This document provides an introduction to the Python programming language. It covers Python's basic data types like integers, floats, strings and lists. It also discusses functions, conditionals, loops, modules and libraries. Example code is provided to demonstrate Python syntax for variables, arithmetic, string operations, conditionals, functions and more. Key aspects of Python like dynamic typing, indentation, comments and documentation strings are also explained.

1. -Hardik Malhotra

2. Python introduction
Basic programming: data types, conditionals, looping
Data structures: Lists and dictionaries
Functions
Example codes
Based on presentation from www.cis.upenn.edu/~cse391/cse391_2004/PythonIntro1.ppt

3. A programming language with powerful typing and object oriented features.
◦Commonly used for producing HTML content on websites.
◦Useful built-in types (lists, dictionaries).
◦Clean syntax

4. Natural Language ToolKit
AI Processing: Symbolic
◦Python‘s built-in datatypes for strings, lists, and more.
◦Java or C++ require the use of special classes for this.
AI Processing: Statistical
◦Python has strong numeric processing capabilities: matrix operations, etc.
◦Suitable for probability and machine learning code.

5. Shell for interactive evaluation.
Text editor with color-coding and smart indenting for creating python files.
Menu commands for changing system

6. x = 34 - 23 # A comment.
y = “Hello” # Another one.
z = 3.45
if z == 3.45 or y == “Hello”:
x = x + 1
y = y + “ World” # String concat.
print x
print y

7. x = 34 - 23 # A comment.
y = “Hello” # Another one.
z = 3.45
if z == 3.45 or y == “Hello”:
x = x + 1
y = y + “ World” # String concat.
print x
print y

8. Assignment uses = and comparison uses ==.
For numbers +-*/% are as expected.
◦Special use of + for string concatenation.
◦Special use of % for string formatting.
Logical operators are words (and, or, not) not symbols (&&, ||, !).
The basic printing command is ―print.‖
First assignment to a variable will create it.
◦Variable types don‘t need to be declared.
◦Python figures out the variable types on its own.

9. Integers (default for numbers)
z = 5 / 2 # Answer is 2, integer division.
Floats
x = 3.456
Strings
Can use ―‖ or ‗‘ to specify. ―abc‖ ‗abc‘ (Same thing.)
Unmatched ones can occur within the string. ―matt‘s‖
Use triple double-quotes for multi-line strings or strings than contain both ‗ and ― inside of them: ―――a‗b―c‖‖‖

10. Whitespace is meaningful in Python: especially indentation and placement of newlines.
◦Use a newline to end a line of code. (Not a semicolon like in C++ or Java.) (Use when must go to next line prematurely.)
◦No braces { } to mark blocks of code in Python… Use consistent indentation instead. The first line with a new indentation is considered outside of the block.
◦Often a colon appears at the start of a new block. (We‘ll see this later for function and class definitions.)

11. Start comments with # – the rest of line is ignored.
Can include a ―documentation string‖ as the first line of any new function or class that you define.
The development environment, debugger, and other tools use it: it‘s good style to include one.
def my_function(x, y):
“““This is the docstring. This function does blah blah blah.””” # The code would go here...

12. Python determines the data types in a program automatically. ―Dynamic Typing‖
But Python‘s not casual about types, it enforces them after it figures them out. ―Strong Typing‖
So, for example, you can‘t just append an integer to a string. You must first convert the integer to a string itself.
x = “the answer is ” # Decides x is string.
y = 23 # Decides y is integer.
print x + y # Python will complain about this.

13. Names are case sensitive and cannot start with a number. They can contain letters, numbers, and underscores.
bob Bob _bob _2_bob_ bob_2 BoB
There are some reserved words:
and, assert, break, class, continue, def, del, elif, else, except, exec, finally, for, from, global, if, import, in, is, lambda, not, or, pass, print, raise, return, try, while

14. You can also assign to multiple names at the same time.
>>> x, y = 2, 3
>>> x
2
>>> y
3

15. We can use some methods built-in to the string data type to perform some formatting operations on strings:
>>> “hello”.upper()
„HELLO‟
There are many other handy string operations available. Check the Python documentation for more.

16. Using the % string operator in combination with the print command, we can format our output text.
>>> print “%s xyz %d” % (“abc”, 34)
abc xyz 34
―Print‖ automatically adds a newline to the end of the string. If you include a list of strings, it will concatenate them with a space between them.
>>> print “abc” >>> print “abc”, “def”
abc abc def

17. Your program can decide what to do by making a test
The result of a test is a boolean value, True or False
Here are tests on numbers:
◦< means “is less than”
◦<= means “is less than or equal to”
◦== means “is equal to”
◦!= means “is not equal to”
◦>= means “is greater than or equal to”
◦< means “is greater than”
These same tests work on strings

18. Boolean values can be combined with these operators:
◦and – gives True if both sides are True
◦or – gives True if at least one side is True
◦not – given True, this returns False, and vice versa
Examples
◦score > 0 and score <= 100
◦name == "Joe" and not score > 100

19. The if statement evaluates a test, and if it is True, performs the following indented statements; but if the test is False, it does nothing
Examples:
◦if grade == "A+": print "Congratulations!"
◦if score < 0 or score > 100: print "That’s not possible!" score = input("Enter a correct value: ")

20. The if statement can have an optional else part, to be performed if the test result is False
Example:
◦if grade == "A+": print "Congratulations!" else: print "You could do so much better." print "Your mother will be disappointed."

21. The if statement can have any number of elif tests
Only one group of statements is executed—those controlled by the first test that passes
Example:
◦if grade == "A": print "Congratulations!" elif grade == "B": print "That's pretty good." elif grade == "C": print "Well, it's passing, anyway." else: print "You really blew it this time!"

22. Indentation is required and must be consistent
Standard indentation is 4 spaces or one tab
IDLE does this pretty much automatically for you
Example:
◦if 2 + 2 != 4: print "Oh, no!" print "Arithmethic doesn't work!" print "Time to buy a new computer."

23. A list
◦Example: courses = ['CIT 591', 'CIT 592', 'CIT 593']
Referring in list
◦Example: courses[2] is 'CIT 593'
The len function
◦Example: len(courses) is 3
Range is a function that creates a list of integers, from the first number up to but not including the second number
◦Example: range(0, 5) creates the list [0, 1, 2, 3, 4]
Range with third number
◦Example: range(2, 10, 3) creates the list [2, 5, 8]

24. A for loop performs the same statements for each value in a list
◦Example: for n in range(1, 4): print "This is the number", n prints This is the number 1 This is the number 2 This is the number 3
The for loop uses a variable (in this case, n) to hold the current value in the list

25. A while loop performs the same statements over and over until some test becomes False
◦Example: n = 3 while n > 0: print n, "is a nice number." n = n – 1 prints 3 is a nice number. 2 is a nice number. 1 is a nice number.
If the test is initially False, the while loop doesn't do anything.
If the test never becomes False, you have an "infinite loop." This is usually bad.

26. A function is a section of code that either (1) does some input or output, or (2) computes some value.
◦A function can do both, but it's bad style.
◦Good style is functions that are short and do only one thing
◦Most functions take one or more arguments, to help tell them what to do
Here's a function that does some input: age = input("How old are you? ") The argument, "How old are you?", is shown to the user
Here's a function that computes a value (a list): odds = range(1, 100, 2) The arguments are used to tell what to put into the list

27. 1.def sum(numbers):
2. """Finds the sum of the numbers in a list."""
3. total = 0
4. for number in numbers:
5. total = total + number
6. return total
7.def defines a function numbers is a parameter: a variable used to hold an argument
8.This doc string tells what the function does
6.A function that computes a value must return it sum(range(1, 101)) will return 5050

28. Dictionary is an unordered set of key: value pairs
The keys are unique (within one dictionary)
Use of dictionary:
Example codes
Based on presentation from www.cis.upenn.edu/~cse391/cse391_2004/PythonIntro1.ppt

29. Arithmetic: + - * / % < <= == != >= >
Logic (boolean): True False and or not
Strings: "Double quoted" or 'Single quoted'
Lists: [1, 2, 3, 4] len(lst) range(0, 100, 5)
Input: input(question) raw_input(question)
Decide: if test: elif test: else:
For loop: for variable in list:
While loop: while test:
Calling a function: sum(numbers)
Defining a function: def sum(numbers): return result

30. Things to read through
―Dive into Python‖ (Chapters 2 to 4) http://diveintopython.org/
Python 101 – Beginning Python http://www.rexx.com/~dkuhlman/python_101/python_101.html
Things to refer to
The Official Python Tutorial http://www.python.org/doc/current/tut/tut.html
The Python Quick Reference http://rgruet.free.fr/PQR2.3.html