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Python Programimg Language

Python is high-level programming language, with applications in numerous areas,including web programming, scripting, scientific computing and artificial intelligence.

Python is processed at runtime by the interpreter, There is no need to compile your program before executing it.

GitHub

IPython

Version : 3.x

Python has several different implementations, written in various languages. CPython, is the most popular by far.

Print Statement [Output Text]

print('Hello World')
print  "Hello World"

The actual syntax of the print() function is :

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

print(1,2,3,4, sep='/', end=';')
# Output: 1/2/3/4;

Output formatting

print('I love {0} and {1}'.format('php','java'))
# Output: I love php and java

print('Welcome, {firstname} {lastname}'.format(firstname = 'Jeevan', lastname = 'Lal'))
# Output: Welcome, Jeevan Lal

f = 17.89585252
print('The value of f is %3.2f' %f)
# Output: The value of f is 17.90

print('The value of f is %3.4f' %f)
# Output: The value of f is 17.8959

Operations

Python has the capability of carrying out calculations.

>>> 2 + 2
4
>>> 5 + 4 - 3
6
>>> 2 * (3 + 4)
14
>>> 10 / 2
5.0 # return float
>>> 5 ** 2 # Power (Exponentiation)
25

Using a single slash to divide numbers produces a decimal (or float, as it is called in programming). We'll have more about floats in a letter lesson.

Dividing by zero in Python produces an error, as no answer can be calculated.

>>> 11 / 0
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

Input and Variables

Here is a program to show examples of variables:

a =  123.4
b23 =  'Span'
first_name =  "Jeevan"
b =  432
c = a + b

Assigning multiple values to multiple variables

a, b, c = 5, 3.2, "Hello"
print(a, b, c)

Multi-line statement

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

Command-line String Input

number =  input("Type in a number: ")
text =  raw_input("Type in a String :")
print  "Number is a", type(number)

type(variable) : Show variable data type

Here is the list of some string operations:

Operation Symbol Example
Repertition * "i" * 5 == "iiiii"
Concatenation + "Hello, " + "World" == "Hello, World"

Python NumbersThere are three numeric types in Python:

  • int
  • float
  • complex
x = 1  # int  
y = 2.8  # float  
z = 1j # complex

To verify the type of any object in Python, use the type() function

  • Float can also be scientific numbers with an "e" to indicate the power of 10. ex : y = 35e3
  • Complex numbers are written with a "j" as the imaginary part. ex ; y = 5j

Python Casting

Conversion between data types

int(10.6) # Output : 10
int(-10.6) # Output : -10
float(5) # Output : 5.0
float('2.5') # Output : 2.5
str(25) # Output : '25'
set([1,2,3]) # Output : {1, 2, 3}
tuple({5,6,7}) # Output : (5, 6, 7)
list('hello') # Output : ['h', 'e', 'l', 'l', 'o']
dict([[1,2],[3,4]]) # Output : {1: 2, 3: 4}
dict([(3,26),(4,44)]) # Output : {3: 26, 4: 44}

Python Strings

String literals in python are surrounded by either single quotation marks, or double quotation marks.

'hello' is the same as "hello".

Methods

  • Get the character at position
a = "Hello, World!"  
print(a[1]) # Output : e
print(b[2:5]) # Output : llo
  • The strip() method removes any whitespace from the beginning or the end.
  • The len() method returns the length of a string.
  • The lower() method returns the string in lower case.
  • The upper() method returns the string in upper case.
  • The replace() method replaces a string with another string.
  • The split() method splits the string into substrings if it finds instances of the separator
a = " Hello, World! "  
print(a.strip()) # returns "Hello, World!"
print(len(a))
print(a.lower())
print(a.upper())
print(a.replace("H", "J"))
print(a.split(",")) # returns ['Hello', ' World!']

Python Operators

Python Comparison Operators

Comparison operators are used to compare two values:

==	,Equal,	x == y	
!=	,Not equal,	x != y	
>	,Greater than,	x > y	
<	,Less than,	x < y	
>=	,Greater than or equal to,	x >= y	
<=	,Less than or equal to,	x <= y

Python Logical Operators

Logical operators are used to combine conditional statements:

and ,Returns True if both statements are true,	x < 5 and  x < 10	
or  ,Returns True if one of the statements is true,	x < 5 or x < 4	
not ,Reverse the result, returns False if the result is true, not(x < 5 and x < 10)

Python Identity Operators

Identity operators are used to compare the objects, not if they are equal, but if they are actually the same object, with the same memory location:

is 	,Returns true if both variables are the same object,	x is y	
is not	,Returns true if both variables are not the same object,	x is not y

Python Membership Operators

Membership operators are used to test if a sequence is presented in an object:

in 	,Returns True if a sequence with the specified value is present in the object,	x in y	
not in	,Returns True if a sequence with the specified value is not present in the object,	x not in y

Decisions

if statement

n = input("Number ?")
if n <  0:
    print "The absolute valu of", n, "is", -n
else:
    print "The absolute valu of", n, "is", n

if, elif, else : Multi if else statement

Short Hand If

If you have only one statement to execute, you can put it on the same line as the if statement.

if a > b: print("a is greater than b")

Short Hand If ... Else

One line if else statement

print("A") if a > b else  print("B")

One line if else statement, with 3 conditions

print("A") if a > b else  print("=") if a == b else  print("B")

Python Functions

def hello():
    print "Hello"

def area(w, h):
    return w * h

def print_welcome(name):
    print "Welcome", name

Python Collections (Arrays)

There are four collection data types in the Python programming language:

  • List is a collection which is ordered and changeable. Allows duplicate members.
  • Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
  • Set is a collection which is unordered and unindexed. No duplicate members.
  • Dictionary is a collection which is unordered, changeable and indexed. No duplicate members.

Python Lists

A list is a collection which is ordered and changeable. In Python lists are written with square brackets.

thislist = ["apple", "banana", "cherry"]
print(thislist[1]) # Access Items
thislist[1] =  "blackcurrant"  # Change Item Value

Loop Through a List

You can loop through the list items by using a for loop:

thislist = ["apple", "banana", "cherry"]
for x in thislist:
    print(x)

Output :

apple
banana
cherry

Check if Item Exists

thislist = ["apple", "banana", "cherry"]
if  "apple"  in thislist:
    print("Yes, 'apple' is in the fruits list")

Methods

  • To add an item to the end of the list, use the append() method.
  • To add an item at the specified index, use the insert() method.
  • The remove() method removes the specified item.
  • The pop() method removes the specified index, (or the last item if index is not specified).
  • The del keyword removes the specified index.
  • The clear() method empties the list.
  • To determine how many items a list have, use the len() method.

The del keyword can also delete the list completely

thislist = ["apple", "banana", "cherry"]

print(len(thislist)) # Output : 3
thislist.append("orange")
thislist.insert(1, "orange")
del thislist[0]
del thislist # The del keyword can also delete the list completely
thislist.clear()
print(thislist)

Python Tuples

A tuple is a collection which is ordered and unchangeable. In Python tuples are written with round brackets.

thistuple = ("apple", "banana", "cherry")
print(thistuple)
print(thistuple[1]) # Access Tuple Items
  • Once a tuple is created, you cannot change its values. Tuples are unchangeable.
  • Tuples are unchangeable, so you cannot remove items from it, but you can delete the tuple completely.

Python Sets

A set is a collection which is unordered and unindexed. In Python sets are written with curly brackets.

thisset = {"apple", "banana", "cherry"}
print(thisset)

Note: Sets are unordered, so the items will appear in a random order.

Add Items

  • To add one item to a set use the add() method.
  • To add more than one item to a set use the update() method.
thisset = {"apple", "banana", "cherry"}
thisset.add("orange")
thisset.update(["orange", "mango", "grapes"])

Remove Item

To remove an item in a set, use the remove(), or the discard() method.

Note: If the item to remove does not exist, remove() will raise an error.

Note: If the item to remove does not exist, discard() will NOT raise an error.

  • To determine how many

Note: Sets are unordered, so the items will appear in a random order.

Methods

  • To determine how many items a set have, use the len() method.
  • You can also use the pop(), method to remove an item, but this method will remove the last item. Remember that sets are unordered, so you will not know what item that gets removed.

Note: Sets are unordered, so when using the pop() method, you will not know which item that gets removed.

  • The clear() method empties the set.
  • The del keyword will delete the set completely. ex: del thisset

Python Dictionaries

A dictionary is a collection which is unordered, changeable and indexed. In Python dictionaries are written with curly brackets, and they have keys and values.

thisdict =	{
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}
print(thisdict)

Accessing Items

x = thisdict["model"]
# There is also a method called `get()` that will give you the same result
x = thisdict.get("model")

Loop Through a Dictionary

You can also use the values() function to return values of a dictionary

for x in thisdict.values():  
    print(x)

Loop through both keys and values, by using the items() function.

for x, y in thisdict.items():  
    print(x, y)

Methods

  • Check if Key Exists : To determine if a specified key is present in a dictionary use the in keyword.
  • Length : To determine how many items (key-value pairs) a dictionary have, use the len() method.
  • Removing Items : The pop() method removes the item with the specified key name.
  • The del keyword removes the item with the specified key name.
  • The clear() keyword empties the dictionary. ex : thisdict.clear()

Python Loops

Python has two primitive loop commands:

  • while loops
  • for loops

The while Loop

i = 1
while i < 6:
    print(i)
    i += 1

Note: remember to increment i, or else the loop will continue forever.

Methods

  • With the break statement we can stop the loop even if the while condition is true.
  • With the continue statement we can stop the current iteration, and continue with the next.

For Loops

A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).

fruits = ["apple", "banana", "cherry"]
for x in fruits:
    print(x)

Even strings are iterable objects, they contain a sequence of characters

for x in "banana":
    print(x)

The range() Function

To loop through a set of code a specified number of times, we can use the range() function,

The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.

for x in range(6):
    print(x)

Note that range(6) is not the values of 0 to 6, but the values 0 to 5.

The range() function defaults to increment the sequence by 1, however it is possible to specify the increment value by adding a third parameter: range(2, 30, 3)

Python Classes and Objects

  • Create a class named MyClass, with a property named x.
  • Now we can use the class named myClass to create objects.
class MyClass:
    x = 5

p1 = MyClass()
print(p1.x)

The init() Function

  • To understand the meaning of classes we have to understand the built-in init() function.
  • All classes have a function called init(), which is always executed when the class is being initiated.
# Create a class named Person, use the __init__() function to assign values for name and age:

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

p1 = Person("John", 36)

print(p1.name)
print(p1.age)

Note: The init() function is called automatically every time the class is being used to create a new object.

Insert a function that prints a greeting, and execute it on the p1 object:

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def myfunc(self):
        print("Hello my name is " + self.name)

p1 = Person("John", 36)
p1.myfunc()

Note: The self parameter is a reference to the class instance itself, and is used to access variables that belongs to the class.

Methods

  • Modify Object Properties: p1.age = 40
  • Delete Object Properties: del p1.age
  • Delete Objects: del p1

Python Iterators

Python Import

A module is a file containing Python definitions and statements. Python modules have a filename and end with the extension .py

import math
import sys

print(math.pi)
print(sys.path)

# Output: 3.141592653589793
# Output:
['',
 'C:\\Users\\G.one\\AppData\\Local\\Programs\\Python\\Python36-32\\Scripts\\ipython.exe',
 'c:\\users\\g.one\\appdata\\local\\programs\\python\\python36-32\\python36.zip',
 'c:\\users\\g.one\\appdata\\local\\programs\\python\\python36-32\\DLLs',
 'c:\\users\\g.one\\appdata\\local\\programs\\python\\python36-32\\lib',
 'c:\\users\\g.one\\appdata\\local\\programs\\python\\python36-32',
 'C:\\Users\\G.one\\.ipython'
]

Python Modules

  • Consider a module to be the same as a code library.
  • A file containing a set of functions you want to include in your application.

Create a Module

To create a module just save the code you want in a file with the file extension .py:

# Save this code in a file named mymodule.py
def greeting(name):
    print("Hello, " + name)

Use a Module

Now we can use the module we just created, by using the import statement:

import mymodule

mymodule.greeting("Jonathan")

Note: When using a function from a module, use the syntax: module_name.function_name.

Variables in Module

Save this code in the file mymodule.py

person1 = {
    "name": "John",
    "age": 36,
    "country": "Norway"
}
import mymodule

a = mymodule.person1["age"]
print(a)

Re-naming a Module

You can create an alias when you import a module, by using the as keyword:

import mymodule as mx

a = mx.person1["age"]
print(a)

Built-in Modules

import platform

print(platform.system())

Using the dir() Function

There is a built-in function to list all the function names (or variable names) in a module. The dir() function:

import platform

x = dir(platform)
print(x)

Note: The dir() function can be used on all modules, also the ones you create yourself.

Import From Module

You can choose to import only parts from a module, by using the from keyword.

# The module named `mymodule` has one function and one dictionary:
def greeting(name):
    print("Hello, " + name)

person1 = {
    "name": "John",
    "age": 36,
    "country": "Norway"
}
# Import only the person1 dictionary from the module:
from mymodule import person1

print (person1["age"])

Python Datetime

A date in Python is not a data type of its own, but we can import a module named datetime to work with dates as date objects.

import datetime

x = datetime.datetime.now() # 2018-11-16 12:11:11.644662
print(x)
print(x.year)
print(x.strftime("%A"))

Creating Date Objects

import datetime

x = datetime.datetime(2020, 5, 17)
print(x)

A reference of all the legal format codes: DateTime

Python JSON

Python has a built-in package called json, which can be use to work with JSON data.

Parse JSON - Convert from JSON to Python

If you have a JSON string, you can parse it by using the json.loads() method.

import json

# some JSON:
x =  '{ "name":"John", "age":30, "city":"New York"}'

# parse x:
y = json.loads(x)

# the result is a Python dictionary:
print(y["age"])

Convert from Python to JSON

If you have a Python object, you can convert it into a JSON string by using the json.dumps() method.

import json

# a Python object (dict):
x = {
    "name": "John",
    "age": 30,
    "city": "New York"
}

# convert into JSON:
y = json.dumps(x)

# the result is a JSON string:
print(y)

You can convert Python objects of the following types, into JSON strings:

import json

print(json.dumps({"name": "John", "age": 30}))
print(json.dumps(["apple", "bananas"]))
print(json.dumps(("apple", "bananas")))
print(json.dumps("hello"))
print(json.dumps(42))
print(json.dumps(31.76))
print(json.dumps(True))
print(json.dumps(False))
print(json.dumps(None))

Format the Result

The example above prints a JSON string, but it is not very easy to read, with no indentations and line breaks.

The json.dumps() method has parameters to make it easier to read the result:

# You can also define the separators, default value is (", ", ": ")
json.dumps(x, indent=4, separators=(". ", " = "))

Order the Result

Use the sort_keys parameter to specify if the result should be sorted or not:

json.dumps(x, indent=4, sort_keys=True)

Python PIP

PIP is a package manager for Python packages, or modules if you like.

Note: If you have Python version 3.4 or later, PIP is included by default.

What is a Package?

A package contains all the files you need for a module.

Modules are Python code libraries you can include in your project.

Check if PIP is Installed

$ pip --version

Install PIP

If you do not have PIP installed, you can download and install it from this page: https://pypi.org/project/pip/

Download a Package

$ pip install camelcase

Using a Package

Once the package is installed, it is ready to use.

import camelcase

c = camelcase.CamelCase()

txt = "hello world"

print(c.hump(txt))

Find Packages

Find more packages at https://pypi.org/.

Remove a Package

Use the uninstall command to remove a package:

$ pip uninstall camelcase

List Packages

Use the list command to list all the packages installed on your system:

$ pip list

Python Try Except

  • The try block lets you test a block of code for errors.
  • The except block lets you handle the error.
  • The finally block lets you execute code, regardless of the result of the try- and except blocks.
# The try block will generate an exception, because x is not defined:
try:
  print(x)
except:
  print("An exception occurred")

Finally

The finally block, if specified, will be executed regardless if the try block raises an error or not.

try:
  print(x)
except:
  print("Something went wrong")
finally:
  print("The 'try except' is finished")

This can be useful to close objects and clean up resources:

# Try to open and write to a file that is not writable:
try:
  f = open("demofile.txt")
  f.write("Lorum Ipsum")
except:
  print("Something went wrong when writing to the file")
finally:
  f.close()