Scripts and Utilities -- Python lecture

This file: http://www.cs.utk.edu/~plank/plank/classes/cs494/494/notes/Python/lecture.html

Lecture links: http://www.cs.utk.edu/~plank/plank/classes/cs494/494/notes/Python/links.html

Python manuals: http://tecfa.unige.ch/guides/python (This lecture is taken from the Python tutorial manual)

Getting Started With Python

python

Using Python as a Calculator

>>> 10 + 20
30
>>> (40 + 50) * 6 - 10
530
>>> 7/3
2
>>> 7.0 / 3
2.33333333333

Results can be assigned to variables using the = sign:

>>> a = 10 + 30
>>> a
40
>>> b = a * 20
>>> b
800

Other Python Data Types

Strings

>>> 'spam eggs'
'spam eggs'
>>> 'doesn\'t'
"doesn't"
>>> "doesn't"
"doesn't"
>>> '"Yes, " he said.'
'"Yes, " he said.'
>>> "\"Yes, \" he said."
'"Yes, " he said.'

Strings can be concatenated using the + operator and repeated using the * operator:

>>> word = 'Help' + 'A'
>>> word
'HelpA''
>>> '<' + word*5 + '>'
''

Strings can be subscripted like arrays, with index 0 denoting the first character in the string:

>>> word[4]
'A'

Indices may also be negative numbers, to start counting from the right. -1 denotes the last character in the string, -2 the next to last character, and so on:

>>> word[-1]
'A'
>>> word[-2]
'p'

Python also provides a slicing mechanism that allows you to easily manipulate substrings. A slice is written as two inidces separated by a colon:

>>> word[0:2]
'He'
>>> word [2:4]
'lp'

If you omit the first index it defaults to 0. If you omit the second index, it defaults to the length of the string:

>>> word[:2]
'He'
>>> word[2:]
'lpA'

len returns the length of a string:

>>> len(word)
5

Strings are immutable so trying to modify an element of a string will result in an error:

>>> word[3] = 'a'
Traceback (innermost last):
  File "", line 1, in ?
TypeError: can't assign to this subscripted object

Lists

>>> a = [10, 20, 'brad', 'knoxville']
>>> a
[10, 20, 'brad', 'knoxville']
>>> a[1]
20
>>> a[1:3]
[20, 'brad']
>>> a[-1]
'knoxville'

The subscripting rules for lists are identical to the subscripting rules for strings.

Unlike strings, lists can be modified:

>>> a[1] = 50
>>> a
[10, 50, 'brad', 'knoxville']
>>> a[1:3] = [80, 'hello']
>>> a
[10, 80, 'hello', 'knoxville']
>>> a[0:1] = [20, 60, 70]    # lists can be expanded or shrunk
[20, 60, 70, 80, 'hello', 'knoxville']

Dictionaries

>>> tel = {'jack': 4098, 'sape': 4139}
>>> tel['guido'] = 4127
>>> tel
{'sape': 4139, 'guido': 4127, 'jack': 4098}
>>> tel['jack']
4098
>>> del tel['sape']
>>> tel['irv'] = 4127
>>> tel
{'guido': 4127, 'irv': 4127, 'jack': 4098}
>>> tel.keys()
['guido', 'irv', 'jack']
>>> tel.has_key('guido')
1
>>>

Control Flow Tools

Conditionals

>>> if x < 0:
...      x = 0
...      print 'Negative changed to zero'
... elif x == 0:
...      print 'Zero'
... elif x == 1:
...      print 'Single'
... else:
...      print 'More'

 Loops 

 For Loops 
>>> # Measure some strings:
    a = ['cat', 'window', 'defenestrate']
>>> for x in a:
        print x, len(x)
>>>


How to insert items into a list--It is not safe to insert into a list
while iterating over it:
>>> for x in a[:]: # make a slice copy of the entire list
        if len(x) > 6: a.insert(0, x)

>>> a
['defenestrate', 'cat', 'window', 'defenestrate']
>>>


>>> sum = 0
>>> for i in range(10):
        sum = sum + i
>>> print sum


 While Loop 

>>> a = range(10)
>>> i = 0
>>> while (i < 10):
...     if a[i] == 5:
...	    print i
...	    break
...     else:
...	    i = i + 1
... else:
...     print 'could not find 5'


 Functions 
>>> true = 1
>>> false = 0

>>> def dfs(node):
...    node.visited = true
...    for neighbor in node.neighbors:
...	   if neighbor.visited == false:
...	       dfs(neighbor)


 Classes 
>>> class node:
...     pass


Without initialization function:
>>> x = node()
>>> x.visited = false
>>> x.neighbors = []


With initialization function:
>>> class node:
...    def __init__(self):
...        self.visited = false
...  	   self.neighbors = []

>>> x = node()