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Flask is a micro web framework that is written in Python. It is classified as a microframework because it does not require specific tools or libraries, does not have a default database, and does not have an authentication mechanism. Despite these, it is equipped with all necessary tools needed to build a robust web application.

Introduction to Flask

Flask is based on Werkzeug WSGI toolkit and Jinja2 template engine and was released in April 2010 as part of the Pocoo projects. This flexible framework is known for its simplicity and fine-grained control, unlike Django that follows a batteries-included approach.

When building your web application, Flask allows you to decide which components you would like to use. This extends your flexibility and helps you to avoid unnecessary dependencies.

Installing and Using Flask

To start with Flask, you need to install it. You can do this by running the pip install flask command in your terminal. Once you are done installing, you can proceed to build your web application.

A common way to start a new Flask web app involves setting up a basic structure including a Flask instance, defining routes, and creating view functions.

Here is a basic example of a Flask app.

from flask import Flask
app = Flask(__name__)

def hello_world():
    return 'Hello, World!'

Understanding Flask Code

The code begins by importing the Flask module and creating a Flask web server from the Flask module. The Flask constructor takes the name of the current module (__name__) as an argument. The route() function of the Flask class is a decorator, which tells the application which URL should call the associated function.

In the above code, ‘/’ URL is bound with `hello_world()` function. Hence, when the home page of the webserver is opened in a browser, the `hello_world()` function is executed and previews ‘Hello, World!’ on the browser page.

Flask Redirects and Errors

Flask provides a powerful remapping feature. URL remapping is a technique to make a web page available under several URL addresses. It allows us to freely remap URLs to different handlers based on the application condition.

from flask import Flask, redirect, url_for, render_template, request
app = Flask(__name__)

def hello_world(user):
    return render_template('hello.html', name = user)

In the code above, the `/` URL triggers the `hello_world` function and passes the as a variable to the function. The function then uses this user to render a template called `hello.html`.

Flask also provides a way to handle errors, HTTP errors, and user-made errors can be handled and rectified using Flask error handlers.

Understanding Flask, its basic codes, redirects, and error handlers is the first step to building a robust Python web application with much flexibility as it allows you to make a custom choice for your tools. Moreover, Flask enables you to maintain fewer and cleaner codes, emphasizing convention over configuration.

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