Official site

https://www.python.org/

Technical features

• An object-oriented programming language, comparable to Perl, Ruby, Scheme, or Java.
• Elegant sintax, making the programs you write easier to read.
• Easy-to-use language that makes it simple to get your program working.
• Comes with a large standard library that supports many common programming tasks.
• There is a bundled development environment called IDLE.
• Runs anywhere.

Python keywords

True, False and None are written as it is, ALL THE OTHERS are written in lowercase.

Identifiers

Same rules as Java, C#, etc.

Python is case-sentitive

Multi-line statement

\ explicit line continuation:

a = 1 + 2 + 3 + \
    4 + 5 + 6 + \
    7 + 8 + 9 +

(), [] and {} implied line continuation

a = (1 + 2 + 3
    4 + 5 + 6
    7 + 8 + 9)

Multiple statements in a single line

We use ;

a = 1; b = 2; c = 3

Identation

To define a block of code, Python uses identation.

for i in range(1, 11):
    print(i)
    if i == 5
        break
print('finish')

1, 2, 3, 4, 5, 'finish'

Comments

Use # to comment something in the code

Multi-line comments

You can use triple quotes like ''' or """

Docstring

It's a string that occurs as the first statement in a module, function, class, or method definition. We must write what a function/class does in the docstring. We use triple quotes to write docstrings.

def double(num)
    """ Function to double the value """
    return num * 2
print(double._doc_)

"Function to double the value"

Variables

We don't need to declare a variable before using it.

Multiple assigments

a, b, c = 5, 3.2, "Hello"

x = y = z = "same value"

Data types

Every value in Python has a data type. Everything is an object in Python programming. data types are actually classes and variables are instance(object) of these classes

• integers - int
• floating point numbers float
• complex numbers complex

a = 5
print(a, "is of type", type(a))
a = 2.0
print(a, "is of type", type(a))
a = 1 + 2j
print(a, "is complex number?", isinstance(1 + 2j, complex))

5 is of type <class 'int'>

2.0 if of type <class 'float'>

(1 + 2j) is complex number? True

Integers

Can be any length (limited by the memory available)

Floating point numbers

Is acurate up to 15 decimal places

Complex numbers

Are written in the form: x + yj, where x is the real part and y is the imaginary part

a = 12345678901234567890
print(a)
b = 0.1234567890123456789
print(b)
c = 1 + 2j
print(c)

1234567890123456789

0.12345678901234568

(1 + 2j)

! b got truncated at the final of floating point numbers

List - use []

Ordered sequence of items.

a = [1, 2.2, 'python']

a = [5, 10, 15, 20, 25, 30, 35, 40]
# a[2] = 15
print("a[2] = ", a[2])
# a[0:3] = [5, 10, 15]
print("a[0:3] = ". a[0:3])
# a[5:] = [30, 35, 40]
print("a[5:] = ", a[5:])

a[2] = 15

a[0:3] = [5, 10, 15]

a[5:] = [30, 35, 40]

Lists are mutable

Value of the elements of a list can be altered

a = [1, 2, 3]
a[2] = 4
print(a)

[1, 2, 4]

Python Tuple - use ()

Ordered sequence of items.

Tuples are immutable

• are used to write protected data
• faster than list as it cannot change dynamically

It's defined within parentheses

t = (5, 'program', 1 + 3j)

t = (5, 'program', 1 + 3j)
print("t[1] =", t[1])
print("t[0:3] =", t[0:3])
t[0] = 10

t[1] = program

t[0:3] = (5, 'program', (1 + 3j))

Error:

Traceback (most recent call last): File "", line 4, in t[0] = 10 TypeError: 'tuple' object does not support item assignment

Strings

• Sequence of unicoe characters
• We can use ' or "
• Multi-line Strings, we can use ''' or """

s = 'Hello world'
print("s[4] = ", s[4])
print("s[6:11] = ", s[6:11])
s[5] = 'd'

s[4] = 0

s[6:11] = world

Traceback(most recent call last): File "", line 4, in s[5] = 'd' TypeError: 'str' object does not support item assignment

! strings are immutable

Python set - use {}

Unordered collection of unique items

a = {5, 2, 3, 1, 4}
print("a = ", a)
print(type(a))

a = {1, 2, 3, 4, 5}

<class 'set'>

Set eliminate duplication

a = {1, 2, 3, 4, 5}
print(a)

{1, 2, 3}

There is no indexing

a = {1, 2, 3}
print(a[1])

Traceback(most recent call last) File "", line 3, in print (a[1]) TypeError: 'set' object does not support indexing

Python dictionary - use {}

Unordered collection of key-value pairs

• Generally used when we have a huge amount of data. Dictionaries are optimized for retrieving data

d = {1:'value', 'key': 2}
print(type(d))
print("d[1] = ", d[1])
print("d['key'] = ", d['key'])
print("d[2] = ", d[2])

<class 'dict'>

d[1] = value

d['key'] = 2

Traceback(most recent call last): File "", line 5, in print("d[2] = ", d[2]) KeyError: 2

Convertion through data types

Float to int

It will truncate the value to make it close to 0

print(int(10.6))
print(int(-10.9))

10

-10

String

print(float('2.5'))
print(str(25))
print(int('1p'))

2.5

25

Traceback(most recent call last): File "", line 3, in runcode File "", line 1, in ValueError: invalid literal for int() with base 10: '1p'

Print()

Sintax:

print(*objects, sep'', end='\n', file=sys.stdout, flush=false)

• objects - value(s) to be printed
• sep - separator between the values
• end - it printed after all values are printed
• file - where the values are printed
• sys.stdout - screen

print(1,2,3,4)
print(1,2,3,4,sep='*')
print(1,2,3,4,sep='#', end='&')

1 2 3 4

123*4

1#2#3#4&

Formatting - stg.format()

print('I love {0} and {1}'.format('bread', 'butter'))
print('I love {1} and {0}'.format('bread', 'butter'))

I love bread and butter

I love butter and bread

print('Hello {name}, {greeting}'.format(gretting='good morning', name = 'John'))

Hello John, Good morning

x = 12.3456789
print('The value of x is %3.2f' %x)
print('The value of x is %3.4f' %x)

The value of x is 12.35

The value of x is 12.3457

Input

Sintax: input([prompt]), where prompt is the string we wish to display at the screen

num = input('Enter a number.')
print(num)

Enter a number: 10

'10'

Eval()

It evaluate and executes expressions even if the provided input is a string

int('2+3')

error

eval('2+3')

5

Import

This basically works similar to other languages.

import math
print(math.p)

3.141592653589793

We can also import only some attributes and functions

Operators

• arithmetic operators
• logical operators
  • and, or and not
• assignment operators

Bitwise operators

Act on operands as if they were string or binary digits. It operates bit by bit, hence the name.

• & AND
• | OR
• ~ NOT
• ^ XOR
• >> right shift
• << left shift

x = 10 (0000 1010)
y = 4 (0000 0100)

x & y = 0 (0000 0000)
x | y = 14 (0000 1110)
~ x = -11 (1111 0101)
x ^ y = 14 (0000 1110)
x >> 2 = 2 (0000 0010)
x << 2 = 40 (0010 1000)

Special operators

Identity operators

• is and is not

Used to check if two values (or variables are located on the same part of the memory). True if the operands are identical (refer to the same object)

x1 = 5
y1 = 5
x2 = 'Hello'
y2 = 'Hello'
x3 = [1,2,3]
y3 = [1,2,3]
print(x1 is not y1)
print(x2 is y2)
print(x3 is y3)

false

true

false

The interpreter locates the values.

Membership operators

Used to test wether a value or variable is found in a sequence (string, list, tuple, set and dictionry)

⚠️ In a dictionry we can only test for presence of key, not the value

• in True if value/variable is found in the sequence
• not in True if value/variable is not found in the sequence

x = 'Hello World'
y = {1 : 'a', 2 : 'b'}
print('H' in x)
print('hello' not in x)
print(1 in y)
print('a' in y)

true

true Remember, python is case sensitive

true

false

if, else and elif statement

The body of if statement is indicated by identation. Python interprets non-zero values as true. None and 0 are interpreted as false.

num = 3
if num > 0:
    print(num, "is a positive number.")
print("pineaple")
if num < 0:
    print("don't print this")
print("second pineaple")

3 is a positive number

pineaple

second pineaple

elif if a shortcut for else if

We can use nested statements, this follows the same rule of identation.

for loop

numbers = [6,5,3,8,4,2,5,4,11]
sum = 0
for val in numbers:
    sum = sum + val
print(sum)

Range function

Generate a sequence of numbers

range(10) will generate numbers from 0 to 9 (10 numbers)

range(start, stop, stepsize) defining the start, stop and step size

This function does not store all the values in memory, it would be inefficient. So it remembers the start, stop and stepsize and generates the next number to go. To force this function to output all the items, wen can use the function list().

print(range(10))
print(list(range(10)))
print(list(range(2, 8)))
print(list(range(2, 20, 3)))

range(0, 10)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

[2, 3, 4, 5, 6, 7]

[2, 5, 8, 11, 14, 17]

We can use range function in for loops to iterate through a sequence of numbers. It can be combined with len() function to iterate through a sequence using indexing.

genre = ['pop', 'rock', 'jazz']
for i in range(len(genre)):
    print (genre[i])

pop

rock

jazz

For loop with else | While loop with else

The else part is executed if the items in the sequence used in for loop exhasuts. Break statement can be used to stop a for loop. In such case, the else part is ignored.

While works the same way.

digits[1,2,5]
for i in digits:
    print(i)
else:
    print("No items left")

0

1

5

No items left

Break statement

Terminates the loop containing it. Control the program flows to the statement immediately after the body of the loop.

for val in "string":
    if val == "i":
        break
    print(val)
print("The end")

s

t

r

The end

Continue statement

Is used to skip the rest of the code inside a loop for the current iteration only. Loop does not terminate but continues on with the next iteration

for val in "string":
    if val == "i":
        continue
    print(val)
print("The end")

s

t

r

n

g

The end

Pass statement

Pass is a null statement. The difference between a comment and pass statement in Python is that, while the interpreter ignores a comment entirely, pass is not ignored. However, nothing happens when pass is executed. It results into no operation (NOP). We generally use it as a placeholder. Suppose we have a loop or a function that is not implemented yet, but we want to implement it in the future. They cannot have an empty body. The interpreter would complain. So, we use the pass statement to construct a body that does nothing.

sequence = {'p','a','s','s'}
for val in sequence
    pass

def function(args):
    pass

Function

Is a group of related statements that perform a specific task.

Sintax:

def function_name(parameters):
    """docstring"""
    statements(s)

Function definition:

1. keyword def marks the start of function header
2. a function name to uniquely identify it.
3. parameters (arguments) through which we pass values to a function
4. a colon (:) mark the end of function header
5. optional documentation (docstring) to describe what the functio does
6. one or more valid python statements that make up the function body. Statements must have the same identation level.
7. an optional return statement to return a value from the function

Example:

def greet(name):
    """This function greets to the person passed in a parameter"""
    print("Hello, " + name + "Good morning!")

⚠️ we need to take care about scopes (inside/outside functions) We can declare global variable using the keyword global

Default arguments

we can use use default values on a function, with the keyword =, But remember, all the arguments on the right of the first default argument should have a default value.

Artbitrary arguments

When we don't know in advance the number of arguments that will be passed into a function, we use arbitrary arguments

def greet(*names):
    for name in names:
        print(name)

greet("A", "B", "C", "D")

Output:

A

B

C

D

Built-in functions

There are 68 built-in functions in Python 3.5.2