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Python yield clear output [duplicate]

I have a view that generates data and streams it in real time. I can't figure out how to send this data to a variable that I can use in my HTML template. My current solution just outputs the data to a blank page as it arrives, which works, but I want to include it in a larger page with formatting. How do I update, format, and display the data as it is streamed to the page?
import flask
import time, math
app = flask.Flask(__name__)
#app.route('/')
def index():
def inner():
# simulate a long process to watch
for i in range(500):
j = math.sqrt(i)
time.sleep(1)
# this value should be inserted into an HTML template
yield str(i) + '<br/>\n'
return flask.Response(inner(), mimetype='text/html')
app.run(debug=True)

You can stream data in a response, but you can't dynamically update a template the way you describe. The template is rendered once on the server side, then sent to the client.
One solution is to use JavaScript to read the streamed response and output the data on the client side. Use XMLHttpRequest to make a request to the endpoint that will stream the data. Then periodically read from the stream until it's done.
This introduces complexity, but allows updating the page directly and gives complete control over what the output looks like. The following example demonstrates that by displaying both the current value and the log of all values.
This example assumes a very simple message format: a single line of data, followed by a newline. This can be as complex as needed, as long as there's a way to identify each message. For example, each loop could return a JSON object which the client decodes.
from math import sqrt
from time import sleep
from flask import Flask, render_template
app = Flask(__name__)
#app.route("/")
def index():
return render_template("index.html")
#app.route("/stream")
def stream():
def generate():
for i in range(500):
yield "{}\n".format(sqrt(i))
sleep(1)
return app.response_class(generate(), mimetype="text/plain")
<p>This is the latest output: <span id="latest"></span></p>
<p>This is all the output:</p>
<ul id="output"></ul>
<script>
var latest = document.getElementById('latest');
var output = document.getElementById('output');
var xhr = new XMLHttpRequest();
xhr.open('GET', '{{ url_for('stream') }}');
xhr.send();
var position = 0;
function handleNewData() {
// the response text include the entire response so far
// split the messages, then take the messages that haven't been handled yet
// position tracks how many messages have been handled
// messages end with a newline, so split will always show one extra empty message at the end
var messages = xhr.responseText.split('\n');
messages.slice(position, -1).forEach(function(value) {
latest.textContent = value; // update the latest value in place
// build and append a new item to a list to log all output
var item = document.createElement('li');
item.textContent = value;
output.appendChild(item);
});
position = messages.length - 1;
}
var timer;
timer = setInterval(function() {
// check the response for new data
handleNewData();
// stop checking once the response has ended
if (xhr.readyState == XMLHttpRequest.DONE) {
clearInterval(timer);
latest.textContent = 'Done';
}
}, 1000);
</script>
An <iframe> can be used to display streamed HTML output, but it has some downsides. The frame is a separate document, which increases resource usage. Since it's only displaying the streamed data, it might not be easy to style it like the rest of the page. It can only append data, so long output will render below the visible scroll area. It can't modify other parts of the page in response to each event.
index.html renders the page with a frame pointed at the stream endpoint. The frame has fairly small default dimensions, so you may want to to style it further. Use render_template_string, which knows to escape variables, to render the HTML for each item (or use render_template with a more complex template file). An initial line can be yielded to load CSS in the frame first.
from flask import render_template_string, stream_with_context
#app.route("/stream")
def stream():
#stream_with_context
def generate():
yield render_template_string('<link rel=stylesheet href="{{ url_for("static", filename="stream.css") }}">')
for i in range(500):
yield render_template_string("<p>{{ i }}: {{ s }}</p>\n", i=i, s=sqrt(i))
sleep(1)
return app.response_class(generate())
<p>This is all the output:</p>
<iframe src="{{ url_for("stream") }}"></iframe>

5 years late, but this actually can be done the way you were initially trying to do it, javascript is totally unnecessary (Edit: the author of the accepted answer added the iframe section after I wrote this). You just have to include embed the output as an <iframe>:
from flask import Flask, render_template, Response
import time, math
app = Flask(__name__)
#app.route('/content')
def content():
"""
Render the content a url different from index
"""
def inner():
# simulate a long process to watch
for i in range(500):
j = math.sqrt(i)
time.sleep(1)
# this value should be inserted into an HTML template
yield str(i) + '<br/>\n'
return Response(inner(), mimetype='text/html')
#app.route('/')
def index():
"""
Render a template at the index. The content will be embedded in this template
"""
return render_template('index.html.jinja')
app.run(debug=True)
Then the 'index.html.jinja' file will include an <iframe> with the content url as the src, which would something like:
<!doctype html>
<head>
<title>Title</title>
</head>
<body>
<div>
<iframe frameborder="0"
onresize="noresize"
style='background: transparent; width: 100%; height:100%;'
src="{{ url_for('content')}}">
</iframe>
</div>
</body>
When rendering user-provided data render_template_string() should be used to render the content to avoid injection attacks. However, I left this out of the example because it adds additional complexity, is outside the scope of the question, isn't relevant to the OP since he isn't streaming user-provided data, and won't be relevant for the vast majority of people seeing this post since streaming user-provided data is a far edge case that few if any people will ever have to do.

Originally I had a similar problem to the one posted here where a model is being trained and the update should be stationary and formatted in Html. The following answer is for future reference or people trying to solve the same problem and need inspiration.
A good solution to achieve this is to use an EventSource in Javascript, as described here. This listener can be started using a context variable, such as from a form or other source. The listener is stopped by sending a stop command. A sleep command is used for visualization without doing any real work in this example. Lastly, Html formatting can be achieved using Javascript DOM-Manipulation.
Flask Application
import flask
import time
app = flask.Flask(__name__)
#app.route('/learn')
def learn():
def update():
yield 'data: Prepare for learning\n\n'
# Preapre model
time.sleep(1.0)
for i in range(1, 101):
# Perform update
time.sleep(0.1)
yield f'data: {i}%\n\n'
yield 'data: close\n\n'
return flask.Response(update(), mimetype='text/event-stream')
#app.route('/', methods=['GET', 'POST'])
def index():
train_model = False
if flask.request.method == 'POST':
if 'train_model' in list(flask.request.form):
train_model = True
return flask.render_template('index.html', train_model=train_model)
app.run(threaded=True)
HTML Template
<form action="/" method="post">
<input name="train_model" type="submit" value="Train Model" />
</form>
<p id="learn_output"></p>
{% if train_model %}
<script>
var target_output = document.getElementById("learn_output");
var learn_update = new EventSource("/learn");
learn_update.onmessage = function (e) {
if (e.data == "close") {
learn_update.close();
} else {
target_output.innerHTML = "Status: " + e.data;
}
};
</script>
{% endif %}

Display new graphs in Flask

I have been struggling trying to find a way to update graphs on a flask webserver. I stored the images in the static file of my directory and accessed them by {{url_for('static',filname=ph_plot.png) }} but everytime I would send a post request to fetch a new range of data the graph would not update on my webserver but on my filesystem it would. I know I can change the name of the file everytime I save it to make it appear but I dont know if that is an optimal way to display a dynamically changing photo.
Currently I have been using the send_from_directory method in flask but with it hasnt worked for me either. Below is my code.
I have been working on this for a while and would love some help! Thank you
Notes: all_plots.ph_plot() is calling a function from another python program.
FLASK CODE:
#app.route('/read_ph', methods=["GET", "POST"])
def ph_plot():
if request.method == "POST":
a = request.form['read_ph']
all_plots.ph_plot(a)
time.sleep(3)
ph_plot = os.path.join(os.getcwd(), "images/ph_plot.png")
return render_template('live_stream.html', image_name=ph_plot)
#app.route('/read_ph/<ph_plot>', methods=["GET", "POST"])
def send_ph(ph_plot):
return send_from_directory("images", ph_plot)
HTML:
<html>
<body>
<h1>Data Monitoring Station</h1>
<h2>PH</h2>
<form method="POST" action="read_ph" >
<input name="read_ph" placeholder="Instances" type="text">
</form>
<button type="button">PH Graph</button>
<img src="{{ url_for('send_ph',ph_plot=image_name) }}" id="plot" width ="220" height ="220">
<hr>
<h5> Return to main page RETURN</h5>
<hr>
</body>
</html>

send_from_directory is generally for files that have actually been uploaded into a directory from a user. This is not what you're actually trying to do; you're:
Generating the plot data
Creating a plot and spending time with matplotlib rendering it
Saving this plot image to disk
Loading that image back off disk and sending it to a user
Cut out the middleman here of disk storage: create the data and send it straight to the template. Here's a crude example using plotly.js in a single file to get the data rendered on the front end. You can keep refreshing the page to get different graphs. But note, each plot is interactive; you can zoom, for example, export etc. things with the menu in the top right, show/hide the plot (which would make more sense if there were multiple traces). You don't get any of that by rendering a plot image.
from flask import Flask, render_template_string
import random
app = Flask(__name__)
# Normally you'd have this in the templates directory but for simplicity of
# putting everything into one file for an example, I'm using a template string
template = """
<script src="https://cdn.plot.ly/plotly-latest.min.js"></script>
<div id="plot_div" style="width: 100%; height: 100%"></div>
<script type="text/javascript">
var trace1 = {
x: {{ plot_data.x_axis }},
y: {{ plot_data.y_axis }},
type: 'scatter',
name: 'Example'
};
var layout = {
title: "Test",
titlefont: {
family: 'Poppins',
size: 18,
color: '#7f7f7f'
},
showlegend: true,
xaxis: {
title: 'Axis Unit',
},
yaxis: {
title: 'Other Axis Unit',
},
margin: {
l: 70,
r: 40,
b: 50,
t: 50,
pad: 4
}
};
var data = [trace1];
Plotly.newPlot("plot_div", data, layout);
</script>
"""
def get_plot_data():
x = list(range(10))
y = [random.randint(0, 100) for i in range(10)]
return {'x_axis': x, 'y_axis': y}
#app.route('/')
def home():
plot_data = get_plot_data()
return render_template_string(template,
plot_data=plot_data)
if __name__ == '__main__':
app.run()
A common issue here is in passing datetime strings because they will be escaped. In this case, you'll need to use x: {{ plot_data.x_axis | safe }}. See Passing HTML to template using Flask/Jinja2 and How to make html markup show up?

How to use AJAX in Flask to iterate through a list

Problem
I am trying to display image-files using AJAX in Flask. More specifically, I want to display an image once a button is clicked and display the next image once the button is clicked again (like a slide-show). The filenames of the images are stored in my database. I query the database to get a list of filenames for the current user and combine each filename with the rest of the path (path to where the images are stored on disk) in order to display the image.
So far, I manage to get the first image of the current user. However, I can't figure out a way to keep track of which image is the next one to show.
I tried using a global variable as a counter (file_counter) which should serve as an index. I want to increase file_counter by 1 each time an ajax request is made in order to get the next file but the counter does not increase upon subsequent calls nor does it throw an error.
Question
How do I need to initialize the global variable (file_counter) in order for it to store its value across multiple calls? Furthermore, is the usage of global variables the correct way of doing this?
HTML
<div id="ajax-field"></div>
<button class="btn btn-block" id="next-button"><p>Next Image!</p></button>
AJAX:
$('#next-button').click(function(){
$("#ajax-field").text("");
$.ajax({
url: "/get_data",
type: "POST",
success: function(resp){
$('#ajax-field').append(resp.data);
}
});
});
Routing:
global filenames
global file_count
#app.route("/get_data", methods=['POST'])
def get_data():
try: # Is intended to fail on the first run in order for the global variables to be initialized. However it keeps failing on subsequent runs
display_img = filenames[file_count]
file_count +=1
except:
filenames = []
# current_user.uploads returns all file-objects of the current user
user_uploads = current_user.uploads
for file in user_uploads:
# file.filename returns the respective filename of the image
filenames.append(file.filename)
#filenames is now a list of filenames i.e. ['a.jpg','b.jpg','c.jpg'...]
display_img = filenames[0]
file_count = 1
path = "image_uploads/4_files/"+display_img
return jsonify({'data': render_template('ajax_template.html', mylist = path)})
ajax_template.html:
<ul>
{% block content %}
<li>
<img id="selected-image-ajax" src="{{url_for('static',filename=mylist)}}" class="img-thumbnail" style="display:block; margin:auto;"></img>
</li>
{% endblock content %}
</ul>

As #roganjosh pointed out, a session is the optimal way to store information across multiple requests. This solution presents an implementation of the photo display using flask.session to store the counter:
import flask, random, string
app = flask.Flask(__name__)
app.secret_key = ''.join(random.choice(string.printable) for _ in range(20))
#to use flask.session, a secret key must be passed to the app instance
#app.route('/display_page', methods=['GET'])
def display_page():
'''function to return the HTML page to display the images'''
flask.session['count'] = 0
_files = [i.filename for i in current_user.uploads]
return flask.render_template('photo_display.html', photo = _files[0])
#app.route('/get_photo', methods=['GET'])
def get_photo():
_direction = flask.request.args.get('direction')
flask.session['count'] = flask.session['count'] + (1 if _direction == 'f' else - 1)
_files = [i.filename for i in current_user.uploads]
return flask.jsonify({'photo':_files[flask.session['count']], 'forward':str(flask.session['count']+1 < len(_files)), 'back':str(bool(flask.session['count']))})
The display_page function will be called when a user accesses the /display_page route and will set the count to 0. get_photo is bound to the /get_photo route and will be called when the ajax request is sent.
photo_display.html:
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
</head>
<body>
<div class='image_display'>
<img src="{{photo}}" id='photo_display' height="100" width="100">
<table>
<tr>
<td class='back'></td>
<td class='forward'><button id='go_forward' class='navigate'>Forward</button></td>
</tr>
</table>
</div>
</body>
<script>
$(document).ready(function(){
$('.image_display').on('click', '.navigate', function(){
var direction = 'b';
if ($(this).prop('id') === 'go_forward'){
direction = 'f';
}
$.ajax({
url: "/get_photo",
type: "get",
data: {direction: direction},
success: function(response) {
$('#photo_display').attr('src', response.photo);
if (response.back === "True"){
$('.back').html("<button id='go_back' class='navigate'>Back</button>")
}
else{
$('#go_back').remove();
}
if (response.forward === "True"){
$('.forward').html("<button id='go_forward' class='navigate'>Forward</button>")
}
else{
$('#go_forward').remove();
}
},
});
});
});
</script>
</html>
The javascript in display_page.html communicates with the backend, and updates the img tag src accordingly. The script adds or removes the navigation buttons, depending on the current count value.
Demo:
To test the solution above, I created an image folder to store random photographs to display:

Update Value dynamically using repeated Ajax calls from DJango template

I wanted to show my sound sensor readings from a django site (original code as posted in the link). Now as per the situation 2 of the accepted answer, I wanted to make a Javascript function which repeatedly calls the ajax_data function from views template.
But it seems that no repeated calls are being made. And no update in reading reflects either.
My django template till now:
<!doctype html>
<html>
<head>
<title>Noise measurement site</title>
<script src="http://code.jquery.com/jquery-3.0.0.js"
integrity="sha256-jrPLZ+8vDxt2FnE1zvZXCkCcebI/C8Dt5xyaQBjxQIo="
crossorigin="anonymous"></script>
<script language="javascript" type="text/javascript">
function updateValue() {
$.ajax({
url:"D:/Python programs/django_projects/Noise_Measurement/noise_m/views.py/ajax_data/",
//success: updateValue(), for experimenting with a recursive call...
});
}
$(document).ready(function(){
var v = setInterval(updateValue,2000);
});
</script>
</head>
<body>
<p>Hello there</p>
<p>Present noise level : {{noise_level}} dB</p>
</body>
</html>
(I have mentioned rest of the code in my previously asked question. I have read some of the answers on the platform but I'm not getting much results.)
Update
Sorry I made a mistake. I made slight changes in the code and posted output without that only. Now I have made the exact changes as in the previous part. But output is not sorted out yet. (Thanks to comment by CumminUp07)

** Own answer on own question **
Oh sorry, actually it was a misunderstanding of the syntax from my side.
I was first supposed to create method in views.py, which will send the reading from the module taking it. Then for that method, I had to assign an url using path(), in a fashion like:
path('read', views.data_update1, name='readings'),
Then the ajax request was supposed to be made to read link:
$.ajax({
url: "read",
dataType: "json",
contentType: "application/json",
success: function(r) { ... }
});
Then this method is conveniently called using setInterval method.
But finally at the line, the {{ }} didn't help, so the div where the value was to be displayed was assigned an id, whose value was updated on each call of the method.

Flask and Jinja2 url_for error - concatenating json object into url_for

I am using Flask with Jinja2 and MapBox on a project which involves plotting data on a map using GeoJSON derived from model data. Example of how this is loaded:
$.getJSON("{{ url_for(".geojson") }}", function(data) {
var geojson = L.geoJson(data, {
onEachFeature: function (feature, layer) {
//do stuff
}
});
markers.addLayer(geojson);
var map = L.map('map', {maxZoom: 9, minZoom: 3}).fitBounds(markers.getBounds());
baseLayer.addTo(map);
markers.addTo(map);
An example of using this JSON data within my JS:
var feature = e.layer.feature;
//print item name
console.log(feature.properties.name)
//print item latitude
console.log(feature.properties.latitude)
//print item category info
console.log(feature.properties.category.name)
This works great. My dataset has now extended to include image urls (example 09379_580_360.jpg), and the images themselves are hosted in a static/images/eol folder. I'd like to include these as an image within a DIV, of which I am setting dynamically via JS like so...
var commoncontent = '<div class="panel-heading"><h3>'+feature.properties.name+'</h3></div>'
$('#common').html(commoncontent)
However, when I attempt to concatenate my image data into jinja's url_for...
var commoncontent = '<div><img src="{{ url_for("static", filename="images/eol/thumbs/big/'+feature.properties.category.localimageurl.jpg+'") }}"></div>'
... I get this error in my console
GET http://127.0.0.1:5000/static/images/eol/thumbs/big/feature.properties.category.localimageurl.jpg 404 (NOT FOUND)
I know that feature.properties.category.localimageurl is correct as it prints to my console when I console.log() it. However, I have no idea why the interpreter is taking it directly as a string and not concatenating it?

feature is a JavaScript object. Jinja doesn't have access to those; it runs on the server, whereas your JavaScript runs in the client. feature doesn't exist when your template is rendered. You will need to handle the concatenation with JavaScript.
var commoncontent = '<div><img src="{{ url_for("static", filename="images/eol/thumbs/big/") }}' + feature.properties.category.localimageurl.jpg + '"></div>'