Transcript And Now For Something Completely Different

“And Now For Something
Completely Different”
A Quick Tutorial of the Python
Programming Language
Michael Scherger
Department of Computer Science
Kent State University
January 24, 2006
And Now For Something
Completely Different
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Contents
•
•
•
•
•
•
Python Resources
Python Basics and Language Elements
Statements and Control Structures
Input, Output, and File Access
Functions
Classes and Objects
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Python Resources
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Python Resources
• Python Web Site: http://www.python.org
• Check out the tutorial at:
http://www.python.org/doc/tut
• Download current stable version for your
PC
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Running Python
• PC
– c:\>python
– Use Python GUI called ‘IDLE’
• Linux
• % python
• Mac version available
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Running Python
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Running Python
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Python Basics and Language
Elements
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Numbers
• Integers
– Decimal
• At least 32 bit
– Octal and hexadecimal
• Long integers
– Limited by memory size
– Trailing “L”
• Floating point
• Complex
– Use “j” instead of “i”
• Boolean
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>>> 1+1
2
>>> 1+1.0
2.0
>>> 1+1e0
2.0
>>>
>>> 999999999999999
999999999999999L
>>> 9999999999999999999999999999
9999999999999999999999999999L
>>>99999999999999999999999999999999999999999
99999999999999999999999999999999999999999L
>>>
>>> 17
17
>>> 021
17
>>> 0x11
17
>>>
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Variables
• Variables do not have
types
• You do not declare
variables
– They appear when you
assign them
– They disappear when
you do not use them
anymore
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>>> x = 1 + 1
>>> x
2
>>> x = 1 + 1.0
>>> x
2.0
>>>
>>> y
Traceback (most recent call last):
File "<pyshell#20>", line 1, in -toplevely
NameError: name 'y' is not defined
>>> y = None
>>> y
>>> y == None
True
>>> del y
>>> y
Traceback (most recent call last):
File "<pyshell#25>", line 1, in -toplevely
NameError: name 'y' is not defined
>>>
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Lists
• Python’s array like
object is a list
– Uses [ ]
– Subscripted like arrays
in C (zero based)
– Mutable
>>>
>>>
5
>>>
>>>
[4,
>>>
3
>>>
z = [4, 5, 6]
z[1]
z[1] = 7
z
7, 6]
len( z )
– Use len(L) to get the
length of the list
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Lists
• Indexing
– Uses bracket notation
– Indexes start at 0…can have a negative index
name = [“Cleese”, “John”]
print name[1], name[0]
name[0] = “Smith”
x=[[1,2,3],[y,z],[[[]]]]
0
“A”
-5
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1
“B”
-4
# example of strings in a list
# prints “John Cleese”
# example of lists in a list
2
“C”
-3
3
“D”
-2
4
“E”
-1
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Lists
• Slicing is similar to indexing except you
indicate both the start and stop index
separated by a colon
x =
["spam1","spam2","spam3","spam4","spam5","eggs","and","spam6"]
print x[5:7] # Prints the list ["eggs","and"]
0
1
“A”
-5
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-4
2
“B”
3
“C”
-3
-2
5
4
“D”
“E”
-1
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List Methods
• append()
add an element to the
end of the list
• pop()
remove an element
from the end of a list
• insert()
add an element
anywhere in the list
• + and *
list concatenation and
replication
• And a whole bunch more…look them up!
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Strings
• A string is a sequence of
characters between quotes;
either “ “ or ‘ ‘
– There is no character data
type in Python…only string
• Strings are not arrays of
characters as in C/C++!
– Use subscript notation to read
individual characters as a
string
– Strings are immutable
– Use len(S) to get the length of
the string
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>>>
>>>
'a'
>>>
3
>>>
x = "abc"
x[0]
len(x)
x[0] = 'd'
Traceback (most recent
call last):
File "<pyshell#34>",
line 1, in -toplevelx[0] = 'd'
TypeError: object doesn't
support item assignment
>>>
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Converting Values
• String values can be
converted to integers
using the int() function
• Example
• String values can be
converted to floating
points using the float()
function
x = int( “10” )
y = float( “10”)
yy = float (10)
Z = str(3.14159)
• Ints and floats can be
converted to string values
str() function
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String Methods
upper()
Returns the uppercase version of the string
lower()
Returns the lowercase version of the string
swapcase()
Returns a new string where the case of each letter is
switched
capitalize()
Returns a new string with the first letter capitalized and the
remaining letters are in lowercase
title()
Returns a new string with the first letter of each word
capitalized and all other letters are in lower case
strip()
Returns a new string with leading and trailing white space
removed.
replace( old, new, [,max])
Returns a new string where occurrences of the string “old”
are replaced by “new” up to “max” number of times
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Tuples
• Tuples are like lists
– Immutable
– Use ( ) notation instead of [ ]’s
for creation
– Uses [ ] notation for indexing
and slicing
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>>> x = (1, "Mike", 2, Laurie", 3,
"Sarah")
>>> x
(1, 'Mike', 2, 'Laurie', 3, 'Sarah')
>>> x[1] = 'Michael'
Traceback (most recent call last):
File "<pyshell#46>", line 1, in toplevelx[1] = 'Michael'
TypeError: object doesn't support
item assignment
>>>
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Dictionaries
• Dictionaries are unordered lists
– Associative array/list or table or map
– List of “key/value” pairs
– Use “key” to look up the element or value
person = {'first name': "Robin",
'last name’ : "Hood",
'occupation': "Scoundrel" }
person[‘last name’] = “of Locksley”
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Dictionary Methods
• keys()
returns a list of keys
• values()
returns a list of values
• items()
returns a list of key
value pairs
• has_key()
tests if a key exists
• And a whole bunch more…look them up!
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Assignments
• Assignment uses =
• Equality uses ==
• Not Equal uses != or <>
x, y, z = 1, 2, 3
first, second = second, first
a = b = 123
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Other Assignment Operators
• Augmented assignment operators
– A combination of assignment and a mathematical
operation
•
•
•
•
•
*=
/=
%=
+=
-=
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x *= 5
x /= 5
x %=5
x += 5
x -= 5
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x=x*5
x=x/5
x=x%5
x=x+5
x=x-5
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Logical Values
• Python 2.3 has a Boolean type
– Prior versions do not
– False is zero or empty
– True is not False (non-zero or
not empty)
– And, Or, Not are shortcircuited
•
•
•
•
•
•
•
•
•
•
1 < 2
1 > 2
[1,2] < [3,4]
[1,2] == [1,2]
“ab” == “a” + “b”
(1,2) == [1,2]
not []
not [1,2,3]
not ‘’
not ‘ab’
# False
#
#
#
#
#
#
#
#
#
True
False
True
True
True
False
True
False
True
– Can do lexicographic
comparison of sequences
(lists, strings, tuples)
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Statements and Control
Structures
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Blocks
• Blocks are indicated using indentation only
– No BEGIN/END or { }
if x < 5 or (x > 10 and x < 20):
print “The value is OK.”
if x < 5 or 10 < x < 20:
print “The value is OK.”
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If-Elif-Else
if color == Green:
do_green_function()
elif color == Blue:
do_blue_function()
elif color == Red:
do_red_function()
else:
report_error()
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While Loops
• Example of a while loops
x = 10
while x >= 0:
print “x is still negative.”
x = x-1
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For Loops
• To make an “ordinary” for loop use the
built-in function range()
# Print out the values from 0
# to 99 inclusive
for value in range(100):
print value
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For Loops
• More examples of for loops
# print values in a list
for value in [1, 2, 3, 4, 5, 4, 3, 2, 1]:
print value
# print 10 thru 19
for value in range(10,20):
print value
# print 10 12 14 16 18
for value in range(10, 20, 2):
print value
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Input, Output, and File Access
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Getting User Input
• The raw_input function returns a string.
– Be careful!
• 10 is not the same as “10”
• Other functions convert strings to integers and vice
versa.
• Example: Personal Greeter
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A Tiny Algorithm
• Prompt user for a number
• Print out the square of the number
x = input( “Please enter a number: “)
print “The square of that number is”, x * x
• Could also use…
x = raw_input( “Please enter a number: “)
x = int(x)
print “The square of that number is”, x * x
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File Input / Output
• Example
filespec = open( “somefile”, “r” )
s = filespec.readline()
s = filespec.readlines()
# read one line
# read all lines
filespec.close()
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File Input / Output
• Example
filespec = open( “somefile”, “w” )
filespec.writeline(s)
filespec.writelines(l)
# write one string
# write string list
filespec.close()
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Functions
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Functions
• Use the keyword def
–
–
–
–
Passing parameters
Named arguments
Default values
Variable scoping
def square(x):
return x*x
print square(2)
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# prints out 4
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Functions
• When you pass a parameter to a function, you
bind the parameter to the value
• Creates a new reference
def change(some_list):
some_list[1] = 4
x = [1,2,3]
change(x)
print x # Prints out [1,4,3]
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Functions
• Another example
def nochange(x):
x = 0
y = 1
nochange(y)
print y # Prints out 1
• Example: Balanced Parentheses and Fibonocci
Numbers
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Functions
• Example
>>>def sumdif(a, b):
return a+b, a-b
>>>sumdif( 5, 7 )
(12,2)
>>>
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Putting It All Together
• Write a Python version of wc (word count)
– Writes the counts of the number of characters, words,
and lines in a file
• Algorithm
–
–
–
–
open the file
count the characters, words, and lines
write out the counts
close the file
• Example: wc
– % python wc.py somefile.ext
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Classes and Objects
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Classes and Objects
class Basket:
def __init__(self, contents=None):
self.contents = contents or []
def add(self, element):
self.contents.append(element)
def print_me(self):
result = “”
for element in self.contents:
result = result + “ “ + `element`
print “Contains: “ + result
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Classes and Objects
•
All methods (functions in an object) received an additional argument at the
start of the argument list containing the object itself. (Called self which is
customary)
•
Methods are called like this: object.method( arg1, arg2)
•
Some method names, like __init__ (two underscores on each side), are
predefined and mean special things (constructor)
•
Some arguments are optional and given and default value
•
Short circuit evaluation and assignment
•
Backquotes convert an object to its string representation
•
Addition sign (+) is used for string concatenation
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Classes and Objects
• Instead of short circuit to assign contents…could
have written the statement like this
if contents:
self.contents = contents
else:
self.contents = []
def __init__(self,contents=[])
self.contents = contents[:]
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Classes and Objects
• To create an instance of a class (object)
a = Basket()
b = Basket( [“apple”, “lemon”])
b.add( “orange” )
b.print_me()
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Classes and Objects
• Convert object to string
– Use the __str__ method
def __str__(self)
result = “”
for element in self.contents:
result = result + “ “ + `element`
return “Contains: “ + “ “ + result
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Classes and Objects
•
Example: Basket Class
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
>>> b = Basket()
>>> b
<__main__.Basket instance at 0x00A09B98>
>>> print b
Contains:
>>> b.add( "Foo" )
>>> print b
Contains: 'Foo'
>>> b.add (["Foo", "Bar"])
>>> b.print_me()
Contains: 'Foo' ['Foo', 'Bar']
>>> b.add ((1,2,3))
>>> print b
Contains: 'Foo' ['Foo', 'Bar'] (1, 2, 3)
>>>
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