Python(Functional abstraction)

Libraries, modules, namespaces

• Library is a generic term for a collection of (useful) functions, data structures, etc.
• In python, libraries are called modules.
• Importing a module, eg:

import math
import robot

• Imported names are prefixed with the module name, as in math.pi, robot.lift up, etc.
• How does python find modules?
  - Standard modules (e.g., math) are installed in a specific location on the file system.
  - Non-standard modules (e.g., robot) must be in the current working directory (cwd).

Functional abstraction

• In programming, a function (also known as “procedure” or “subroutine”) is a piece of the program that is given a name.
  - The function is called by its name.
  - A function is defined once, but can be called any number of times.
• Why use functions?
  - Abstraction: To use a function, we only need to know what it does, not how.
  - Break a complex problem into smaller parts.

Function definition in python

def move_to_next_stack():
    robot.drive_right()
    robot.drive_right()

• def is a python keyword (“reserved word”).
• The function’s name is followed by a pair of parentheses and a colon.
  - Inside the parentheses are the function’s parameters (more on this in coming lectures).
• The function suite is the sequence of statements that will be executed when the function is called.
• In python, a suite is delimited by indentation.
  - All statements in the suite must be preceded by the same number of spaces/tabs(standard is 4 spaces).

Syntax

• The syntax of a (programming) language is the rules that define what is a valid program.
• A python program is a sequence of statements:
  - defining a function:

def move_to_next_stack():
    robot.drive_right()
    robot.drive_right()

- calling a function:

move twice()
robot.lift_up()

- importing a module:

import math

Whitespace

• Spaces, tabs and end-of-line are known as whitespace.
• The whitespace before a statement is called indentation.
• In python, whitespace has two special roles:
  - end-of-line marks the end of a statement(some exceptions, more later in the course);
  - indentation defines the extent of a suite of statements.
• Other than this, whitespace is ignored.

Permitted names in python

• A function name in python may contain letters, numbers and underscores ( ), but must begin with a letter or undescore.
• Reserved words cannot be used as names.
• Names are case sensitive: upper and lower case letters are not the same.
  

Comments

• A hash sign (#) marks the beginning of a comment; it continues to end-of-line.
• Comments are ignored by the interpreter.
  - Comments are for people.
  - Use comments to state what is not obvious.
• If it was hard to write, it’s probably hard to read. Add a comment. (Punch & Enbody, Rule 6)

Testing and debugging

• How do we know our program works?
  - Specify the assumptions under which the program (or function) is meant to work.
  - Test it with a variety of cases that fall under those assumptions.
  - Particularly, “edge cases”.
• Some common errors:
  - SyntaxError:
  You have broken the rules of python syntax.
  - NameError or AttributeError:
  You have used a (function) name that doesn’t exist. Check for typos.
  - IndentationError:
  Too much or too little indentation.
  * - All statements in a function suite must have the same indentation.
  * - All statements outside function definitions must have no indentation.