Python PrimerΒΆ

Think of python as a program. what it does is takes human readable source code (.py) and parses, compiles and executes in real time.

Python 2.7 Documentation

Best place to start. Python Tutorial

You can run python in 2 ways. (there is actually many ways to run from the command line but

only 2 are used frequently)

Open terminal

  1. with no arguments. opens the python interpreter

>>> python

2. with a path to python script (absolute or relative path). executes the script >>> python hello_world.py

Using Python Interpreter Zen of Python

>>> import this

you can use the command line as a powerful calculator see http://docs.python.org/tutorial/introduction.html

>>> x=1
>>> y=2
>>> x+y
3
>>> x=50

Use the up and down arrows to cycle thru previous commands

>>> x+y
53
>>> y=x+y
>>> x+y
103
>>> i=0
>>> i+=1    #same as i = i+1 works for all math operators i -=1, i*=2 etc...

strings are defined using ', ", or ''' blocks. ' and " are equivalent. convenient when needing to nest quotes.

>>> s='foo'
>>> b='bar'
>>> print s,b
foo bar
>>> s
'foo'
>>> b
'bar'
>>> "foo" == 'foo' == '''foo'''
True

to make a list of items use a list or tuple see http://docs.python.org/tutorial/datastructures.html

>>> l1=[1,2,3,4]
>>> l2=['foo','bar']
>>> t1=(1,2,3)
>>> t2=('foo','bar')

lists are mutable, tuples are immutable

>>> l1[0]=10
>>> l1
[10,2,3,4]
>>> t1[0]=10
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment

get the length of the sequence use builtin len

>>> len(l1)
4

to generate a list of numbers use builtin range

>>> range(10)
[0,1,2,3,4,5,6,7,8,9]

>>> range(0,10,2)
[0,2,4,6,8]

get item from list

>>> l1[0]
1

get the last item

>>> l1[-1]
9

get a sublist

>>> l1[0:2]    #list[startindex:stopindex:step]
                #each parameter is optional but at least needs to be set
                #startindex defaults to 0
                #stopindex defaults to the last index
                #step defaults to 1
                #l1[0:2] same as l1[0:2:1] and l1[:2] (preferred)

same slicing operations work on strings. just think of them as a list of characters

>>> s= 'hello world'
>>> s[:5]
'hello'
>>> s[6:]
'world'
>>> s[-5:]
'world'

you can split and join strings easily

>>> s.split(' ') #str.list(character to  split on) returns a list
['hello', 'world']
>>> ', '.join(s.split(' ')) #join_str.join(list of strings to join)
hello, world
>>> '\n'.join(['this is a good','way to write multi','line text'])
this is a good
way to write multi
line text

Dictionaries are key:value containers. There are two syntaxes for creating a dictionary

>>> d=dict(name='Jake', office=316, building='MSEC')
>>> d2 = {'name':'Jake','office':316, 'building':'MSEC'} #convenient when the keys are variables as well
>>> key1='person'
>>> key2='id'
>>> val1='John'
>>> val2=10394303
>>> d3 = {key1:val1, key2:val2}

to get a value from the dictionary you specifiy a key. To get the definition of a word you find the word (key) in are dictionary and read the associated entry

>>> d['name']
Jake

entries can be modified

>>> d['name']='Jake Ross'
>>> d['name']
Jake Ross

String formating is awesome in python. Lets say you want to display some text with your results

>>> 'the result of {} plus {} is {}'.format(x,y,x+y)
'the result of 50 plus 53 is 103'
>>> 'the result of {1} plus {0} is {2}'.format(x,y,x+y)
'the result of 53 plus 50 is 103'

you can use pass in a key:pairs

>>> "{name}'s office is {building} {office}".format(name='Jake',building='MSEC',office=316)

or better

>>> "{name}'s office is {building} {office}".format(**d2)
"Jake's office is MSEC 316"