python
at the Unix
command prompt.
[dm533@woglinde ~/tutorial]$ python
Python 2.5.2 (r252:60911, Oct 5 2008, 19:24:49)
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
IPython can be used
as a system shell replacement, especially on Windows, which has a
minimally capable shell. It offers enhanced introspection, additional
shell syntax, code highlighting, tab completion string completion and
rich history. It is a component of the SciPy package.
The Python interpeter can be used to evaluate expressions, for example simple arithmetic expressions.
If you enter such expressions at the prompt (>>>
) they will
be evaluated and the result wil be returned on the next line.
>>> 1 + 1
2
>>> 2 * 3
6
>>> 2 ** 3
8
The **
operator in the last example corresponds to exponentiation.
+
operator is overloaded
to do string concatenation on string values.
>>> 'artificial' + "intelligence"
'artificialintelligence'
>>> 'artificial'.upper()
'ARTIFICIAL'
>>> 'HELP'.lower()
'help'
>>> len('Help')
4
' '
or double quotes " "
to surround string.
>>> s = 'hello world'
>>> print s
hello world
>>> s.upper()
'HELLO WORLD'
>>> len(s.upper())
11
>>> num = 8.0
>>> num += 2.5
>>> print num
10.5
dir
and help
commands:
>>> s = 'abc'
>>> dir(s)
['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', '__ge__',
'__getattribute__', '__getitem__', '__getnewargs__', '__getslice__', '__gt__', '__hash__', '__init__','__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__','__repr__', '__rmod__', '__rmul__', '__setattr__', '__str__', 'capitalize', 'center',
'count', 'decode', 'encode', 'endswith', 'expandtabs', 'find', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'replace', 'rfind','rindex', 'rjust', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']
>>> help(s.find)
Help on built-in function find:
find(...)
S.find(sub [,start [,end]]) -> int
Return the lowest index in S where substring sub is found,
such that sub is contained within s[start,end]. Optional
arguments start and end are interpreted as in slice notation.
Return -1 on failure.
>> s.find('b')
1
Try out some of the string functions listed in dir
(for now, ignore those with underscores '_' around the method name).
>>> fruits = ['apple','orange','pear','banana']
>>> fruits[0]
'apple'
We can use the +
operator to do list concatenation:
>>> otherFruits = ['kiwi','strawberry']
>>> fruits + otherFruits
>>> ['apple', 'orange', 'pear', 'banana', 'kiwi', 'strawberry']
Python also allows negative-indexing from the back of the list.
For instance, fruits[-1]
will access the last
element 'banana'
:
>>> fruits[-2]
'pear'
>>> fruits.pop()
'banana'
>>> fruits
['apple', 'orange', 'pear']
>>> fruits.append('grapefruit')
>>> fruits
['apple', 'orange', 'pear', 'grapefruit']
>>> fruits[-1] = 'pineapple'
>>> fruits
['apple', 'orange', 'pear', 'pineapple']
fruits[1:3]
which returns a list containing
the elements at position 1 and 2. In general fruits[start:stop]
will get the elements in start, start+1, ..., stop-1
. We can
also do fruits[start:]
which returns all elements starting from the start
index. Also fruits[:end]
will return all elements before the element at position end
:
>>> fruits[0:2]
['apple', 'orange']
>>> fruits[:3]
['apple', 'orange', 'pear']
>>> fruits[2:]
['pear', 'pineapple']
>>> len(fruits)
4
The items stored in lists can be any Python data type. So for instance
we can have lists of lists:
>>> lstOfLsts = [['a','b','c'],[1,2,3],['one','two','three']]
>>> lstOfLsts[1][2]
3
>>> lstOfLsts[0].pop()
'c'
>>> lstOfLsts
[['a', 'b'],[1, 2, 3],['one', 'two', 'three']]
dir
and
get information about them via the help
command:
>>> dir(list)
['__add__', '__class__', '__contains__', '__delattr__', '__delitem__',
'__delslice__', '__doc__', '__eq__', '__ge__', '__getattribute__',
'__getitem__', '__getslice__', '__gt__', '__hash__', '__iadd__', '__imul__',
'__init__', '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__',
'__new__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__',
'__rmul__', '__setattr__', '__setitem__', '__setslice__', '__str__',
'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse',
'sort']
>>> help(list.reverse) Help on built-in function reverse: reverse(...) L.reverse() -- reverse *IN PLACE*
>>> lst = ['a','b','c']
>>> lst.reverse()
>>> ['c','b','a']
Note: Ignore functions with underscores "_" around the names; these are private helper methods.
>>> pair = (3,5)
>>> pair[0]
3
>>> x,y = pair
>>> x
3
>>> y
5
>>> pair[1] = 6
TypeError: object does not support item assignment
The attempt to modify an immutable structure raised an exception. This is how many errors will manifest: index out of bounds errors, type errors, and so on will all report exceptions in this way.
>>> studentIds = {'aria': 42.0, 'arlo': 56.0, 'john': 92.0 }
>>> studentIds['arlo']
56.0
>>> studentIds['john'] = 'ninety-two'
>>> studentIds
{'aria': 42.0, 'arlo': 56.0, 'john': 'ninety-two'}
>>> del studentIds['aria']
>>> studentIds
{'arlo': 56.0, 'john': 'ninety-two'}
>>> studentIds['aria'] = [42.0,'forty-two']
>>> studentIds
{'aria': [42.0, 'forty-two'], 'arlo': 56.0, 'john': 'ninety-two'}
>>> studentIds.keys()
['aria', 'arlo', 'john']
>>> studentIds.values()
[[42.0, 'forty-two'], 56.0, 'ninety-two']
>>> studentIds.items()
[('aria',[42.0, 'forty-two']), ('arlo',56.0), ('john','ninety-two')]
>>> len(studentIds)
3
As with nested lists, you can also create dictionaries of dictionaries.
Python data types also include set
that eliminates
duplicate entries:
>>> set(['a','b','a'])
set(['a', 'b'])
Exercise: Use dir
and help
to learn about the functions you can call on dictionaries.
Exercise: How would you use the dictionary type
in order to represent a set (rather than a list) of unique items?
for
loop. Open the file called foreach.py and update it with the
following code:
# This is what a comment looks like
fruits = ['apples','oranges','pears','bananas']
for fruit in fruits:
print fruit + ' for sale'
fruitPrices = {'apples': 2.00, 'oranges': 1.50, 'pears': 1.75}
for fruit, price in fruitPrices.items():
if price < 2.00:
print '%s cost %f a pound' % (fruit, price)
else:
print fruit + ' are too expensive!'
At the command line, use the following command in the directory
containing foreach.py:
[dm533@woglinde ~/tutorial]$ python foreach.py
apples for sale
oranges for sale
pears for sale
bananas for sale
oranges cost 1.500000 a pound
pears cost 1.750000 a pound
apples are too expensive!
nums = [1,2,3,4,5,6]
plusOneNums = [x+1 for x in nums]
oddNums = [x for x in nums if x % 2 == 1]
print oddNums
oddNumsPlusOne = [x+1 for x in nums if x % 2 ==1]
print oddNumsPlusOne
Put this code into a file called listcomp.py and run the script:
[dm533@woglinde ~/tutorial]$ python listcomp.py
[1,3,5]
[2,4,6]
There are two ways you can use Emacs to develop Python code. The
most straightforward way is to use it just as a text editor: create and
edit Python files in Emacs; then run Python to test the code somewhere
else, like in a terminal window. Alternatively, you can run Python
inside Emacs: see the options under "Python" in the menubar, or type
C-c !
to start a Python interpreter in a split screen.
(Use C-x o
to switch between the split screens). See here for Emacs keybindings.
For advanced debugging, you may want to use an IDE like Eclipse. In that case, you should refer to PyDev.
Exercise: Write a list comprehension which, from a list, generates a lowercased version of each string that has length greater than five. Solution
if 0 == 1: print 'We are in a world of arithmetic pain' print 'Thank you for playing'will output
Thank you for playing
But if we had written the script as
if 0 == 1:
print 'We are in a world of arithmetic pain'
print 'Thank you for playing'
there would be no output. The moral of the story: be careful how you indent! Its best to use a single tab for
indentation.
fruitPrices = {'apples':2.00, 'oranges': 1.50, 'pears': 1.75}
def buyFruit(fruit, numPounds):
if fruit not in fruitPrices:
print "Sorry we don't have %s" % (fruit)
else:
cost = fruitPrices[fruit] * numPounds
print "That'll be %f please" % (cost)
# Main Function
if __name__ == '__main__':
buyFruit('apples',2.4)
buyFruit('coconuts',2)
Rather than having a main
function as in Java, the __name__ == '__main__'
check is
used to delimit expressions which are executed when the file is called as a
script from the command line.
Read this
thread on how to write main
functions.
Save this script as fruit.py and run it:
[dm533@woglinde ~/tutorial]$ python fruit.py
That'll be 4.800000 please
Sorry we don't have coconuts
Be aware that in Python, all argument passing is
Exercise (for submission): Add some more fruit to the
fruitPrices
dictionary and add a buyLotsOfFruit(orderList)
function which takes a list of (fruit,pound)
tuples and returns
the cost of your list. If there is some fruit
in the list which
doesn't appear in fruitPrices
it should print an error message and
return None
.
Solution
This function should be defined in a file called buyLotsOfFruits.py
.
A stub implementation is provided here. Note that
the fruitPrices
variable must be set exactly as it is in the stub.
Test Case:You can "sanity check" this portion of your code by testing that
buyLotsOfFruits.buyLotsOfFruit([ ('apples', 2.0), ('pears',3.0), ('limes',4.0) ]) == 12.25
Advanced Exercise: Write a quickSort
function in
Python using list comprehensions. Use the first element as the
pivot. The solution should be very short.
Solution
class FruitShop:
def __init__(self, name, fruitPrices):
"""
name: Name of the fruit shop
fruitPrices: Dictionary with keys as fruit
strings and prices for values e.g.
{'apples':2.00, 'oranges': 1.50, 'pears': 1.75}
"""
self.fruitPrices = fruitPrices
self.name = name
print 'Welcome to the %s fruit shop' % (name)
def getCostPerPound(self, fruit):
"""
fruit: Fruit string
Returns cost of 'fruit', assuming 'fruit'
is in our inventory or None otherwise
"""
if fruit not in self.fruitPrices:
print "Sorry we don't have %s" % (fruit)
return None
return self.fruitPrices[fruit]
def getPriceOfOrder(self, orderList):
"""
orderList: List of (fruit, numPounds) tuples
Returns cost of orderList. If any of the fruit are
"""
totalCost = 0.0
for fruit, numPounds in orderList:
costPerPound = self.getCostPerPound(fruit)
if costPerPound != None:
totalCost += numPounds * costPerPound
return totalCost
def getName(self):
return self.name
The FruitShop
class has some data, the name of the shop and the prices per pound
of some fruit, and it provides functions, or methods, on this data. What advantage is there to wrapping this data in a class? There are two
reasons: 1) Encapsulating the data prevents it from being altered or used
inappropriately and 2) The abstraction that objects provide make it
easier to write general-purpose code.
So how do we make an object and use it? Download the FruitShop
implementation from here and save it to a file called shop.py
. We can use the FruitShop
as follows:
Copy the code above into a file called shopTest.py (in the same
directory as shop.py) and run it:
import shop
name = 'DM533'
fruitPrices = {'apples':2.00, 'oranges': 1.50, 'pears': 1.75}
myFruitShop = shop.FruitShop(name, fruitPrices)
print myFruitShop.getCostPerPound('apples')
otherName = 'DM811'
otherFruitPrices = {'kiwis':1.00, 'bananas': 1.50, 'peaches': 2.75}
otherFruitShop = shop.FruitShop(otherName, otherFruitPrices)
print otherFruitShop.getCostPerPound('bananas')
So what just happended? The
[dm533@woglinde ~/tutorial]$ python shopTest.py
Welcome to the DM533 fruit shop
2.0
Welcome to the DM811 fruit shop
1.5
import shop
statement told Python to load all of the functions and classes in shop.py.
These import statements are used more generally to load code modules. The line myFruitShop = shop.FruitShop(name, fruitPrices)
constructs an instance of the FruitShop
class defined in shop.py, by calling the __init__
function in that class. Note that we only passed two arguments
in, while __init__
seems to take three arguments: (self, name, fruitPrices)
. The reason for this is that all methods in a class have self
as the first argument. The self
variable's value is automatically set by the interpreter; when calling a method, you only supply the remaining arguments. The self
variable contains all the data (name
and fruitPrices
) for the current specific instance, similar to this
in Java.
Exercise (for submission): Write a function, shopSmart(orders,shops)
which takes an orderList
(like the kind passed in to FruitShop.getCostOfOrder
and a list of FruitShop
and returns the FruitShop
where your order costs
the least amount in total. Solution
This function should be defined in a file called shopSmart.py
.
A stub implementation is provided here.
Note that the shop.py
implementation is provided as a "support" file, so please do not submit it.
Test Case:We will check that, with the following variable definitions:
orders1 = [('apples',1.0), ('oranges',3.0)]
orders2 = [('apples',3.0)]
dir1 = {'apples': 2.0, 'oranges':1.0}
shop1 = shop.FruitShop('shop1',dir1)
dir2 = {'apples': 1.0, 'oranges': 5.0}
shop2 = shop.FruitShop('shop2',dir2)
shops = [shop1, shop2]
The following are true:
shopSmart.shopSmart(orders1, shops).getName() == 'shop1'
and
shopSmart.shopSmart(orders2, shops).getName() == 'shop2'
range
to generate a sequence of integers, useful for generating traditional indexed for
loops:
for index in range(3): print lst[index]
reload
command:
>>> reload(shop)
@ $ %=
Last modified: Tue Jul 6 21:36:57 CEST 2010