1. Pamela Moore & Zenia Bahorski
Python 1
SIGCS Intro to Python
March 16, 2011
Presented by
Pamela Moore & Zenia Bahorski

2. Scripting Languages Useful for system administration Web programming
text processing, etc.

3. Scripting Languages Casual with regard to the typing of variables
Variables can be different types depending on the different information stored in them.
Different types were especially important when memory constraints were a bigger issue than they are today.
Integers take less memory to store than real numbers which take less memory than arrays.

4. Scripting Languages
Arrays (which we haven’t defined yet) can use elements of different types - unheard of in most procedural languages
E.g.:
Hello
68 widgets
$333.99

5. Scripting Languages Lots of built in high-level operations
String concatenation (making one large string by "fusing" two smaller strings)
Pushing an item onto a stack
Popping an item off of a stack

6. Scripting Languages Interpreted, rather than compiled
An interpreted language is executed "on the fly", one statement at a time
Can be faster
Often uses less memory
Easier for using on machines with different operating systems

7. Scripting Languages Interpreted, rather than compiled
A compiled language is first translated into the machine language of the host machine and then executed.
Limited by the host machine in use
Uses more memory since it has to hold the entire program before it is executed

8. Why Python?
Python & the Python-like Ruby have momentum in the scripting world.
Cleaner
More elegant
Popular with developers – (Google uses Python)
Easier to learn

9. Why Python? Pythonista - a Python aficionado
Pythonistas advocate the use of Python for all programming, not just scripting

10. Interactive Mode
Interactive mode - executing Python statements immediately
Useful for quick checks of commands
% python
>>> prompt for a command
… prompt when in a block
… <return> to finish
Stop and have them do some!

11. Case sensitive! Python is case sensitive
Upper and lower case letters are perceived as different things
Be careful of variables - especially if you use a text processing program that automatically capitalizes sentences
Ex: num & Num
Ex: print & Print

12. Formatting Notes Python is picky about some characters
Indenting must be with spaces, not tabs
Quotes must be non-smart double quotes (even though some versions say single or double quotes)
Ex: "Hello"
Ex: “Hello” or ‘Hello’ will each generate a syntax error

13. Definitions Block indicated by a : at the end of the line.
Everything after MUST be indented the same amount. Be consistent.
Sub-blocks (blocks within blocks) are possible very useful.

14. Definitions
List
A sequence of objects (can be different things) separated by commas and enclosed in square brackets.
Ex: [1, 2, 4, num]
Ex: ["hello", "goodbye"]
Ex: [23.5, $16.42, [0, -3], "string"]

15. Definitions Variable Letter or series of letters used to hold a value
Use descriptive variable names whenever possible
Makes debugging SO much easier!
Use single letter variables for loop control only! (my rule – not a Python rule)

16. Definitions Functions Call to a pre-defined mini program
Usually returns a value
Call with
Funcname(args)
Ex: range(2) returns the list [0,1]
Ex: type(3) returns <type ‘int’>

17. Definitions Counting In Python, counting starts at 0
Becomes important for some functions (like range)

18. Definitions Long lines Continue on next line with a back slash Ex:
print "This is a very very very very very long"\
"line of text continued on the next line"

19. Order of Operations
Mathematical calculations are executed in the following order:
Expressions within ( )
Function calls
Exponentiation - raise to a power - **
Multiply & divide
* & /
Add & subtract
+ & -

20. Order of Operations Ex: x= 3 print x**3 + 5 - 6.3*2
print 12 - (max[x, 6] /3)

21. Comments
To put comments in a Python program just put # as the first char
Rest of the line is ignored
Ex:
# ignore this line of code
# print the results 10 times
Comments will be necessary in ALL programs! (my rule)

22. For loop preview
For loop - loop will execute a definite amount of times
Ex: for i in [1, 2, 3, 4]:
print "i=", i
Ex: for x in range(9):
print "loop"