Found a Flask template bug caused by PyObject_HEAD size in Python for 32bit Windows.

Today, I was facing an issue when using Flask template.

If use an dash “-“ linked variable name in Flask template, it will cause Flask to hang without
throwing any exception.

At beginning, I guess it may be an issue of Jinja template rending. But after traced in IDE
(Pycharm community 2019.3), I saw that templating raised the exception and Werkzeug is handling.

At first I uses Flask debug mode and IDE debug run to step into codes to dig it. IDE gets
slower and slower and the debugger hung in Werkzerg codes.

Eventually, I found an endless loop in Python\Lib\traceback.py to walk trackback stacks. The
trackbacks are stored in a link which nodes point to next. In flask\templating.py,
flask\app.py and werkzeug\debug\tbtools.py, the exception was catched. In
handle_exception() of flask\app.py, Werkzeug wants to log it and print it out.
The trackback link has looped reference as below picture.

Simple code to re-produce it as below (“font-size” variable caused.).

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import sys
from flask import Flask, request, render_template_string, make_response, url_for, redirect


app = Flask(__name__)

template_page = '''
<html>
<head>
<meta charset="{{ encoding }}" />
<meta http-equiv="content-type" content="text/html; charset={{ encoding }}" />
<style type="text/css">
  body, pre, p, div, input, h1,h2,h3,h4,h5 {
    font-family : Consolas, Courier New;
  }
</style>
</head>
<body>
    <div id="wrapper" style="font-size:{{ font-size }}em">
        {{ content | safe }}
    </div>
</body>'''


@app.route('/', methods=['GET'])
def index():
    context = {
        "encoding": 'utf-8',
        "font-size": 0.9,
        "content": 'hello flask'
    }
    resp = make_response(render_template_string(template_page, **context))
    resp.headers['Content-Type'] = 'text/html; charset=utf-8'
    return resp


if __name__ == "__main__":
    # app.run(debug=True, host='0.0.0.0', port=8080)

    app.run()

Raised Github issue at Flask:

Environment:

  • Window 10
  • Python 3.6.8150.1013 (venv)
  • Flask 1.1.1
  • Werkzeug 0.16.1

Possible related Python issue:

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