Conditional Statements in Python

Python programs contains statements that are executed sequentially. Let us understand the usage of conditional statements using an example: Allow booking of tickets to a show only when booked ticket count is less than the seating capacity available for the show. When seating capacity is filled completely display a message "No more Booking allowed". One such conditional statement in python is if.

Now let us write a program to do this:

Source code can be downloaded from GitHub

import sys

'''

    Program to demonstrate conditional statements in python

    Conditional statements in python if else

'''

def BookTickets():

    '''

        Reads Seating  Capacity and ticket Count as command

        line arguments.

        Using if and else condition to control the program flow

    '''

    try:

        SeatingCapacity = int(sys.argv[1])

        TicketCount = int(sys.argv[2])

        if ((SeatingCapacity - TicketCount) >= 0 ):

            print('Seats Available for Booking.. ', SeatingCapacity)

        else:

            print('No More Booking Allowed')

        

    except Exception as error:

        print(error)

    

if (len(sys.argv) < 3):

    print('Program Takes 2 arguments Seating Capacity and Ticket Count')

    exit(0)

BookTickets()

elif clause in python

In the above program we have used if else that is execute statements in if block on expression becoming true or statements in else block are executed. Always the control flow is either if or else. Now to add more control flow in the program we can use elif block.

Source code can be downloaded from GitHub

import sys

x = sys.argv[1]

if (x == 0):

    print('Ground floor')

if (x == 1):

    print("1st floor")

elif (x == 2):

    print ("2nd floor")

elif (x == 3):

    print ("3rd floor")

elif (x == 4):

    print ("4th floor")

elif (x == 5):

    print ("5th floor")

else:

    print ('Roof Top reached' )

  

Here we determine the floor in the building that we are currently based on value of x that is passed as an argument to the program.

Any value that is greater than 5 is passed always executes the else block determining roof top is reached.

Execute the code in elif block when all conditions above the elif are not satisfied. 

finally keyword in python

finally block is always executed in python irrespective of exception is raised or not within the program. Exception handling in python can be referred under chapter Python Exception Handling

In finally block we can handle closing of any open connections, files after the exception has been raised in the program or any additional steps to be performed prior to exiting the program.

Copy the below code into file expfinally.py and execute it or copy the code from GitHub

'''

    Module demonstrates exception Handling 

    and finally keyword

'''

def ConvertToInteger(x):

    ''' ConvertToInteger function 

        Converts passed argument to Integer

        Args:

            Takes an argument and converts to Integer

        Return:

            Returns an integer number.

            In case of Exception error code -1 is returned

    '''

    try:

        num = int(x)

    except:

        errCode = -1

        return errCode

    finally:

        print("finally block executed always with or without exception")

    return num

    

def CountWordsInfile(file):

    ''' CountWordsInfile function 

        Counts the total number of words in file

        Args:

            Takes a single argument fileName 

        Return:

            Return the number of words in the file

    '''

    try:

        fp = open(file, mode='rt', encoding='utf-8')

        data = fp.read()

        words = data.split()

        fp.close()

        print ("words in text file: ",len(words))

        return len(words)

    except Exception as error:

        print("Exception Occured and Exception is:")

        print(error)

    finally:

        print("finally block executed always with or without exception")

For details on docstrings how to use help on module refer to chapter Docstrings and Comments in Python

We have used help on module to refer to the functions inside the module. Demonstrated finally keyword in case of exception and without exception.

expfinally.ConvertToInteger('a') error code of -1 is returned in case of exception and finally block is also executed.

expfinally.ConvertToInteger('29') a valid number is returned after conversion without exception and finally block is also executed.

Python Exception Handling

Exception break normal control flow of the program and terminate the program if not handled. Exceptions are errors raised during execution of program. Python generates exception for errors that are raised and can be handled using exception handling. Exception cause program to crash if not handled and prints the stack trace where and why exception has occurred.

Python uses try and except to handle exceptions within the program. The code or the operation that has possibility of raising an exception is written within the try block and the code to handle the exception raised in try block is written in except block.

For all below demonstration code can be downloaded from GitHub

At the REPL type the below commands to the get the current working directory,

import os

os.getcwd()  #This command will display the current working directory

Let us copy the below code to a file exception.py  and place the file in python current working directory to demonstrate exception handling.

''' Module Converts passed argument to Integer'''

def ConvertToInteger(x):

    num = int(x)

    return num

Now let us import the module exception.py in python REPL and see different exceptions raised.

When passed integer to our conversion function converted successfully. When we passed a character python raised 'ValueError' exception. After the exception program exited immediately.

Handling of Exceptions

Now we will see how to handle exception and ensure the program termination is smooth.

Lets update our current program to handle the exception in our code using try and except block.

''' Module Converts passed argument to Integer

    Args:

        Takes an argument and converts to Integer

    Return:

        Returns an integer number.

        In case of Exception error code -1 is returned

'''

def ConvertToInteger(x):

    try:

        num = int(x)

    except:

        errCode = -1

        return errCode

    return num

Let us run the program for the same input passed earlier and validate the output.

We have enclosed the code num = int(x)  raising exception in try and except block. When a character string is passed and cannot convert, python raised an exception and its handled in except block with error code -1.

Indentation Error, Syntax Error, Name Error are caused by programmer error these have to be fixed by programmer rather than at runtime of program.

Details of Exception Handled:

An exception has been raised and its handled, however we do not know the details of exception that is raised, let us print the details that caused the exception within our code.

We will read the file count the words in file to demonstrate exception details:

def CountWordsInfile(file):

    ''' CountWordsInfile function 

        Counts the total number of words in file

        Args:

            Takes a single argument fileName 

        Return:

            Return the number of words in the file

    '''

    try:

        fp = open(file, mode='rt', encoding='utf-8')

        data = fp.read()

        words = data.split()

        fp.close()

        print ("words in text file: ",len(words))

        return len(words)

    except Exception as error:

        print("Exception Occured and Exception is:")

        print(error)

Lets import the function CountWordsInfile from module exception and pass a random file that does not exist on the machine. Python prints the exception details.

Also let us pass a file that exists and see the output that prints the number of words in the file.

Code can be downloaded from GitHub

For file reading and writing files refer Python Reading Files

pass keyword in python

Python programming do not allow empty block of code within a program. pass is a syntactical requirement of python programming to write future code. Empty block of code is not allowed in loops, function definitions, conditional statements, class definitions.

Let us try few examples with empty block of code and introduce pass later.

Copy the below code into a program called pass.py and execute it. printData function do not have any statements.

def printData():

    

def main():

    printData()

if (__name__ == '__main__'):

    main()

Execution of the program is showing indentation error at function main. Now let us introduce pass in printData function and re-execute the program.

def printData():

    pass

    

def main():

    printData()

if (__name__ == '__main__'):

    main()

After introducing pass keyword program executed without any errors. Similarly empty blocks are not allowed for loops, function definitions, conditional statements, class definitions.

Code can be downloaded from GitHub

While Loop in Python

Loops are used in programming language to perform a particular task repetitively until a condition is met. In this chapter we will cover while loop in  python. syntax of while loop in python programming

while expression:

    Loop Statements

While loop starts with while keyword and expression (condition for while) followed by a colon representing a new block of while. Statements in the loop are executed repetitively until the expression is evaluated to false.

paste the below code into a file while_loop.py and execute the code

import sys

'''

    Program to demonstrate while loop 

    Reads a string and prints each word.    

'''

def printWords(string):

    '''

        print Words function reads the strings, splits on space

        loops through the words and prints the words

        

        Args:

            Takes a string as a input

        Returns:

            no return

    '''

    wordList = string.split()

    wordCount = len(wordList)

    k = 0

    while(k < wordCount):

        print(wordList[k])

        k += 1

        

        

if (__name__ == '__main__'):

    if(len(sys.argv) < 2):

        print('Provide string as an argument')

        exit(0)

    inpStr = sys.argv[1]

    printWords(inpStr)

 

Alternatively code can be downloaded from GitHub

Here we are passing a command line argument to the program while_loop.py . The program parses the argument string splits and prints words to the console using while loop.

Let us see one more simple program using while loop with continue and break.

continue: skips the current iteration and continues with the next one.

i=0

while (i<=10):

    i += 1

    if (i%2 == 0):

        continue

    print(i)

break: stops the loop at the current iteration and exits the loop.

i=0

while (i<10):

    i += 1

    if (i == 5):

        break

    print(i)

loop starts evaluating the statements inside the loop, Immediately after break statement the control comes out of the loop and loop stops executing.

for loop in python

Loops are used in programming language to perform a particular task repetitively until a condition is met. In this chapter we will cover for loop in  python. syntax of for loop in python programming

for data in sequence:

    Loop Statements

for loop starts with while keyword and data in sequence followed by a colon representing a new block of for. Statements in the loop are executed repetitively until the data is available in sequence.

paste the below code into a file for_loop.py and execute the code. Code referred will be same as while chapter, difference will be using for loop in place of while loop.

import sys

'''

    Program to demonstrate while loop 

    Reads a string and prints each word.    

'''

def printWords(string):

    '''

        print Words function reads the strings, splits on space

        loops through the words and prints the words

        

        Args:

            Takes a string as a input

        Returns:

            no return

    '''

    wordList = string.split()

    for word in wordList :

        print(word)

        

if (__name__ == '__main__'):

    if(len(sys.argv) < 2):

        print('Provide string as an argument')

        exit(0)

    inpStr = sys.argv[1]

    printWords(inpStr)

Output produced by while and for loop are same. Refer to usage of while loop in python.

Code snippet of for loop from GitHub

what is __name__

Python has a special builtin variables which are delimited by double under scores. These variables are evaluated at runtime of python program. We will discuss one such variable here that is __name__ . This variable produces different outputs when module is imported  vs when module is executed directly. This is an important feature of python helps writing the module functionality accordingly.

Here we will be reusing the code written in our Module chapter. Reusing this to demonstrate the functionality.

Paste the code into below module words_name.py

'''

    Module to read a text file and convert each word 

    first letter to CAPS.

'''

def readfile(fileName):

    '''

       Reads input file passed as an agrument process the data

       and prints capitalized words to console.

       Args:

            Takes fileName as an argument

    '''

    fp = open(fileName, mode='rt', encoding='utf-8')

    words = fp.read().split()

    newWords = convertToCaps(words)

    print('Words in Capitalized form')

    for word in newWords:

        print (word)

    fp.close()

    

def convertToCaps(wordsList):

    '''

       Reads input fila passed as an agrument process the data

       and prints capitalized words to console.

       Args:

            Takes wordsList as an argument

       Returns:

            returns wordsList with capitalized letter in each word

    '''

    capitalizedWords = []

    for word in wordsList:

        capitalizedWords.append(word.capitalize())

    return capitalizedWords

    

    

print(__name__) # Using the double under score variable

In the last line of above code we are outputting the value of __name__ variable.

Importing the Module

Here we imported the module, output of the __name__ variable is evaluated to module name words_name. 

Executing the Module

We executed the module, output of the __name__ variable is evaluated to __main__

Let use this important feature of detection of import vs execution to program our module words_name

Code in the file words_name.py is updated to read file as an argument and process the data in the file when the module is executed. Special variable __name__ is used to determine the execution and corresponding functions to process the data are called.

Code is not executed when the module is imported as the flow does not evaluate special __name__ to __main__

Paste the below code into words_name module or copy the code from GitHub

import sys

'''

    Module to read a text file and convert each word 

    first letter to CAPS.

    

    Execution of module takes a single argument that is 

    file to read and convert.

'''

def readfile(fileName):

    '''

       Reads input file passed as an agrument process the data

       and prints capitalized words to console.

       Args:

            Takes fileName as an argument

    '''

    try:

        fp = open(fileName, mode='rt', encoding='utf-8')

        words = fp.read().split()

        newWords = convertToCaps(words)

        print('Words in Capitalized form')

        for word in newWords:

            print (word)

        fp.close()

    except Excpetion as error:

        print(error)

def convertToCaps(wordsList):

    '''

       Reads input fila passed as an agrument process the data

       and prints capitalized words to console.

       Args:

            Takes wordsList as an argument

       Returns:

            returns wordsList with capitalized letter in each word

    '''

    capitalizedWords = []

    for word in wordsList:

        capitalizedWords.append(word.capitalize())

    return capitalizedWords

def main(inputfile):

    readfile(inputfile)

if(__name__ == '__main__'):

    if (len(sys.argv) < 2):

        print('Module takes file as input provide a file name:')

        exit (0)

    main(sys.argv[1])

Executed the module in the first instance without file, program is requesting to provide file name. In the second instance module is executed with file as input, data is processed and output to console.

Importing the module has no impact

Module words_name is imported and no request for file input or execution of module.

Module in Python

In this chapter you will learn what is module, how to define a custom module, importing custom module in python program and use them.

Lets start by defining a module in python, Module is piece of code that does a specific functionality in programming. All similar functionalities are grouped into source code files called module. 

For Example math module of python, different math functions like Trigonometric functions, logarithmic functions, angular functions, numeric functions, hyperbolic functions are grouped into a single math module.

All python source file use an extension .py lets create a python source file words.py to read data from a file and convert first letter of each word to caps. 

Copy below code to a file and execute

'''

    Module to read a text file and convert each word 

    first letter to CAPS.

'''

def readfile(fileName):

    '''

       Reads input file passed as an agrument process the data

       and prints capitalized words to console.

       Args:

            Takes fileName as an argument

    '''

    fp = open(fileName, mode='rt', encoding='utf-8')

    words = fp.read().split()

    newWords = convertToCaps(words)

    print('Words in Capitalized form')

    for word in newWords:

        print (word)

    fp.close()

    

def convertToCaps(wordsList):

    '''

       Reads input fila passed as an agrument process the data

       and prints capitalized words to console.

       Args:

            Takes wordsList as an argument

       Returns:

            returns wordsList with capitalized letter in each word

    '''

    capitalizedWords = []

    for word in wordsList:

        capitalizedWords.append(word.capitalize())

    return capitalizedWords

Python module name is same as file name excluding the extension .py , our module name is words as file name is words.py lets import the module and execute it. Module is imported using the import keyword

Docstrings in Python

Functions in Python

Reading Files in Python

Download the code from GitHub

Importing the same module times has no effect, once module is imported, to impact any changes made to module after its imported quit python interpreter and import the module on a fresh python interpreter.

Lets create a test module to demonstrate the same. create a file test.py and copy the below code into it

'''

    Module to demonstrate importing same

    module multiple times and its effect

'''

print(__name__) 

we will define __name__ in our next chapter, for now lets execute and see the output.

Code inside module is executed immediately after its imported. When module is imported __name__ is evaluated to module name. Output test is printed only once, Multiple imports of same module has no effect.

Lets execute the module directly and see the output of __name__

Here same __name__ is evaluated to __main__ when executing the module.

This is an important feature of python same __name__ built in type showing different outputs when executing the module vs importing module. __name__ in detailed in our next chapter. 

Functions in Python

A function is a block of code that does a specific task. Large programs are broken to functions to perform a particular task. Functions are executed when it is called with in a program. Functions take input data using arguments and returns a result. Now let us how to define functions and use in python

Functions in python are defined using the 'def' keyword followed by function name and arguments to the function and a colon to start a new block.

def functionName(arguments):

return [expression] is used to return a value from the function.

Python program uses indentation to start a new block. A indentation of 4 spaces is generally used to form the block of statements.

To break from a function use return with no expression, this breaks and exits from the function.

return with no expression returns 'None' , Capture the return of function into a variable and check.

Download the code from GitHub

Python Writing Files

Files are used for reading and writing text and binary data from files. Here will learn how to read, write and manage file resources efficiently. To perform any operation on files the file has to opened first. To open the file we use the built-in python open() function.

open function takes multiple arguments, most commonly used arguments are

file: path to the file to be opened

mode: mode in which the file is to be opened read/write/append and binary/text optional

encoding: Its better to define the encoding of the file, if no encoding is specified python will choose a  default encoding by sys.getdefaultencoding on that machine.

Now lets see how to write data to file using python programming.

Character	Meaning
'r'	Open for reading (default)
'w'	Open for writing, truncating the file if file exists with same name
'a'	Open for writing, append to the end of file if the file exists
'b'	binary mode
't'	text mode (default)

optional modes b,t can be combined with modes r,w,a. so resultant modes might be like 

rb : read binary

wb: write binary

ab : append binary

at  : append text

rt  : read text

wt : write text

fp = open('dummyfile.txt',  mode='wt',  encoding='utf-8')

fp.write('Programming helps to automate the process of manual work.')

fp.write('Automation makes work less error prone.\n')

fp.write('practice programming')

fp.close()

A file pointer fp is created to hold the file object that is opened using the open function, we passed file name, mode of the file and encoding of data in the file. Data is written to the file using the write function. Finally after completion of writing all the required data the file pointer has to be released, this is done by calling close function on the file object.

Number that is displayed after each write function above is the number of bytes that is written to the file.

Download the code from GitHub

Python Reading Files

Files are used for reading and writing text and binary data from files. Here will learn how to read, write and manage file resources efficiently. To perform any operation on files the file has to opened first. To open the file we use the built-in python open() function.

open function takes multiple arguments, most commonly used arguments are

file: path to the file to be opened

mode: mode in which the file is to be opened read/write/append and binary/text

encoding: Its better to define the encoding of the file, if no encoding is specified python will choose a  default encoding by sys.getdefaultencoding on that machine.

Now lets see how to read data from file using python programming.

Character	Meaning
'r'	Open for reading (default)
'w'	Open for writing, truncating the file if file exists with same name
'a'	Open for writing, append to the end of file if the file exists
'b'	binary mode
't'	text mode (default)

optional modes b,t can be combined with modes r,w,a. so resultant modes might be like 

rb : read binary

wb: write binary

ab : append binary

at  : append text

rt  : read text

wt : write text

Please read the post Python Writing Files before continuing with reading of file.

fp = open('dummyfile.txt',  mode='rt',  encoding='utf-8')

A file pointer fp is created to hold the file object that is opened using the open function, we passed file name, mode of the file and encoding of data in the file. Here the mode of the file is set to read text.

fp.read() 

read function reads the complete file as a single string. 

Resetting the file pointer to the beginning is done using the seek function. fp.seek(0)

To read only specific number of characters from a file using read function, pass the number of characters to be read to the read function. To read first 10 characters from file use fp.read(10)

After this operation file pointer is advanced to the position to the point it has completed reading. To read the remaining file use fp.read()

fp.read() function returns an empty string on file pointer reaching end of file.

Reading file line by line

To read the file line by line use the python readline function. fp.readline()

fp.readline() function returns an empty string on file pointer reaching end of file.

Reading all lines of file into List

To read all lines of file into a single list use python readlines function. fp.readlines() 

readlines function returns a list of all lines within the file.

Close the file

Finally after completion of writing all the required data the file pointer has to be released, this is done by calling close function on the file object.

fp.close()

Download the code from GitHub

Docstrings and Comments in Python

Docstrings is used for documenting the code in python. Doc strings are literal strings at the beginning used for documenting a module or function . We use triple quoted strings for single line and multi line documentation strings and even add more detail.

Paste the below code into a file named docstring.py

"""

   Reads a number and returns even or odd

   

    Usage:

        python docstring.py <number>

"""

import sys

def even_or_odd(n):

    """

        Evaluates if a passed number is even or odd

        

    Args:

        Takes one argument as number

    Return:

        returns the result even or odd

    """

    if(n%2 == 0):

        print("Even")

    else:

        print("Odd")

def main(x):

    """Calls even_or_odd function passing the argument

    """

    even_or_odd(x)

    

if "__name__" == "__main__":

    main(sys.argv[1])

Download the code from GitHub

After copying the above code into a file, import the file and run help(docstring) we will see a nice documentation of our module.

Comments in python start with hash symbol #. Often in code we need to explain what approach is followed to achieve the required functionality.

Begin with # symbol and continue till end of line

if "__name__" == "__main__":

    main(sys.argv[1]) # passing a command line argument

Operations on Python Lists

An empty python list is created using empty square braces with the following syntax list5=[], here an empty list list5 is created.

Inserting an Element to Python List:

Element is inserted into the list using the insert function. It takes two arguments index and element.

list.insert(index,element)

insert function can also be used to insert an element at particular position, now we insert O into fourth position to make the string python.

Appending an Element to Python List:

To append an element to the list use append function. It takes one argument.

list.append(element)

append function always adds element to the end of the current list. Here we inserted a space followed by PRO.

To append a new list to the current list use the extend function. It takes list as an argument 

list.extend(newList)

In the above example a new list list6 is added to list5.

Replacing an Element in Python List:

To replace element with a new value in python is done by assigning a new value at corresponding index

location.

Here element at position 0 is replaced by J. string has converted from "PYTHON PROGRAMMING" to "JYTHON PROGRAMMING" . Similarly all other elements can be replaced.

Removing an Element from Python List:

Removing or deleting an element from python lists using remove function. remove function takes element as an argument to be removed.

list5.remove(element)

The element to be removed is present multiple times in a list, only the first occurrence is removed.

 

To delete an item from python we use del keyword. del can be used to delete a 

single element : del list5[0] , del list5[-1] 

multiple elements : del list5[2:5]

and also entire list : del list5 

count:

Count returns number of occurrences of that element in a list. 

pop:

pop function takes a single argument that is index. pop removes the item at the given index. 

pop index defaults to -1 when no index is passed to pop function. 

pop returns a value and its the popped item form the list.

reverse:

reverse function updates the list elements in the reverse order.

list.reverse()

Elements of animals are displayed in the reverse order after the reverse function.

sort:

sort function is used to sort the elements in the ascending order of items in the list.

After the sort operation on animals list, items of the list are sorted in ascending order. 

clear:

Clear functions clears all the elements present inside the list.

Python Lists

Python has a builtin type name list. Lists values(elements/items) are enclosed within the square braces [] and separated by commas. Python lists are mutable and elements within the list can be replaced, removed. new elements can be inserted and appended. 

Python lists are heterogeneous, A list can contain a combination of different data types.

Here the list contains a integer, float and string

Accessing elements of List in Python:

Items/Elements of a list in python is accessed via index. List index starts from Zero, first element is accessed using zero, second element by 1 and so on. 

To access the elements from end the index starts from minus one (-1), last element in the list accessed using -1, second element from end is accessed using -2 and so on.To access fifth element from end use -5.

Accessing a List using Index Range:

Elements of a list can be accessed using the slicing operator : . Range of indexes can be specified in a list where to start and end.

list[startIndex : endIndex : Step]

elements of the list are displayed from start of Index till end of Index. Index element 6 is excluded while accessing using the range.

In the above example list1 we have passed list1[2:10:2], here the elements from the position 2 till position 10 with a step of 2. step determines the position of next element to be evaluated after current position element. Here the element at index position 2 is 3 next element to be evaluated is determined using the step size. step size is 2 so the next element is 5, next element is 7 and so on.. 

Step size is equal to 3, so every third element is evaluated from start position of index till end position of Index.