Agenda

Control flows
Functions
Data structures
Modules

Python at a glance

A sample code from C

    for (int i = 0; i <= 6; i++) {
        if (i % 2 == 0) {
            printf("Even number: %d\n", i);
        }
    }

A sample code from Python



In [1]:

    
for number in [0, 1, 2, 3, 4, 5, 6]:
    if number % 2 == 0:
        print "Even number:", number









    



Even number: 0
Even number: 2
Even number: 4
Even number: 6

What do we see?

No semicolon
No parenthese () or brackets {}
Wait... How do we express code block like in C?
- Using indentation
No static typing

Basic of Python

Number

Expression syntax and the operators: +, -, *, %, / work just like in most other languages.



In [2]:

    
(50 - 5.0 * 6) / 4









    Out[2]:





5.0

** operator can be used to calculate powers



In [3]:

    
5 ** 2









    Out[3]:





25

String

Enclosed in single quotes '...' or double quotes "...".
\ can be used to escape characters



In [4]:

    
"I can eat glass it doesn't hurt me"









    Out[4]:





"I can eat glass it doesn't hurt me"



In [5]:

    
'I can eat glass it doesn\'t hurt me'









    Out[5]:





"I can eat glass it doesn't hurt me"

Print a string using print statement.
If you don’t want characters prefaced by \ to be interpreted as special characters, you can use raw strings by adding an r before the first quote



In [6]:

    
print 'I can eat glass.\nIt doesn\'t hurt me'
print '---------------'
print r'I can eat glass.\n It doesn\'t hurt me'









    



I can eat glass.
It doesn't hurt me
---------------
I can eat glass.\n It doesn\'t hurt me

String literals can span multiple lines using triple-quotes: """...""" or '''...'''



In [7]:

    
print '''I can eat glass
It doesn't hurt me'''









    



I can eat glass
It doesn't hurt me

Concatenate string using +
Repeat a string using *



In [8]:

    
print 'AA' + 'BB'
print 'AA' * 3









    



AABB
AAAAAA

Access a character in string using index, first index is 0
Indices may also be negative numbers, to start counting from the right



In [9]:

    
word = 'python'
print word[0]
print word[-1]
print word[-2]









    



p
n
o

Slicing is also supported.



In [10]:

    
#  Get characters from position 1 to position 4 (4 characters)
print word[1:5] # exclude 5
# Get characters from position 1 to the end
print word[1:]
# Get all characters except the final character
print word[:-1]
# Get 3 last characters
print word[-3:]









    



ytho
ython
pytho
hon

Python strings cannot be changed - they are immutable



In [11]:

    
# get error when trying to change 
word[0] = 'J'









    



---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-11-31dcbf8b41b8> in <module>()
      1 # get error when trying to change
----> 2 word[0] = 'J'

TypeError: 'str' object does not support item assignment

But if you need a different string, you can create a new one



In [12]:

    
print 'J' + word[1:]









    



Jython

len() function can be used to calculate length of string



In [13]:

    
print len(word)

List

You can think it as array.
Can be defined as list of comma-separated values between square brackets
Might contain items of different types.



In [14]:

    
squares = [1, 4, 9, 16, 25]
print squares









    



[1, 4, 9, 16, 25]

Like strings (and all other built-in sequence types), list can be indexed and sliced:



In [15]:

    
print squares[0] # Indexing returns the item



In [16]:

    
print squares[-1]



In [17]:

    
print squares[-3:] # Slicing returns a new list









    



[9, 16, 25]

However, unlike strings, list are mutable type. It is possible to change their content



In [18]:

    
cubes = [1, 8, 27, 65, 125] # something's wrong here
print cubes









    



[1, 8, 27, 65, 125]



In [19]:

    
cubes[3] = 64
print cubes









    



[1, 8, 27, 64, 125]

Concatenate lists using + operator
Add new items at the end of the list by using append() method
Calculate length of list using len() function



In [20]:

    
print squares + [36, 49]









    



[1, 4, 9, 16, 25, 36, 49]



In [21]:

    
squares.append(36)
print squares









    



[1, 4, 9, 16, 25, 36]



In [22]:

    
print len(squares)

Assignment to slices is also possible
- It can change the size of the list



In [2]:

    
letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
print letters









    



['a', 'b', 'c', 'd', 'e', 'f', 'g']



In [3]:

    
letters[2:5] = ['C', 'D', 'E']
print letters









    



['a', 'b', 'C', 'D', 'E', 'f', 'g']



In [4]:

    
letters[2:5] = ['-']
print letters









    



['a', 'b', '-', 'f', 'g']

Unpacking

Unpack a sequence to the variables



In [37]:

    
a, b, c = [1, 2, 3]
print a, b, c

Number of variables and length of sequence must match



In [36]:

    
a, b, c = [1, 2]









    



---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-36-9d4181223c7f> in <module>()
----> 1 a, b, c = [1, 2]

ValueError: need more than 2 values to unpack



In [39]:

    
a, b = 1, 2
a, b = b, a
print a, b

2 1

Agenda

Control flows

Functions

Data structures

Modules

Python at a glance

Basic of Python

Number

String

List

Unpacking