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Intro to Python

Intro to Python. IC322 Fall 2013. Starting Python. Command-line: C:Python32python.exe >>> GUI: ->All Programs->Python3.2->IDLE (Python GUI). Running Python. Interactive mode: >>>print (‘Hello’) Running python files: Start IDLE (It opens in Interactive Mode.)

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Intro to Python

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  1. Intro to Python IC322 Fall 2013

  2. Starting Python • Command-line: C:\Python32\python.exe >>> • GUI: ->All Programs->Python3.2->IDLE (Python GUI)

  3. Running Python • Interactive mode: >>>print (‘Hello’) • Running python files: • Start IDLE (It opens in Interactive Mode.) • File->New Window (Creates a new editor window.) • In the editor, type: print (‘Hello’) • File->Save As • Desktop/hello.py • Run->Run Module (or F5) to run.

  4. What is Python • Interpreted language • vs. compiled languages like C++ • Simple to use • Designed for programmer’s convenience, not the processor’s • Easy string and list manipulation * • Easy scoping rules • No memory management • Simplified variable typing • Extensible – you can create you own C++ libraries to import • Portable between Windows and Linux * >>> x = ‘this is a test’ >>> dir(x) >>> x.split()

  5. First program • Create the following program in your IDLE editor: print (‘Hello’) for i in range(10): print (i) • Press F5 to run

  6. Variables • Types include: • int • float • bool • string • objects • Type is implied, not specified • Types may be cast • Operations may include variables of different types: • float + int = float • float + string = <error> >>> x = 1 >>> type(x) >>> y = 1.1 >>> type(y) >>> z = ‘1’ >>> type(z) >>> x + y >>> type (x+y) >>> x + z >>> a = (int)(y) >>> type (a) >>> x + (int)(z)

  7. Whitespace matters • Leading whitespace in Python == {} in C++/Java • Run each of these programs and compare the output: • Program A print (‘Hello’) for i in range(10): print (i) print (i * 1000) • Program B print (‘Hello’) for i in range(10): print (i) print (i * 1000) • Now change the indentation randomly to see where it breaks

  8. Conditional Statements • Run the following: x = 1 if(x < 10): print (‘success’) else: print (‘failure’) • Try changing the second line to the following: if(x): if(x == 1): if(x != 2): if(x <= 1): if(not x==2): If(x == 1 or x == 2):

  9. Looping • Option 1: for i in range(10): print (i) • Option 2: x = 1 while(x < 10): print (x) x = x + 1 • Option 3: x = 1 while(1): print (x) x = x + 1 if (x>=10): break Removing which line(s) would cause an infinite loop? Remove it and press F5. Use Ctrl-C to stop the program.

  10. Functions • We can define our own functions: • Run the following: def countdown(x): for i in range (x, 0, -1): print (i) countdown(20) Type the following in the interpreter to learn about the range() arguments: >>> help(range) Note the complete lack of type-checking in the function definition. Try calling the countdown() function with a string or float value to see what happens.

  11. Importing libraries (modules) • We can import libraries that other people have written. • Libraries contain functions, classes, variables and libraries. • Start an interactive console and run the following: >>> import os >>> dir (os) >>> os.listdir (says listdir is a function) >>> os.listdir()(path is optional in Python 3) >>> help(os.listdir) >>> os.__file__ >>> os.__file__()(error) >>> type(os.__file__)

  12. Playing nicely with modules • Python does not have ‘private’ or ‘constant’ objects. • __private__  treat variables with __ as private • CONSTANT treate ALL_CAPS as constants • You can change anything in a module, but should not. • Start an interactive console and run the following: >>> dir() >>> dir (__builtins__) (what is in this module?) >>> __builtins__ = 1 >>> dir()(Why does this give you an error?) • Tip – do not set module values directly. Use module functions to set them.

  13. Two ways to import libraries • Run the following in a new python shell: >>> dir() >>> import socket >>> dir() >>> socket.gethostname() • Run the following in a new python shell: >>> dir() >>> from socket import * >>> dir() >>> gethostname() This method is preferred. Can you figure out two reasons why?

  14. Finding help on modules • List of all modules available for import: >>> help(‘modules’) • List of all functions/variables in a library: >>> import socket >>> dir(socket) • Description of entire library: >>> help(socket) • Description of a specific function or variable in library: >>> help(socket.gethostname) • Online reference for Python 3.2: http://docs.python.org/py3k/py-modindex.html

  15. Next Steps • The installed documentation has a good intro to python: •  ->All Programs->Python3.2->Python Manuals The Python Tutorial  An Informal Introduction • Here is the same information online: http://docs.python.org/py3k/tutorial/ Start in Chapter 3: An Informal Introduction to Python • You will need to be able to use the socket module for next week’s lab. Read about it here: http://docs.python.org/library/socket.html#module-socket

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