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: Search for “Guido”

4 How Python is Managed Guido collects and writes Python Enhancement Proposals (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 EnglishPython integerint “real” numberfloat picturePicture pixelPixel colourColor string of lettersstr

6 Python Numeric Types Mathematical types: int float long bool English names: integer, floating-point number, long integer, boolean int range: … float values: about …

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 use the same amount of memory Floating-point number have a fixed size Consequence: can’t represent every possible value Long integers can take up an arbitrarily large amount of memory

9 Python Operators Usual meaning: * + > = - ( ) New operators power: 2 ** 5 testing for equality: 3 == x remainder: 8 % 3 division: 8 / 3 assignment: num_bananas = 0

10 You’re Not My Type Operations involving different types use this hierarchy for the type of the result: float > long > int > bool 45.3 * L # result: float L - 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 -xPositive, negative * / % // Multiplication, division, remainder, integer division + -Addition, subtraction Operator precedence, highest to lowest

12 Names Variable names, function names, and every other name you’ll 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:

13 Naming Conventions thEre’S 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 def function_name(parameters): body def keyword Zero or more, comma-separated One or more statements

15 Return Statement Form: return expression This must appear in a function body How it’s executed 1. Evaluate the expression 2. 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 You’ve 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 What’s Next Functions