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Chapter 01 | The Basics | Getting Started Quickly

Hello World

Hello world is the traditional way to start the first program in any programming language. In Python hello world can just be written as

print("Hello World")

Since it is this simple, we will start the journey of python with a meatier example.

from datetime import datetime

odds = [1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31,33,35,37,39,41,43,45,47,49,51,53,55,57,59,]

right_this_minute = datetime.today().minute

if right_this_minute in odds:
    print("This minutes seems a litte odd.")
else:
    print("Not an odd minute.")

Python Imports

  • Python execution starts from the first line in the file, python does not need any special function like main() to start execution.
  • Python interpreter runs the code directly, there is no conversion to any intermediate form like in Java's .class file

Now lets be the Python Interpreter for the code we wrote above.

from datetime import datetime
  • imports helps in brining some pre-existing software module already present in Python standard library to be used for our use the program.
  • In the above we are importing a datetime submodule from a module named datetime.
  • Python standard library is full with lot of reusable code, that is the reason it is called batteries included.
  • Python community also supports third party modules.

Data Structures

  • Python have some very powerful built-in data structures. These built-in data structures provide additional functionality from anything present in other programming language.

List

List in python is denoted by [], this is similar to array in other programming language, but have additional functionality.

  • List in python can store heterogeneous data type and not limited with only homogeneous data type like in other programming language.

In the above example this line creates a list.

odds = [1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31,33,35,37,39,41,43,45,47,49,51,53,55,57,59,]
  • = is called an assignment operator.
  • Variables type need not be pre-defined in python, as it is a dynamically typed language.
  • There is no special line character to signal end of line in python.

Invoking Methods

Coming to the next line in the above code.

right_this_minute = datetime.today().minute
  • today() is a function invoked on the submodule datetime which itself is part of the module datetime
  • today() returns a time object, which conveys lot of information about current time.
  • . dot operator helps in accessing this information returned by today(). we are using the minute attribute of current time.

To understand further we can split the above code into this.

this_now = datetime.today()
right_this_minute = this_now.minute
  • Since we may not use the this_now variable in the future code, it is preferred in python not to create such temporary variable.

Conditionals

  • in is a special operator which checks if one thing is inside another.
  • in returns True or False
if right_this_minute in odds:
    print("This minutes seems a little odd.")
else:
    print("Not an odd minute.")
  • The if statement in the above code uses the in operator to check if right_this_minute value is inside odds value.
  • Once it evaluates to True the first block of code is executed. print("This minutes seems a little odd.")
  • The above code has two code block, and it can be easily identified as this is always indented.
  • Python uses indentation to denote a block of code.
  • :, the colon character, which denotes the start of block for control statements like if, else, for.
  • Compiler raises an error if the next line following : is not right indented.
  • Block of code can further be indented to show the different levels of code flow.
  • else block is executed with the if condition evaluates to false.
    • Kindly note there is : just after else
  • elif is also special else if condition, which check multiple conditions.

Bullet Points

  • The Python interpreter runs the code in the file from top to bottom, executing one line at a time. There is no notion of a main() function.
  • Python come with a powerful standard library, which gives access of lots of reusable modules including third party.
  • There is a collection of standard data structures available in Python with advanced functionality then other programming language. list being one of them.
  • Python is dynamically typed language, as a result, variables need not be declared in advance with its type. As soon as we assign value to a variable it comes into existence.
  • if/elif/else statements are decision making statements. These statements are followed by block of code which gets executed based on the conditions.
  • Block of code are easy to be identified in Python as it is always indented.
  • : colon, always signifies the start of block.

Hello World Improved

We will try to better the above Hello World which we wrote. The improvements like make the code run multiple time, and sleep for a random time.

Python since is an REPL(READ, EVAL, PRINT, LOOP), every enhancement for a Python code can follow in these two approach. Though Python helps in the approach of experimentation.

  • Experimentation
  • Through design.

Loops

  • The for loop is perfect for controlling looping when we know ahead of time how many iterations is needed.
    • for i in [1, 2, 3]:
  • while is used when we do not know how many iterations in advance.
  • for loop can iterate over any sequence, list, strings all are sequence.
    • A sequence is an ordered collection of objects.
  • for loop does not need to be told how big the sequence is, they figure the length.
    • for ch in "hello!":
  • range() function helps in generating a sequence of numbers on which for loop can iterate.
    • In most basic form range() accepts only one integer argument, telling how many times the loop will run.
    • range() actually takes 3 parameters
      • start: Number where range starts
      • stop: Number where range ends
      • step: default is 1, it defines for step to take.

import

  • import statement can be used in two ways.
    • from datetime import datetime
      • This imports a named function into our program's namespace.
      • This helps in importing without having to link the function back to the imported module.
      • If two module has same function in a different submodule, than this way of importing will cause namespace collation.
    • import time
      • This just imports the time module.
      • In this way we have to use the dot-notation syntax to access the module's functionality.
      • This module has a function called sleep(), which stops the execution of program for amount of time passed as argument.

Random

  • dir(): it displays all the attributes associated with anything in Python.
  • help(): it gives Python documentation of related to the name.
  • random.randint(): gives a random number between the given number.

Bullet Points

  • Python programmer favor experimenting with code snippets before giving the solution.
  • The for loop can be used to iterate a fixed number of times.
    • If you know ahead of time how many times you need a loop, use for
  • When you don't know ahead of time how often you're going to iterate, use Python while loop.
  • The for loop can iterate over any sequence, as well as execute a fixed number of times.
  • sleep() pauses the execution for the number of seconds passed as arguments.
  • from time import sleep: imports specific function sleep from time module.
  • import time: importing module we need to give the module name to call the function.
  • random has a good function called randint which gives a random integer within a specified range.
  • dir() and help() help in learning about Python function and module.
  • Negative numbers in range() last arguments changes the direction of range.