An Introduction to Python

1
An Introduction to Python

• Dr. Nancy Warter-Perez
• April 15, 2004

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

3
Programming Language and Development Software

• In this program, well 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!)

4
Program Development
Problem solving
Problem specification
Algorithm design
Test by hand
Code in target language
Implementation
Test code / debug
Program
5
Python Basics - Comments

• Python comments
• line comment
• Header comments
• Description of program
• Written by
• Date created
• Last Modified

6
Python Basics - Variables

• Python variables are not declared.
• To assign a variable, just type
  identifierliteral
• 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)

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Numbers

• Numbers
• Normal Integers represent whole numbers
• Ex 3, -7, 123, 76
• Long Integers unlimited size
• Ex 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 34j, 3.04.0j, 3J

8
Python Basics arithmetic operations
Operators Example

• add
• - subract
• multiply
• / divide
• modulus/remainder

y5 z3 x y z x y z x y z x y
/ z x y z
x 8 x 2 x 15 x 1 x 2
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Python Basics arithmetic operations
Operators Example

• ltlt shift left
• gtgt shift right
• raise to power

y5 z3 x y ltlt 1 x y gtgt 2 x y z
x 10 x 1 x 125
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Python Basics Relational and Logical Operators

• Relational operators
• equal
• !, ltgt not equal
• gt greater than
• gt greater than or
• equal
• lt less than
• lt less than or equal

• Logical operators
• and and
• or or
• not not

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Python Basics Relational Operators

• Assume x 1, y 4, z 14

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Python Basics Logical Operators

• Assume x 1, y 4, z 14

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Strings

• Enclosed in single or double quotes
• Ex Hello! , Hello!, 3.5, a, a
• Sequence of charactersmystringhello world!
• mystring0 -gt h mystring1 -gt e
• mystring2 -gt l mystring-1 -gt !

-1 is last, -2 next to last, etc
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String operations
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Strings (2)

• substrings can be reassignedmystringspoonsmy
  string5!mystring -gt spoon!
• slicingmystring2 -gt oon!mystring3 -gt
  spo note last element is never
  included!mystring13-gt po

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Strings (3)

• operatorsort of fill in the blanks
  operationmystrings has n marbles
  (John,35) mystring -gt John has 35 marbles
• s replace with string
• n,i replace with integer
• f replace with float

17
Lists
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Tuples

• Tuples sequence of valueslike lists, but
  cannot be changed after it is createdmytuple(1,2
  ,3,4)mytuple(1,a,bc,3,87.2)mytuple13
• Used when you want to pass several variables
  around at once

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Dictionaries

• Dictionaries map keys to values
• like lists, but indices can be of any type
• Also, keys are in no particular order
• Egmydictb3, a4, 752.85mydictb -gt
  3mydict75 -gt 2.85mydicta -gt 4

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Dictionaries
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Dictionaries other considerations

• Slicing not allowed
• Referencing invalid key is an error
• gtgtgt mydict8.5 8, 'a' 75, 'r' 1, 'g' 2, 'y'
  3.5, 9 'nine'
• gtgtgt mydict"red"
• Traceback (most recent call last) File
  "ltinteractive inputgt", line 1, in ?
• KeyError 'red
• Use mydict.get(red) instead, it returns None if
  key is not found

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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)-gtconvert to int (if possible)
• float(mystring)-gtconvert to float (if possible)

gtgtgt mystrraw_input("Enter a string") Enter a
stringHello World! gtgtgt mystr 'Hello World!'
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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 abc
• abc

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Output Example

• gtgtgt print "hello","world"print "hello","again"
• hello world
• hello again
• gtgtgt print "hello","world",print "hello","again"
• hello world hello again
• gtgtgt print "hello s world" "cold and cruel"
• hello cold and cruel world
• gtgtgt print "hello","cold" " "
  "and","cruel","world"
• hello cold and cruel world

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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
• Dont override!! this is how python tells when
  block statements end!
• If doesnt indent to proper location indicates
  bug

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Running your Program

• To build your program
• Under File-gtRun
• Select No Debugging in the drop-down window
• Fix any errors, then run again

27
Programming Workshop 1

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

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