Python Basics

1
Python Basics
2
Python History

• Late 1970s programming language called ABC at
  the Centrum voor Wiskunde en Informatica in the
  Netherlands
• Audience included physicists, social scientists,
  and linguists
• 1983 Guido van Rossum joined the ABC team
• Late 1980s Guido needed a language for a
  different project based it on ABC, removed warts
• Python, after Monty Python

3
Python History

• Guido Benevolent Dictator for Life (BDFL)
• Neat set of interviews http//www.artima.com/
  intv/
• Search for Guido

4
How Python is Managed

• Guido collects and writes Python Enhancement
  Proposals (PEPs)
• http//www.python.org/dev/peps/
• Interested parties comment
• Guido makes final decisions
• Team of programmers (many of them volunteers!)
  implement the features

5
Python Types

• Every Python value has a type that describes what
  sort of value it is
• Built-in function type will tell you the type of
  an expression

English Python
integer int
real number float
picture Picture
pixel Pixel
colour Color
string of letters str
6
Python Numeric Types

• Mathematical types int float long bool
• English names
• integer, floating-point number, long integer,
  boolean
• int range -2147483648 2147483647
• float values about -10308 10308

7
Python Numeric Types

• long values unlimited (any number of available
  locations in memory)
• int values that grow too large are automatically
  converted to long
• One more bool (from Boolean) True, False

8
Sizes in Memory

• Integers have a fixed size
• -1 and 983471 use the same amount of memory
• Floating-point number have a fixed size
• Consequence cant represent every possible value
• Long integers can take up an arbitrarily large
  amount of memory

9
Python Operators

• Usual meaning gt lt lt gt - ( )
• New operators
• power 2 5
• testing for equality 3 x
• remainder 8 3
• division 8 / 3
• assignment num_bananas 0

10
Youre Not My Type

• Operations involving different types use this
  hierarchy for the type of the result
• float gt long gt int gt bool
• 45.3 400000000L result float
• 400000000L - 45 result long
• 3 True result int (more on combining ints and
  bools later)

11
Mathematical Operator Precedence

• Operators in same box group left to right
• Override using parentheses

Exponentiation
x -x Positive, negative
/ // Multiplication, division, remainder, integer division
- Addition, subtraction
Operator precedence, highest to lowest
12
Names

• Variable names, function names, and every other
  name youll see
• Begin with a letter or underscore (a-z, A-Z, _)
• Are made up of letters, underscores, and numbers
• Valid _moo_cow, cep3, I_LIKE_TRASH
• Invalid 49ers, _at_home

13
Naming Conventions

• thEreS a GoOD rEasON wHy WorDs haVE A StaNDaRd
  caPITaLizAtIon sCHemE
• Python convention for function names and
  variables pothole_case
• CamelCase is sometimes seen, but not for
  functions and variables
• When in doubt, use lowercase_pothole

14
Function Definition
Zero or more, comma-separated
def function_name(parameters) body
One or more statements
def keyword
15
Return Statement

• Form
• return expression
• This must appear in a function body
• How its executed
• Evaluate the expression
• Exit the function, using the result of the
  expression as the value of the function call

16
Useful __builtins__

• dir produce a list of available functions and
  variables
• help display documentation
• type produce the type of an expression
• int, str, float, bool type conversion
• min, max, abs
• raw_input

17
Statements

• Youve seen these statements
• assignment variable value
• print print expression
• function definition def function()
• function call function()
• return return expression
• for loop for variable in list
• If statement if expression

18
Whats Next
Functions