Introduction to Python

Python Introduction

• An interpreted, compiled, and interactive, object-oriented, dynamic, imperative, and open source programming language.
• Created in early 90's by Guido von Rossum at Stichting Mathematisch Centrum in the Netherlands.
• The name comes from the Monty Python and not from the snake.
• There is a big community of Python programmers, with conferences and magazines:
  http://pycon.org/
• Web site: www.python.org.

Features of Python

• Interactive: one can launch a Python console and run instructions directly it.
• Portable: available on most existing systems. It only requires a C compiler to be ported to any new platform.
• Structure: functions, classes, modules.
• It is easy to embed Python with C and C++.
• The user can write their own code in C or C++ and compile it as Python modules or functions. That makes Python extensible.
• Usual applications: scripts including CGI scripts, GUIs, scientific computing.
• Many existing libraries for all sort of purposes.

Syntax Rules

• The syntax is designed to be simplified as compared to other languages like C/C++.
• Every compound instruction ends with ":"
• There are no blocks of code; blocks are implicitly created by indentation.
• Expressions: usual arithmetic operators, named logic operators: and, or, not.
• Assignments use the = sign but they don't have to end with ";"
• Comments start with # as in shell scripting.
• Variables are declared by assigning them a value and they are local to the function where they appear first.

Control Structures

• Conditional:

  if condition:
      instructions
  elif condition: #*
      instructions
  else: # optional
      instructions

• Loops:

  while condition:
      instructions
  else:  # optional
      instructions
  for var in S:
      instructions
  else:  # optional
      instructions 
  for i in range(n):
      instructions 

Built-in Data Structures

• Lists: linked lists implementing the subscript operator:
  

  x = [1,2,3]
  x.append(4)
  print x[2]       # result: 3 

• Tupples: constant kind of arrays

  x = (1,2,3)

• Dictionaries: association lists

  x = {}
  x["word"] = reference
  for k in x.keys():
      print x[k]

Functions and Parameters

• Function definition:

  def function_name (par1, par2, ...):
      body of the function

• It supports default values for parameters.
• All parameters are value parameters.
• Any variable storing a complex data structure contains a reference to it. Any changes to the content of such a data structure in the function will affect the variable passed in the function call.
• Assignments involving a complex data structure don't make a copy of it.

More Built-in Functions

• Function type: returns the type of an object.
  type(0) - returns <type 'int'>
• Checking if something is an integer:
  if type(x) == type(0): ...
• Reading a value from the terminal: input()
  x = input()
• Returning a value from a function:
  return True

Example of Conditional

def check_type(x):
    if type(x) == type(0):
        print x, "is an integer"
    elif type(x) == type(1.0):
        print x, "is a float"
    elif type(x) == type(""):
        print x, "is a string"
    elif type(x) == type([]):
        print x, "is an array"
    ...

Example of while/else

def Euler(a, b):
    if b==0:
        return a
    r = a % b
    while r:
        a = b
        b = r
        r = a % b
    else:
        print "a divisible by b"
        return b
    return r

Booleans

• Truth values: True and False.
• False is equivalent with 0, and empty list [], an empty dictionary {}.
• Anything else is equivalent to True.
• Example:

  x = 0
  if not x:
  print "0 is False"

Default Values for Parameters

• Default values:

  def function (var1 = value, var2 = value, ...):

• Just like in C++, all the parameters that have default values must be grouped at the end.

  def GCD1(a=10, b=20): ...
  GCD1() -> 10
  GCD1(125) -> 5
  GCD1(12, 39) -> 3