1B-Introduction_to_python.ppthdsadfzadfe

1B-Introduction_to_python.ppthdsadfzadfe

• 1.
  Introduction to Python Genome559: Introduction to Statistical and Computational Genomics Prof. William Stafford Noble
• 2.
  Why Python? • Pythonis – easy to learn, – relatively fast, – object-oriented, – strongly typed, – widely used, and – portable. • C is much faster but much harder to use. • Java is about as fast and slightly harder to use. • Perl is slower, is as easy to use, but is not strongly typed.
• 3.
  Getting started onthe Mac • Start a terminal session. • Type “python” • This should start the Python interpreter > python Python 2.4.2 (#2, Apr 10 2006, 16:28:28) [GCC 3.2.3 20030502 (Red Hat Linux 3.2.3-53)] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> print “hello, world!” hello, world!
• 4.
  The interpreter • Tryprinting various things – Leave off the quotation marks. – Print numbers, letters and combinations. – Print two things, with a comma between. – Enter a mathematical formula. – Leave off the word “print”. • The interpreter allows you to try things out interactively and quickly. • Use the interpreter to test syntax, or to try commands that you’re not sure will work when you run your program.
• 5.
  Your first program •In your terminal, Ctrl-D out of python. • Type “pwd” to find your current working directory. • Open TextWrangler. • Create a file containing one line: print “hello, world!” • Be sure that you end the line with a carriage return. • Save the file as “hello.py” in your current working directory. • In your terminal, type “python hello.py” > python hello.py hello, world! Notice that, once you save the file with “.py” as the extension, WordWrangler automatically colors the text according to the syntax.
• 6.
  Objects and types •We use the term object to refer to any entity in a python program. • Every object has an associated type, which determines the properties of the object. • Python defines six types of built-in objects: Number 10 String “hello” List [1, 17, 44] Tuple (4, 5) Dictionary {‘food’ : ‘something you eat’, ‘lobster’ : ‘an edible, undersea arthropod’} Files • Each type of object has its own properties, which we will learn about in the next several weeks. • It is also possible to define your own types, comprised of combinations of the six base types.
• 7.
  Literals and variables •A variable is simply another name for an object. • For example, we can assign the name “pi” to the object 3.14159, as follows: >>> pi = 3.14159 >>> print pi 3.14159 • When we write out the object directly, it is a literal, as opposed to when we refer to it by its variable name.
• 8.
  The “import” command •Many python functions are only available via “packages” that must be imported. >>> print log(10) Traceback (most recent call last): File "<stdin>", line 1, in ? NameError: name 'log' is not defined >>> import math >>> print math.log(10) 2.30258509299
• 9.
  The command line •To get information into a program, we will typically use the command line. • The command line is the text you enter after the word “python” when you run a program. import sys print "hello, world!" print sys.argv[0] print sys.argv[1] • The zeroth argument is the name of the program file. • Arguments larger than zero are subsequent elements of the command line.
• 10.
  Sample problem #1 •Write a program called “print-two-args.py” that reads the first two command line arguments after the program name, stores their values as variables, and then prints them on the same line with a colon between. > python print-two-args.py hello world hello : world
• 11.
  Solution #1 import sys arg1= sys.argv[1] arg2 = sys.argv[2] print arg1, ":", arg2
• 12.
  Sample problem #2 •Write a program called “add-two-args.py” that reads the first two command line arguments after the program name, stores their values as variables, and then prints their sum. • Hint: To read an argument as a number, use the syntax “arg1 = float(sys.argv[1])” > python add-two-args.py 1 2 3.0
• 13.
  Solution #2 import sys arg1= float(sys.argv[1]) arg2 = float(sys.argv[2]) print arg1 + arg2
• 14.
  Reading • Chapter 1-3of Learning Python (3rd edition) by Lutz.