Intro To Python

• version
  • using 3.5.2
    • on lab boxes
    • diff 2<->3
    • unicode support
    • backward incompatible changes to syntax and default libraries

quick facts

• comments - lines start with #

• print()
  • prints a value to console over stdout
  • same as cout
• declare and assign in a single step x = 5, not int x; x = 5;

no compile step

Types

• Numbers
  • Int (Integer) - 5,6,7,etc
  • Float (Floating point) - 5.6
• Boolean
  • True
  • False
• str - String
  • single or double quotes
    • “frog”, ‘frog’
    • ’’,””
    • “frog’s leg” <– double quotes good for apostrophe
  • Iterable
  • no character type, just single-char strings
• None
  • None

Built-in Data Structures

List (list)

• Iterable
• Mutable sequence of elements
  • x=[1]
  • x.append(2) – x==[1,2]
• Elements may be of differing types
  • Ex: [1,2,3] [1,2,3,"frog"] []
• Indexing
  • begins at 0
  • index must be an Int
• slices
  • x[0:2] [included(0):excluded]
  • returns a shallow copy
  • start copying at the first slice index
  • stop before end
  • x[3:1] –[], needs other parameter to step backwards
  • x[3:3] –[], evaluation stops before including first element
• Negative indexing
  • counts from end of list
  • x[-1] –last element

Tuple (tuple)

• Iterable
• immutable sequence of elements
• elements can have different types
• (x,) – comma required
• () – valid tuple, for some reason
• tuples can contain other tuples
• lists in tuples can be modified
• slicing works the same as a list

Dictionary (dict)

• heavily abused
• Iterable(key by default)
• maps keys to values
  • both keys and values can vary in type
• not a sequence
  • cannot depend on keys in any order
• examples:

    x={'a':'frog','b':'giraffe','c':'apple'}
    x['a'] -- "frog"
    x['nope'] -- KeyError
  

Custom Types

• they exist
• Python has class-based objects

Assignment

• Variables are dynamically typed
  • It’ll let you do this
  • It’ll work
    • Until it doesn’t.

Unpacking

• multiple assignments on one line
  • ex

     (a,b) = (1,2)
     a,b = 1,2
     a,b = [1,2]
     a,b = b,a
    

• numbers need to match
  • a,b,c = 1,2 –PROBLEM