Ali Hocam, elinize sağlık. Ancak bir problemle karşılaştım, bağlanamıyorum, hata veriyor. Neden olduğunu anlayamadım.
Pycharm kullanıyorum. "celikKiris" diye yeni bir proje oluşturdum. Projenin altında main.py adında bos bir python dosyası oluşturdum. Sonra sizin 'main.py' dosyasının içindeki kodu benimkine kopyaladim. Flask'ı ve mysql.connect'i kurdum. Programı çalıştırdım. İlk olarak şöyle bir çıktı verip beklemeye başladı:

/home/ozgun/PycharmProjects/celikKiris/venv/bin/python /home/ozgun/PycharmProjects/celikKiris/main.py
VERİTABANI YÜKLENİYOR..

• Serving Flask app 'main' (lazy loading)
• Environment: production
  WARNING: This is a development server. Do not use it in a production deployment.
  Use a production WSGI server instead.
• Debug mode: off
• Running on all addresses.
  WARNING: This is a development server. Do not use it in a production deployment.
• Running on http://192.168.0.21:8080/ (Press CTRL+C to quit)

Bu durumda "http://192.168.0.21:8080/" adresine tıklayınca şöyle bir hata verdi:

Internal Server Error

The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.

Firewall'u kapatıp açtım gene olmadı. Portun
değerini 3306 yapıp tekrar denedim olmadı.
Kullandığım işletim sistemi: Linux
Tarayıcı: Firefox

Pycharm konsolunda da şu hatayı verdi:

[2021-05-24 12:14:21,142] ERROR in app: Exception on / [GET]
Traceback (most recent call last):
File "/home/ozgun/PycharmProjects/celikKiris/venv/lib/python3.9/site-packages/flask/app.py", line 2070, in wsgi_app
response = self.full_dispatch_request()
File "/home/ozgun/PycharmProjects/celikKiris/venv/lib/python3.9/site-packages/flask/app.py", line 1515, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/home/ozgun/PycharmProjects/celikKiris/venv/lib/python3.9/site-packages/flask/app.py", line 1513, in full_dispatch_request
rv = self.dispatch_request()
File "/home/ozgun/PycharmProjects/celikKiris/venv/lib/python3.9/site-packages/flask/app.py", line 1499, in dispatch_request
return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
File "/home/ozgun/PycharmProjects/celikKiris/main.py", line 20, in giris_ekran
return render_template("a_tablo.html")
File "/home/ozgun/PycharmProjects/celikKiris/venv/lib/python3.9/site-packages/flask/templating.py", line 148, in render_template
ctx.app.jinja_env.get_or_select_template(template_name_or_list),
File "/home/ozgun/PycharmProjects/celikKiris/venv/lib/python3.9/site-packages/jinja2/environment.py", line 1068, in get_or_select_template
return self.get_template(template_name_or_list, parent, globals)
File "/home/ozgun/PycharmProjects/celikKiris/venv/lib/python3.9/site-packages/jinja2/environment.py", line 997, in get_template
return self._load_template(name, globals)
File "/home/ozgun/PycharmProjects/celikKiris/venv/lib/python3.9/site-packages/jinja2/environment.py", line 958, in _load_template
template = self.loader.load(self, name, self.make_globals(globals))
File "/home/ozgun/PycharmProjects/celikKiris/venv/lib/python3.9/site-packages/jinja2/loaders.py", line 125, in load
source, filename, uptodate = self.get_source(environment, name)
File "/home/ozgun/PycharmProjects/celikKiris/venv/lib/python3.9/site-packages/flask/templating.py", line 59, in get_source
return self._get_source_fast(environment, template)
File "/home/ozgun/PycharmProjects/celikKiris/venv/lib/python3.9/site-packages/flask/templating.py", line 95, in _get_source_fast
raise TemplateNotFound(template)
jinja2.exceptions.TemplateNotFound: a_tablo.html
192.168.0.21 - - [24/May/2021 12:14:21] "GET / HTTP/1.1" 500 -

Ali Hocam ne yapmalıyım? Nerde hata yapıyorum acaba? Kerem hocam, Sıla hocam siz de deneyebilir misiniz?