We can control 404 errors and bad requests using the abort() function provided by flask.
What it does is that it lets us end a request.

[app.py]

from flask import (Flask, g, render_template, flash, redirect, url_for, 
                    abort)
...

@app.route('/stream')
@app.route('/stream/')
def stream(username=None):
    template = 'stream.html'
    if username and username != current_user.username:
        try:
            user = models.User.select().where(models.User.username**username).get()
        except models.DoesNotExist:
            abort(404)
        else:
            stream = user.posts.limit(100)
    else:
        stream = current_user.get_stream().limit(100)
        user = current_user
    if username:
        template = "user_stream.html"
    return render_template(template, stream=stream, user=user)


@app.route('/follow/')
@login_required
def follow():
    try:
        to_user = models.User.get(models.User.username**username)
    except models.DoesNotExist:
        abort(404)
    else:
        try:
            models.Relationship.create(from_user=g.user._get_current_object(), to_user=to_user)
        except IntegrityError:
            pass
        else:
            flash("You have successfully followed {}!".format(to_user.username), "success")
    return redirect(url_for('stream', username=to_user.username))


@app.route('/unfollow/')
@login_required
def unfollow():
    try:
        to_user = models.User.get(models.User.username**username)
    except models.DoesNotExist:
        abort(404)
    else:
        try:
            models.Relationship.get(from_user=g.user._get_current_object(), to_user=user).delete_instance()
        except IntegrityError:
            pass
        else:
            flash("You have unfollowed {}.".format(to_user.username), "success")
    return redirect(url_for('stream', username=to_user.username))


@app.route('/post/')
def view_post(post_id):
    if post_id:
        posts = models.Post.select().where(models.Post.id==post_id)
        if posts.count() == 0:
            abort(404)
        return render_template('stream.html', stream=posts)


@app.errorhandler(404)
def not_found(error):
    return render_template('404.html'), 404
...

abort(404)
A function that Flask uses to end a view early and throw an error.
If you know HTTP status code, 404 is File Does Not Exist

if posts.count() == 0:
   abort(404)
If we don’t get any records that match the post id we won’t get the models.DoesNotExist error, because we’re using a .select().where() filter instead of .get(). So we have to check for the record using this approach.

@app.errorhandler(404)
def not_found(error)
Anytime a 404 is triggered, our app will use the not_found function, where it renders the 404.html template and passes the 404 error.

[404.html]

{% extends "layout.html" %}

{% block content %}

<h1>404</h1>
<p>Sorry, that page does not exist.</p>
<p><a href="{{ url_for('index') }}">Try again</a></p>

{% endblock %}

Nothing out of the ordinary here.