Introduction to Python Workshop

From MtlPy Wiki

Contents 1 Interactive Introduction to Python Workshop 1.1 Preparation 1.2 Interactivity 1.3 First steps with Python 1.4 Small Program 1.4.1 Things of note 1.5 Different Operators Mean Different Things 1.6 Conditionals 1.7 Containment 1.8 Reading files the Python way 1.9 User input and Array slicing 1.10 Lists 1.10.1 Having fun with lists 1.11 Tuples 1.12 Sets 1.13 Dictionaries 1.14 Iteration and Looping 1.15 Functions

[edit] Interactive Introduction to Python Workshop

This workshop was given on January 25th, 2012 as part of a series of workshops for Winter 2012.

Author: Mathieu Perreault (mathieu.perreault[at]gmail.com) based on material by Alexandre Bourget and from the Python Essential Reference book.

[edit] Preparation

• Make sure everyone has a python interpreter.
  • Windows: Install Python and use shortcut in Start menu or Start -> Run... -> cmd -> python
  • Mac: Utilities -> Terminal -> python
  • Linux: Terminal -> python
• Make sure everyone knows how to create a file and run it.
• Create file workshop.py and run it from the command line.

[edit] Interactivity

For every code snippet:

• People must write the code in the Python interpreter and run it.
• Instructor may pause briefly to make sure everyone has typed it.
• Instructor may ask: Who got an error, what is the error?
• Instructor may ask if someone can describe what is going on (makes them think about the code).
• Instructor should explain what is going on.

[edit] First steps with Python

 print “Hello world”
 6000 + 4523.50 + 134.12

Different types of values: String, Integer, Floating Point (double, float)

[edit] Small Program

 year = 2000
 while year < 2005:
   print year	# Don’t forget the spaces for the indented block.
   year += 1

[edit] Things of note

• Variable assignment
• Indentation
• Conditional statement
• Printing a variable
• Incrementing the variable

[edit] Different Operators Mean Different Things

 print “Hello Mr. %s” % (“Lazhar”)
 print 7 % 2

Different operators mean different things in different contexts (3 % signs here, only one is the modulo operator). String substitution similar to C and other languages.

[edit] Conditionals

 a = 1
 b = 2
 if a < b:
   print “a is smaller than b”
 else:
   print “b is smaller than a”

[edit] Containment

 monty = ‘Egg and spam’
 has_spam = “spam” in monty
 print has_spam

 if ‘spam’ in monty:  # replace with has_spam
   print “I’ve got spam!”
 else:
   print “Where’s my spam!”

• No Difference between single quote and double quote (and triple quotes!) but be consistent.
• Introducing the “in” operator.
• Introducing the boolean. See that it comes from the “in” operator.

[edit] Reading files the Python way

 f = open(“hello.py”)
 for line in f:
   print line

That can be brought back to

 for line in open(‘hello.py’):
   print line,

Opening a file, reading every line. Pythonic way!

[edit] User input and Array slicing

 >>> a = raw_input(“What’s your name: “)
 What’s your name: _______

What’s in a now?

 a = “Hello World” # You can put your name here :)
 a[7] = “o”
 a[0:5] = “Hello”
 a[6:] = “World”
 a[3:8] = “lo Wo”

a is a sequence of characters, that we can access easily characters in specific positions.

[edit] Lists

 names = [“Dave”, “Mark”, “Ann”, “Phil”]
 f.readlines()		      # Get the lines of a file. 
 a = names[2]		      # Access different elements.
 names[0] = “Jeff”	      # Change some elements in the list.
 names.insert(2, “Thomas”)   # Insert something in a list.

 lastnames = []              # Create a new list.
 names.append(“Smith”)       # Append a string to the end of the list.

[edit] Having fun with lists

 [name.upper() for name in names]          # List comprehension
 alist = [[1,2,3], [4,5,”Mark”], [7,8,9]]       # Go and get the number 8 (a[2][1])
 allnames = names + lastnames 		# concatenating lists

[edit] Tuples

 stock = (‘GOOG’, 100, 600.00)		# Defining a tuple
 company, shares, price = stock 	# Variable unpacking

• Although the tuple supports the same methods as a list, the contents are immutable.
• Tuple is more memory efficient if you know that you are not going to modify your sequence.

[edit] Sets

 languages = set([“Python”, “Java”, “Ruby”])
 stones = set([“Ruby”, “Amethyst”])
 snakes = set([“Python”, “Cobra”])

 >>> languages.union(stones) 		# languages | stones
 >>> languages.intersection(snakes)  # languages & snakes
 >>> languages.difference(snakes)	# languages - snakes

Different methods avaiable for set s: s.add(), s.remove(), s.clear()

[edit] Dictionaries

 stock = {“name”: “GOOG”, “shares”: 100, “price” : 490.10}

 company = stock[‘name’]
 value = stock[‘shares’] * shares[‘price’]

 stock[‘price’] = 600.0

 >>> stock_keys = list(stock)		# List of keys
 [‘name’, ‘shares’, ‘price’]

[edit] Iteration and Looping

 for n in [1,2,3,4,5,6]:		# Can be replaced by range(1,7)
   print “2 to the %d power is %d” % (n, 2**n)

 ws = “Python workshop”
 wslist = [‘Python’, ‘Workshop’]

 for c in ws: # Iterate over a string will give you the characters

 for el in wslist: # Iterate over the elements in the list (strings).

[edit] Functions

 def reverse(s): 
   return "".join([s[-i-1] for i in range(len(s))])

Talk about functions, and how you can define a function.