An Introduction to Python Dr. Nancy Warter-Perez April 15, 2004.

An Introduction to Python

Dr. Nancy Warter-PerezApril 15, 2004

4/15/04 Introduction to Python 2

Overview Overview of program development Python Basics Python Types and Operators

Numbers and Arithmetic operators Strings Lists Dictionaries

Input & Output Example amino acid search program Programming Workshop #1


Programming Language and Development Software In this program, we’ll use Python

Interpretive Language Development software

IDLE python gui Pythonwin (recommended)

Do your work on either the hard disk or zip disk (not floppy disk, A: drive – too slow!)


Program DevelopmentProblem specification

Algorithm design

Test by hand

Code in target language

Test code / debug

Program

Problem solving

Implementation


Python Basics - Comments Python comments

# line comment Header comments

#Description of program#Written by:#Date created:#Last Modified:


Python Basics - Variables Python variables are not “declared”.

To assign a variable, just type: identifier=literal Identifiers

Have the following restrictions: Must start with a letter or underscore (_) Case sensitive Must consist of only letters, numbers or underscore Must not be a reserved word (LP pg 137)

Have the following conventions: All uppercase letters are used for constants Variable names are meaningful – thus, often multi-word

Convention 1: alignment_sequence Convention 2: AlignmentSequence

Python specific conventions: Avoid _X, __X__, __X, _, (LP pg 138)


Numbers Numbers

Normal Integers –represent whole numbers Ex: 3, -7, 123, 76

Long Integers – unlimited sizeEx: 9999999999999999999999L

Floating-point – represent numbers with decimal places

Ex: 1.2, 3.14159,3.14e-10 Octal and hexadecimal numbers

Ex: O177, 0x9ff, Oxff Complex numbers

Ex: 3+4j, 3.0+4.0j, 3J


Python Basics – arithmetic operations

+ add- subract* multiply/ divide% modulus/remainder

y=5; z=3x = y + z x = y – z x = y * z x = y / z x = y % z

x = 8x = 2x = 15x = 1x = 2

OperatorsExample


Python Basics – arithmetic operations

<< shift left

>> shift right** raise to power

y=5; z=3x = y << 1 x = y >> 2 x = y ** z

x = 10x = 1x = 125

OperatorsExample


Python Basics – Relational and Logical Operators

Relational operators== equal!=, <> not equal>greater than>= greater

than or equal

<less than<= less than or

equal

Logical operatorsand andor ornot not


Python Basics – Relational Operators Assume x = 1, y = 4, z = 14

Expression Value Interpretation

x < y + z 1 True

y == 2 * x + 3

0 False

z <= x + y 0 False

z > x 1 True

x != y 1 True


Python Basics – Logical Operators Assume x = 1, y = 4, z = 14

Expression Value Interpretation

x<=1 and y==3 0 False

x<= 1 or y==3 1 True

not (x > 1) 1 True

not x > 1 0 False

not (x<=1 or y==3)

0 False


Enclosed in single or double quotesEx: ‘Hello!’ , “Hello!”, “3.5”, “a”, ‘a’

Sequence of characters:mystring=“hello world!”

mystring[0] -> “h” mystring[1] -> “e”

mystring[2] -> “l” mystring[-1] -> “!”

Strings

-1 is last,

-2 next to last, etc…


String operations

mystring = “Hello World!”

Expression Value Purposelen(mystring) 12 number of characters in

mystring

“hello”+“world” “helloworld” Concatenate strings

“%s world”%“hello” “hello world” Format strings (like sprintf)

“world” == “hello”

“world” == “world”

0 or False

1 or True

Test for equality

“a” < “b”

“b” < “a”

1 or True

0 or False

Alphabetical ordering


Strings (2) substrings can be reassigned:mystring=“spoons”mystring[5]=“!”mystring -> “spoon!”

slicing:mystring[2:] -> “oon!”mystring[:3] -> “spo” #note last element is never included!

mystring[1:3]-> “po”


Strings (3) “%” operator:

sort of “fill in the blanks” operation:mystring=“%s has %n marbles” % (“John”,35)

mystring -> “John has 35 marbles”

%s replace with string %n,%i replace with integer %f replace with float

Values to put in blanks

“blanks”


Lists

mylist=[“a”,”b”,3.58,”d”,4,0]mylist[0]mylist[2]

a3.58

Indexing

mylist[-1]mylist[-2]

04

Negative indexing (counts from end)

mylist[1:4] [“b”,3.58,”d”] Slicing (like strings)

“b” in mylist“e” not in mylist

1 or True1 or True

mylist.append(8) [“a”,”b”,3.58,”d”,4,0,8]

Add to end of list


Tuples Tuples – sequence of values

like lists, but cannot be changed after it is createdmytuple=(1,2,3,4)mytuple=(1,”a”,”bc”,3,87.2)mytuple[1]=“3”

Used when you want to pass several variables around at once

Error!


Dictionaries Dictionaries – map ‘keys’ to ‘values’

like lists, but indices can be of any type Also, keys are in no particular order Eg:mydict={‘b’:3, ’a’:4, 75:2.85}mydict[‘b’] -> 3mydict[75] -> 2.85mydict[‘a’] -> 4


Dictionaries

mydict={“r”:1,”g”:2,”y”:3.5,8.5:8,9:”nine”}mydict.keys() ['y', 8.5, 'r', 'g', 9] List of the keys

mydict.values() [3.5, 8, 1, 2, 'nine'] List of the values

mydict[“y”] 3.5 Value lookup

mydict.has_key(“r”) True or 1 Check for keys

mydict.update({“a”:75})

{8.5: 8, 'a': 75, 'r': 1, 'g': 2, 'y': 3.5, 9: 'nine'}

Add pairs to dictionary


Dictionaries – other considerations Slicing not allowed Referencing invalid key is an error:>>> mydict={8.5: 8, 'a': 75, 'r': 1, 'g': 2, 'y':

3.5, 9: 'nine'}>>> mydict["red"]Traceback (most recent call last):

File "<interactive input>", line 1, in ?KeyError: 'red‘

Use mydict.get(“red”) instead, it returns None if key is not found


Input/Output Function raw_input() designed to read a line of

input from the user 1 optional argument: string to prompt user If int or float desired, simply convert string:

int(mystring)->convert to int (if possible)

float(mystring)->convert to float (if possible)

>>> mystr=raw_input("Enter a string:")Enter a string:Hello World!>>> mystr'Hello World!'


Output Function print

Prints each argument, followed by space

After all arguments, prints newline

Put comma after last arg to prevent newline

“add” strings to avoid spaces

print “a”,”b”,”c”a b c

print “a”,”b”,”c”,a b c

print “a”+”b”+”c”abc

Newline!

No Newline!

No spaces!


Output Example>>> print "hello","world";print "hello","again"

hello world

hello again

>>> print "hello","world",;print "hello","again"

hello world hello again

>>> print "hello %s world" % "cold and cruel"

hello cold and cruel world

>>> print "hello","cold"+ " " + "and","cruel","world"

hello cold and cruel world


Creating a Python Program Enter your program in the editor

Notice that the editor has a color coding Comments Key words Etc…

Also notice that it automatically indents Don’t override!! – this is how python tells when

block statements end! If doesn’t indent to proper location – indicates bug


Running your Program To build your program

Under File->Run… Select No Debugging in the drop-down

window Fix any errors, then run again


Programming Workshop #1

Write a Python program to compute the hydrophobicity of an amino acid

Amino Acid Hydrop. VALUEA 1.8C 2.5D -3.5E -3.5F 2.8G -0.4H -3.2I 4.5K -3.9L 3.8M 1.9N -3.5P -1.6Q -3.5R -4.5S -0.8T -0.7V 4.2W -0.9Y -1.3

Program will prompt the user for an amino acid and will display the hydrophobicity

An Introduction to Python Dr. Nancy Warter-Perez April 15, 2004.

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