PyTraceToIX is an expression tracer designed for debugging Jinja2 templates, Flask web apps, lambdas, list comprehensions, method chaining, and expressions in general.

Code editors often cannot set breakpoints within these kinds of expressions, which requires significant code modifications to debug effectively.

For Jinja2 templates, the debug extension can be used, but it typically dumps the entire context, making it difficult to isolate specific issues. PyTraceToIX solves this by allowing developers to trace and write specific data directly to sys.stdout or a stream without altering the design or making any changes to the web application.

Additionally, PyTraceToIX can capture multiple inputs and their results, displaying them all in a single line, making it easier to view aggregated data and trace the flow of values.

PyTraceToIX offers a straightforward solution to these challenges, simplifying debugging while preserving the integrity of the original codebase.

It was designed to be simple, with easily identifiable functions that can be removed once the bug is found.

PyTraceToIX has 2 major functions:

c__ capture the input of an expression input. ex: c__(x)
d__ display the result of an expression and all the captured inputs. ex: d__(c__(x) + c__(y))

And 2 optional functions:

init__ initializes display format, output stream and multithreading.
t__ defines a name for the current thread.

Installation

pip install pytracetoix

Jinja2 templates Usage

In this example:

A flask web app uses a Jinja2 template
It generates a shopping card html table with product, quantity and final price

Product	Qty	Final Price
Smartphone	5	2500
Wireless B	50	49960
Smartphone	20	1990

The product name is only the first 11 characters, but we need to know the full name.
It only shows the final price which is Price * Qty - discount.
The discount is dependent of the quantity.
c__ captures the complete name but doesn't change the design.
c__ captures the qty and labels it as Qty.
c__ captures the discount value.
d__ outputs to sys.stdout all the captured inputs and the final price.

The stdout will display these lines:

i0:`Smartphone 128GB` | qty:`5` | i2:`500` | discount:`0` | _:`2500`
i0:`Wireless Bluetooth Headphones` | qty:`50` | i2:`1000` | discount:`40` | _:`49960`
i0:`Smartphone 64GB Black` | qty:`20` | i2:`100` | discount:`10` | _:`1990`

Jinja2 template:

<html lang="en">
<head><link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0-alpha3/dist/css/bootstrap.min.css" rel="stylesheet"></head>
<body>
    <div class="container mt-5">
        <h1>Shopping Cart</h1>
        <table class="table table-striped">
            <tr><th>Product</th><th>Qty</th><th>Final Price</th></tr>
            {% for item in purchases %}
            {% set product = products[item['product']] %}
            <tr>
                <td>{{ c__(product['name'])[0:10] }}</td>
                <td>{{ c__(item['qty'], name='qty') }}</td>
                <td>{{ d__(c__(product['price']) * item['qty']
                    - c__(discount(item['qty']), name='discount')) }}</td>
            </tr>
            {% endfor %}
        </table>
    </div>
</body>
</html>

app.py:

from flask import Flask, render_template
from pytracetoix import c__, d__

app = Flask(__name__)

app.Jinja2_env.globals['d__'] = d__
app.Jinja2_env.globals['c__'] = c__

DISCOUNTS = {50: 40, 20: 10, 10: 5, 0: 0}
PRODUCTS = {
    'WB50CC': {'name': 'Wireless Bluetooth Headphones', 'price': 1000},
    'PH20XX': {'name': 'Smartphone 128GB', 'price': 500},
    'PH50YY': {'name': 'Smartphone 64GB Black', 'price': 100}
}

PURCHASES = [
    {'product': 'PH20XX', 'qty': 5},
    {'product': 'WB50CC', 'qty': 50},
    {'product': 'PH50YY', 'qty': 20}
]


def discount(qty): return next((k, v) for k, v in DISCOUNTS.items() if k <= qty)[1]


@app.route('/', methods=['GET'])
def index():
    return render_template('index.html', products=PRODUCTS, purchases=PURCHASES, discount=discount)


if __name__ == '__main__':
    app.run(debug=True)

If the previous example, we add c__ to the discount function on app.py:

def discount(qty): return c__(next((k, v) for k, v in DISCOUNTS.items() if k <= qty))[1]

It will add richer discount information to the output:

i0:`Smartphone 128GB` | qty:`5` | i2:`500` | i3:`(0, 0)` | discount:`0` | _:`2500`
i0:`Wireless Bluetooth Headphones` | qty:`50` | i2:`1000` | i3:`(50, 40)` | discount:`40` | _:`49960`
i0:`Smartphone 64GB Black` | qty:`20` | i2:`100` | i3:`(20, 10)` | discount:`10` | _:`1990`

PyTraceToIX - How to debug Jinja2 templates, Flask web apps without breaking the design or code changes

Installation

Jinja2 templates Usage