not if they are strings.
• How does Python know what type a variable is?
  • It remembers it when the variable is assigned.

Expressions

• / does floating point division, not integer division.
  • Even 15 / 5 returns a float.
  • This may be different from other languages you know.
  • use // to perform integer division.
    • Show 17 / 5 vs 17 // 5
• ** is an exponent (Not all languages provide this operator)
• Remember your order of operations: 5 + 2 * 3
• Compound operators (+=, *=, etc.)
• Remember that mod % gives a remainder.

Modules

• When writing larger programs, we usually put Python code in a file.
  • It makes it easier to run and re-run.
• Larger programs often break code into multiple files.
  • It makes the code easier to manage.
  • You can also share parts of your code with others to reduce duplication.
• A file containing code to be used elsewhere is called a module
• To use code from a module in another file, use the import statement.
• For example, the file math.py contains many useful functions (sin, cos, sqrt, etc.)
  • Rather than having every programmer have to write these functions herself, you can say import math.
  • Then you have access to the objects inside math.py like this math.sqrt(49)
  • https://docs.python.org/3/library/math.html
• Python comes with a standard library of modules to import. These modules contain functions that are so commonly used that it makes sense to bundle them in the language. Available modules include
  • math
  • random
  • https://docs.python.org/3/library/

IC 2: Change for a dollar Euclidean distance