DEV Community: kster

Invert Binary Tree

kster — Thu, 13 Apr 2023 00:38:37 +0000

Given the root of a binary tree, invert the tree, and return its root.

First we will have to look at the root node, take its children, swap the positions of the children, then recursively run invert on both right and left subtrees. DFS will allow us to solve this problem

Depth For Search (DFS) - is an algorithm for traversing or searching tree. The algorithm starts at the root node and explores as far as possible along each branch before backtracking.

class Solution:
   def invertTree(self, root: TreeNode) -> TreeNode:
      if not root:
         return None
      # swap the positions of the children
      #saving the left value in a temporary variable
      tmp = root.left 
      #replacing the left value(root.left) to with right value(root.right)
      root.left =root.right 
      #replacing the right value(root.right) with the left which we saved as "tmp"
      root.right = tmp 

      #invert left subtree
      self.invertTree(root.left) 
      #invert right subtree
      self.invertTree(root.right) 
      return root

Namespaces & Scope in Python

kster — Sat, 29 Oct 2022 02:17:20 +0000

The three main types of namespaces in python are Built-in Name spaces, Global Namespaces, and Local Namespaces.

Lets break down Namespace. Name is the identifier and space is an address in the main memory associated with that object.

Built-in Namespace
Global Namespace
Local Namespace

Built-in Namespace,

_Built-in namespaces You can use the following statement in order to see all of the standard built-in namespace!

print(dir(__builtins__))

You will see something like this

['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', 'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning', 'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError', 'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning', 'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False', 'FileExistsError', 'FileNotFoundError', 'FloatingPointError', 'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError', 'ImportWarning', 'IndentationError', 'IndexError', 'InterruptedError', 'IsADirectoryError', 'KeyError', 'KeyboardInterrupt', 'LookupError', 'MemoryError', 'ModuleNotFoundError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented', 'NotImplementedError', 'OSError', 'OverflowError', 'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError', 'RecursionError', 'ReferenceError', 'ResourceWarning', 'RuntimeError', 'RuntimeWarning', 'StopAsyncIteration', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning', 'ValueError', 'Warning', 'WindowsError', 'ZeroDivisionError', '__build_class__', '__debug__', '__doc__', '__import__', '__loader__', '__name__', '__package__', '__spec__', 'abs', 'all', 'any', 'ascii', 'bin', 'bool', 'breakpoint', 'bytearray', 'bytes', 'callable', 'chr', 'classmethod', 'compile', 'complex', 'copyright', 'credits', 'delattr', 'dict', 'dir', 'divmod', 'enumerate', 'eval', 'exec', 'exit', 'filter', 'float', 'format', 'frozenset', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex', 'id', 'input', 'int', 'isinstance', 'issubclass', 'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview', 'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property', 'quit', 'range', 'repr', 'reversed', 'round', 'set', 'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars', 'zip']

Global Namespace,

Global namespaces contain any names defined at the program or module level. Python creates the global namespace when the main program of the body starts, and exists until the program is terminated by the python interpreter.

Local namespace,

Local namespace is defined for a function, a class, a loop, or any block of code. The names defined in a block of code are local which means the variable names cannot me accessed outside of the block of code. local namespace is created when the block of code or the function starts executing and will terminate when the block of code or function terminates.

Mutable and Immutable in Python

kster — Thu, 20 Oct 2022 23:41:07 +0000

In Python there are 2 types of objects.

Mutable objects & Immutable objects

Every object that is created is given a number that uniquely identifies it. The type of the object is defined at runtime and cannot be changed afterwards unless if the object is mutable, then that means that the state can be modified even after it is defined.

Mutable Objects
A Mutable object can be altered.

Lists
Sets
Dictionaries

Immutable Objects
Immutable objects cannot be altered after its been initially defined.

Numbers (int,float,bool,ect..)
Strings
Tuples
Frozen sets

Lets see what happens when we try to alter an immutable object.

Here's the scenario: A company hired a new employee and needs to add them to the employee database.

first_name = 'Jane'
last_name = 'Soe'

UH OH! It looks like in the process of adding this new employee there was a typo! Instead of "Jane Doe" they put "Jane Soe". Because strings are immutable we can't just change an individual character. But lets give it a try.

first_name = 'Jane'
last_name = 'Soe'
last_name[0] = 'D'

This is what we would get in the terminal

last_name[0] = 'D'
TypeError: 'str' object does not support item assignment

Now... there's a way around this.
We can concatenate the string "D" with a slice of the last_name to fix this typo.

fix_typo = 'D' + last_name[1:]

Lists in Python

kster — Tue, 18 Oct 2022 03:01:14 +0000

Python has a built-in list type named 'list'. Lists are ordered, changeable, and allow duplicate values.

Lets try creating a list

heights = [162, 162, 152, 182, 213]

To create a list we have to create a variable. In this case heights is our variable that will store a list of integers into a list.
Lists begin and end with square brackets.
Each of the items in the lists are separated with a comma.
*It's considered good practice to insert a space after each comma

Now, what if we wanted to add another heights to the list? We can use append() method, this will add an item to the end of the list.

Syntax to add an item to a list using .append

listname.append(x)

To remove an item from the list you can use .remove() this removes the first item from the list whose value is equal to x. An ValueError will be raised if there is no such item.

listname.remove(x)

Control Flow Python ☂️🌧️

kster — Wed, 12 Oct 2022 01:34:55 +0000

From our day to day we make some decisions that rely on certain situations and based on that we take further action. For example if it is raining, then bring an umbrella. In programming we find similar situations.

if statement
Using if statements we can decide whether a certain statement will be executed or not.

if is_raining:
  print("Bring an umbrella")

the : tells the computer to execute what is coming next only if the condition is met.

lets try another one

if 5 == 4 + 1:
  print("True") # Output "True"

This prints out True because the condition of the if statement is true.

if-else
What if we wanted to do something else if the condition is false? With an else statement we can run a block of code when the if statement is false.

is_raining = "No"

if is_raining == "Yes":
  print("Bring an umbrella today!")
else:
  print("No need for an umbrella today!") # This would be printed

elif
Using elif we can use multiple options.

weather = "Cold"

if weather == "Raining":
    print("Bring an umbrella today")
elif weather =="Cold":
    print("Wear a warm jacket today")
else:
    print("Weather looks great today")

The output would be "Wear a warm jacket today" since weather = "Cold"

4 Types of Inheritance in Python

kster — Wed, 05 Oct 2022 02:49:13 +0000

Inheritance in medicine, describes the passing of genetic information from parent to child through genes. In programming it pretty much the same.
Inheritance in Python allows us to define a class that inherits all methods and properties from another class.

Base class (parent class) is the class being inherited from

Derived class (child class) is the class that inherits from another class

We will be going over 5 types of inheritances

Single Inheritance
Multiple Inheritance
Multilevel Inheritance
Hierarchical Inheritance

Single Inheritance ,
Single inheritance- when a derived(child) class is derived from a single base(parent) class.

Syntax

# Base class
class Parent:
   def fun1(self):
      print("This is the parent class.")
class Child:
   def fun2(Parent):
      print("This is the child class.")

Creating the base class is the same creating any other class
To create a class that inherits the functionalities from another class, you will need to input the parent class as a parameter when creating the child class.

Multiple Inheritance,
Multiple inheritance- when derived(child) class is derived from more than one base(parent) class.

class Prey:
    def flees(self):
        print("This animal is a hunter")

class Predator:
    def fighter(self):
        print("This animal is a hunter")

class Rabbit(Prey):
    pass

class Lion(Predator):
    pass

class Fish(Prey, Predator):
    pass

rabbit = Rabbit()
lion = Lion()
fish = Fish()

rabbit.flees() # output This animal is a hunter
lion.fighter() # output This animal is a hunter
fish.flees() # output This animal is a hunter
fish.fighter() # output This animal is a hunter

Multilevel Inheritance, #Multilevel Inheritance
Multilevel inheritance - The inheritance of a derived(child) class from another derived(child) class.

class Base:
    pass

class Derived1(Base):
    pass

class Derived2(Derived1):
    pass

Here we created a Base class, Derived1 class inherits from Base class and Derived2 class inherits Derived1.

Hierarchical Inheritance ,

Hierarchical Inheritance is multiple derived classes are created from the same base class. In the image above we see that there is 1 parent class A that has multiple children B, C, D.

# Base class here
class Parent:
    def fun1(self):
        print("This function is in base class.")
#Derived class A
class ChildA(Parent):
    def fun2(self):
        print("This function is the derived class B")
#Derived class B here
class ChildB(Parent):
    def fun3(self):
        print("This function is the derived class C.")
#Derived class C here
class ChildC(Parent):
    def fun4(self):
        print("This function is the derived class D.")

object1 = ChildA()
object2 = ChildB()
object3 = ChildC()


object1.fun1()#Output This function is in base class.
object1.fun2()#Output This function is the derived class B
object2.fun1()#Output This function is in base class.
object2.fun3()#Output This function is the derived class C.
object3.fun1()#Output This function is in base class.
object3.fun4()#Output This function is the derived class D.

Here we brought our diagram to life.

Python Polymorphism

kster — Sun, 02 Oct 2022 05:43:20 +0000

Polymorphism is from the greek roots "poly" (many) and "morphe" (form).

In the context of programming, Polymorphism means that a single type entity (method, operator, or object) can behave in multiple ways.

Polymorphism in additional operator.

For integer data types, + operator is used to perform arithmetic addition operation

num1 = 1 
num2 = 2
print(num1+num2) # this program outputs '3'

For string data types, + operator is used to perform concatenation

string1 = "Good"
string2 = "Morning"
print(string1+string2) # this prints 'Good Morning'

We can see that a single operator + has been used to carry out different operations from distinct data types.

Function Polymorphism

There are some functions in Python which are compatible to run multiple data types.

One of Python's built-in functions is len(), it returns the length of an object

string = "python"
print(len(string)) # this prints 6

list =[1,2,3,4]
print(len(list))# this prints 4

Class and Method Polymorphism

class Pig:
   def diet(self):
      print("Pigs are omnivors")

class Rabbit:
    def diet(self):
        print("Rabbits are herbivors")

obj_pig = Pig()
obj_rabbit = Rabbit()

obj_pig.diet() #Returns "Pigs are omnivors"
obj_rabbit.diet() #Returns "Rabbits are herbivors"

Here we created 2 classes. Pig and Rabbit. Although both classes use the same method name diet, they define those methods differently and wont interact. Objects instantiated from the class will use the method defined in that class.

Python Object Oriented Programming (OOP)

kster — Thu, 29 Sep 2022 00:51:08 +0000

Object Oriented Programming is the process of defining classes and creating objects to represent the responsibilities of a program.

The way you go about creating an object from a class is similar how you would create a home from a blue print. When an object is created, your program asks the operating system for resources, namely memory, to be able to construct the object. On the other hand, when a house is made from a blue print, the contractor will ask what kind of wood, windows, and doors will be use in order to build the home.

Instantiation - breathes 💨 life into a class
When creating an object from a class you will need to instantiate (create) it. What this is doing is asking the operating system to give enough memory to create an object.
To instantiate an object, you add the name of the class followed by parentheses.

class House:
house = House()

'house' is the variable that holds you object's instance.
'House()' is the instantiated object.

Adding attributes to a class
To add attributes to a class you will need to tell the class what attributes it should have at construction time, when an object in instantiated. This is done by a function called a 'constructor' that is used at the moment of creation.

Constructor

In Python, the constructor has the name __init__

The __init__method lets the class initialize the object's attributes. ( This is only used within classes )

You will need to pass a special parameter self to the constructor. (self refers to the objects instance this binds the objects attributes to the arguments received). Any assignment to this keywork ends up on the object instance.

class Car:
    def __init__(self):
        self.make = "Audi"
        self.model = "A4"

car = Car()
print(car.make) 
print(car.model) 
print("Is your car make", car.make, "and model", car.model)

This example shows a class Car with two attributes, the make and model, are assigned to the constructor __init__().

Python Classes

kster — Mon, 26 Sep 2022 23:16:51 +0000

Defining a Class in Python
A class is a blue print of an object. Where the class is the blue print of the house, the object is the actual house built.

Similar to how function definitions begin with a 'def' keyword in Python, class definitions begin with a 'class' keyword.

class MyNewClass:
  '''Docstring goes here. I have created a new class'''
  pass

Here we created a class named 'MyNewClass'. Notice we use the CapWords convention since PEP 8 Style Guide for Python Code recommends this for Class names.
A docstring is not mandatory but highly recommended. It is written so programmers can understand what it does without having to read the details of the implementation.
The 'pass' keyword is used in Python to indicate that the body of the class was intentionally left blank. This prevents an 'IndentationError' from occurring in the terminal.

Python Dictionaries 🍫

kster — Sun, 25 Sep 2022 20:34:45 +0000

A dictionary is an unordered set of key:value pairs.

Dictionaries provide is a way to map pieces of data into eachother so that we can find values that are associated with one another.

Lets say we want to gather all of the calories in various candy bars 🍫

Snickers 215 🍫
Reese's 210 🍫
KitKat 218 🍫

We can create a dictionary called chocolate to store this data:

chocolate = {'Snickers': 215, 'Reese's': 210, 'KitKat': 218}

A dictionary begins with curly brackets { }
Each of the keys are surrounded by quotations " " or '' and each key:value pairs are separated by a colon :

*** It is considered good practice to insert a space after each comma, although it will still run without the spacing. ( BUT don't worry it will still run without the spacing 😊 )

We can also create an empty dictionary:

empty_dict = {}

A single key:value pair can be added to the dictionary like this:

dict[key] = value

Try it yourself ! Open the replit and add a single key:value pair to our empty_dict we created.

Multiple key:value pairs can be added to a dictionary by using the .update() method

empty_dict = {'a':'alpha'}
empty_dict.update({'b':'beta','g':'gamma'})
print(empty_dict)

Output:

{'a':'alpha','b':'beta','g':'gamma'}

Syntax for overwriting existing values:

dictonary_name['key'] = value

Let's go back to our empty_dict example and change the value of 'a' from alpha ---> apple

empty_dict = {'a':'alpha','b':'beta','g':'gamma'}
empty_dict['a'] = apple
print(empty_dict)

Output:

{'a': 'apple ', 'b': 'beta', 'g': 'gamma'}

Python Modules

kster — Sat, 24 Sep 2022 05:37:01 +0000

A module is a collection on Python Declarations intended broadly to use as tools. Modules are also referred to as "packages" or "libraries"

To use a module in a file the syntax needs to be located at the top of that file like this:

from module_name import object_name

'datetime' is a library that comes as part of the Python Standard Library. 'datetime' helps you to be able to work with date and time in Python.
Let's try 'datetime' by importing the 'datetime' module.

from datetime import datetime

time_now = datetime.now

print(time_now)

This outputs a combination of a date and time.
Attributes year, month,day,hour,minute,second,microsecond.

Example:


2022-09-25 01:34:55.601239

Lets try using a cooler module. Wikipedia is a Python library that makes it easy to access and parse data from Wikipedia.

import wikipedia

print(wikipedia.summary("python (programming language)"))

Output

Python is a high-level, general-purpose programming language. Its design philosophy emphasizes code readability with the use of significant indentation.Python is dynamically-typed and garbage-collected. It supports multiple programming paradigms, including structured (particularly procedural), object-oriented and functional programming. It is often described as a "batteries included" language due to its comprehensive standard library.Guido van Rossum began working on Python in the late 1980s as a successor to the ABC programming language and first released it in 1991 as Python 0.9.0. Python 2.0 was released in 2000 and introduced new features such as list comprehensions, cycle-detecting garbage collection, reference counting, and Unicode support. Python 3.0, released in 2008, was a major revision that is not completely backward-compatible with earlier versions. Python 2 was discontinued with version 2.7.18 in 2020.Python consistently ranks as one of the most popular programming languages.

Python Function Arguments

kster — Fri, 23 Sep 2022 02:08:30 +0000

An argument is a value that is passed to a function when it is called. It might be a variable, value, or object passed to a function or method as input.

In python, there are 3 different types of arguments we can give a function.

Default arguments: arguments that are given default values

Positional arguments: arguments that can be called by their position in the function definition

Keyword arguments: arguments that can be called by their name.

Default argument

We can give a default argument by using an assignment operator =. This will happen in the function declaration.
Example:

def find_dinner_total(food, drinks, tip):
  print(food + drinks + tip =15)

Our function is called find_dinner_total() which will calculate the total cost for dinner 🍽️. In our example our default argument is tip =15.
We can either choose to call the function without providing a value (this will result in the default value to be assigned) OR we can overwrite the default argument by entering a different value.

Positional argument

def find_diner_total(food, drinks, tip):
  print(food + drinks + tip)

The first parameter passed is food, second drinks, and third is tip.
When our function is called, the position of the arguments will be mapped based on the positions that are defined in the function declaration.
Example:

# $70 total for food
# $30 total for drinks
# 15 tip
diner_bill(70, 30, 15)

Keyword Arguments

In Keyword Arguments, where we refer to what each argument is assigned to in the function call.
Example:

dinner_bill(drinks=30, food=70, tip=15)