Category: Python

pyenv
pyenv is a popular command-line tool that helps Python developers manage multiple Python versions efficiently. If you work on various Python projects, each requiring a different version of Python, pyenv keeps your workflow smooth by allowing you to install, switch, and manage Python versions effortlessly.

https://github.com/pyenv/pyenv

To install, run
```
curl -fsSL https://pyenv.run | bash
```
Edit ~/.profile
```
vi ~/.profile
```
add at the end of the file
```
export PYENV_ROOT="$HOME/.pyenv"
[[ -d $PYENV_ROOT/bin ]] && export PATH="$PYENV_ROOT/bin:$PATH"
eval "$(pyenv init - bash)"
```
Install build dependency

on Ubunutu, run
```
apt update; apt install make build-essential libssl-dev zlib1g-dev libbz2-dev libreadline-dev libsqlite3-dev curl git libncursesw5-dev xz-utils tk-dev libxml2-dev libxmlsec1-dev libffi-dev liblzma-dev
```
For other operating systems, see

https://github.com/pyenv/pyenv/wiki#suggested-build-environment

Install a specific Python version
```
pyenv install 3.10.7
```
Set the global Python version:
```
pyenv global 3.10.7
```
Set a local version for a project:
```
pyenv local 3.9.13
```
To list all installed python versions, run:
```
pyenv versions
```
Back to Python
July 17, 2025

Serve a Django application using Gunicorn

Create systemd service file

vi /usr/lib/systemd/system/backend.service

Add

[Unit]
Description=Gunicorn instance to serve Django Application
After=network.target

[Service]
User=www-data
Group=www-data
Environment="APP=app"
WorkingDirectory=/path/to/app/backend
Environment="PATH=/path/to/app/backend/venv/bin"
ExecStart=/path/to/app/backend/venv/bin/gunicorn -w 3 --timeout 120 --bind 0.0.0.0:8080 backend.wsgi

[Install]
WantedBy=multi-user.target

On the above, you need to replace “/path/to/app/backend” with the path to your application. In backend.wsgi, backend is the application name here, change it to whatever your Django application name is.

User=www-data
Group=www-data

You can change this to whatever user you use to upload your Django application files.

Enable and start the service

systemctl enable backend.service
systemctl start backend.service

Now your Django application will be available on port 8080. You can configure nginx or apache to proxy to this port.

Here is a sample nginx config

server {
    listen 443 ssl;
    server_name app.serverok.in;
    ssl_certificate /etc/ssl/app.serverok.in.crt;
    ssl_certificate_key /etc/ssl/app.serverok.in.key;
    root /var/www/app.serverok.in/html/;
    index index.html;
    location /media/ {
        alias /var/www/app.serverok.in/html/media/;
    }

    location /api/ {
        include proxy_params;
        proxy_pass http://localhost:8080/api/;
    }
}

Here Django application is served as /api/ folder, you can change this as required.

Back to Gunicorn

March 22, 2023

ModuleNotFoundError: No module named ‘MySQLdb’
When running migration for a Django application with “python manage.py migrate”, I got the following error message
```
ModuleNotFoundError: No module named 'MySQLdb'
```
To fix install
```
pip install mysqlclient
```
This will require MySQL development package. On the Ubuntu server running MariaDB, I installed packaged libmariadb-dev
```
sudo apt install libmariadb-dev
```
Back to Errors
December 9, 2022
error: invalid command ‘bdist_wheel’
When installing a python module on Ubuntu, I got the following error message
```
error: invalid command 'bdist_wheel'
```
The error is fixed by installing
```
apt install -y python3-dev libpq-dev python3-wheel gcc
pip3 install wheel
```
Back to Errors
December 9, 2022

How to install Python 3.10 on CentOS 7

First, install the dependency

yum install -y wget zlib-devel
yum groupinstall -y "Development tools"

Install OpenSSL as the openssl-devel package provided by CentOS is old version. Install OpenSSL from source with

cd /usr/local/src
wget https://www.openssl.org/source/openssl-1.1.1g.tar.gz
tar xvf openssl-1.1.1g.tar.gz
cd openssl-1.1.1g
./config --prefix=/usr/serverok/openssl --openssldir=/usr/serverok/openssl no-ssl2
make
make install

Run following commands

export PATH=/usr/serverok/openssl/bin:$PATH
export LD_LIBRARY_PATH=/usr/serverok/openssl/lib
export LC_ALL="en_US.UTF-8"
export LDFLAGS="-L/usr/serverok/openssl/lib -Wl,-rpath,/usr/serverok/openssl/lib"

Download the latest version of Python from

https://www.python.org/downloads/

To install, run

cd /usr/local/src
wget https://www.python.org/ftp/python/3.10.6/Python-3.10.6.tar.xz
tar xvf Python-3.10.6.tar.xz
cd /usr/local/src/Python-3.10.6
make clean && make distclean
./configure --enable-optimizations  --with-openssl=/usr/serverok/openssl/
make altinstall

Python 10 will be installed in folder

/usr/local/bin/python3.10

To install pip modules, you can use

python3.10 -m pip install -r requirements.txt

Back to Python

August 30, 2022

ModuleNotFoundError: No module named ‘PIL’

When running a python application, I got the error message ModuleNotFoundError: No module named ‘PIL’

(venv) boby@sok-01:~/work/powder (master)$ python main.py 
Traceback (most recent call last):
  File "main.py", line 6, in 
    from extras import *
  File "/home/boby/work/powder/extras.py", line 2, in 
    from PIL import Image, ImageDraw, ImageFont
ModuleNotFoundError: No module named 'PIL'
(venv) boby@sok-01:~/work/powder (master)$

To fix the error install Pillow with

pip install Pillow

June 26, 2022

This version of ChromeDriver only supports Chrome version

When running a python application, I get the error message

(venv) boby@sok-01:~/work/powder (master)$ python main.py 
59 seconds remaining
Process Process-1:
Traceback (most recent call last):
  File "/usr/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
    self.run()
  File "/usr/lib/python3.8/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "main.py", line 51, in main
    driver = webdriver.Chrome(
  File "/home/boby/work/powder/venv/lib/python3.8/site-packages/selenium/webdriver/chrome/webdriver.py", line 69, in __init__
    super().__init__(DesiredCapabilities.CHROME['browserName'], "goog",
  File "/home/boby/work/powder/venv/lib/python3.8/site-packages/selenium/webdriver/chromium/webdriver.py", line 92, in __init__
    super().__init__(
  File "/home/boby/work/powder/venv/lib/python3.8/site-packages/selenium/webdriver/remote/webdriver.py", line 277, in __init__
    self.start_session(capabilities, browser_profile)
  File "/home/boby/work/powder/venv/lib/python3.8/site-packages/selenium/webdriver/remote/webdriver.py", line 370, in start_session
    response = self.execute(Command.NEW_SESSION, parameters)
  File "/home/boby/work/powder/venv/lib/python3.8/site-packages/selenium/webdriver/remote/webdriver.py", line 435, in execute
    self.error_handler.check_response(response)
  File "/home/boby/work/powder/venv/lib/python3.8/site-packages/selenium/webdriver/remote/errorhandler.py", line 247, in check_response
    raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.SessionNotCreatedException: Message: session not created: This version of ChromeDriver only supports Chrome version 98
Current browser version is 103.0.5060.53 with binary path /usr/bin/google-chrome

To fix the error, download the chrome driver for your google chrome browser version from

https://chromedriver.chromium.org/downloads

Extract the file. You will get a file with the name “chromedriver”. Replace your chromedriver file with this one.

sudo mv ~/Downloads/chromedriver_linux64/chromedriver /usr/local/bin/chromedriver

June 26, 2022

OSError: mysql_config not found
When installing the python mysqlclient module with pip, I got the error “OSError: mysql_config not found”.

(venv) root@sok:~# pip install mysqlclient
Collecting mysqlclient
Downloading mysqlclient-2.1.0.tar.gz (87 kB)
|████████████████████████████████| 87 kB 1.2 MB/s
ERROR: Command errored out with exit status 1:
command: /root/venv/bin/python3 -c ‘import sys, setuptools, tokenize; sys.argv[0] = ‘”‘”‘/tmp/pip-install-r5qp0a97/mysqlclient/setup.py’”‘”‘; __file__=’”‘”‘/tmp/pip-install-r5qp0a97/mysqlclient/setup.py’”‘”‘;f=getattr(tokenize, ‘”‘”‘open’”‘”‘, open)(__file__);code=f.read().replace(‘”‘”‘\r\n’”‘”‘, ‘”‘”‘\n’”‘”‘);f.close();exec(compile(code, __file__, ‘”‘”‘exec’”‘”‘))’ egg_info –egg-base /tmp/pip-install-r5qp0a97/mysqlclient/pip-egg-info
cwd: /tmp/pip-install-r5qp0a97/mysqlclient/
Complete output (15 lines):
/bin/sh: 1: mysql_config: not found
/bin/sh: 1: mariadb_config: not found
/bin/sh: 1: mysql_config: not found
Traceback (most recent call last):
File ““, line 1, in
File “/tmp/pip-install-r5qp0a97/mysqlclient/setup.py”, line 15, in
metadata, options = get_config()
File “/tmp/pip-install-r5qp0a97/mysqlclient/setup_posix.py”, line 70, in get_config
libs = mysql_config(“libs”)
File “/tmp/pip-install-r5qp0a97/mysqlclient/setup_posix.py”, line 31, in mysql_config
raise OSError(“{} not found”.format(_mysql_config_path))
OSError: mysql_config not found
mysql_config –version
mariadb_config –version
mysql_config –libs
—————————————-
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
(venv) root@sok:~#

The error is fixed by installing MySQL development library.

On Ubuntu server with MariaDB, i installed
```
apt install libmariadb-dev
```
For MySQL, you can use
```
apt-get install libmysqlclient-dev
```
February 21, 2022
Uvicorn
Uvicorn is a lightning-fast ASGI server used to serve Python ASGI frameworks like FastAPI, Django Channels, Quart, BlackSheep, etc…

https://www.uvicorn.org

To install Uvicorn, run
```
pip install uvicorn
```
To run, you can use
```
uvicorn main:app --host 0.0.0.0 --port 8000 --reload
```
See Python
September 11, 2021
Flask Python framework
Flask is a Python framework. You can find more info at the official website.

https://flask.palletsprojects.com/

How to start a Flask application?

To start flask application, run
```
python app.py
```
How to run flask application on port 80

To run flask application on port 80, use command
```
flask run --host 0.0.0.0 --port 80
```
Or edit app.py, use the following code
```
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=80)
```
Or you can use the following environment variables
```
FLASK_RUN_HOST=0.0.0.0
FLASK_RUN_PORT=80
```
Port 80 can only be used by the root user, so you need to run the above command as root. Only do this for testing as it is insecure to run an application as the root user.

Default flask app name

When you run
```
flash run
```
It looks for app.py file. If you need to change the file name, you can use the environment variable
```
FLASK_APP=application.py
```
See python
August 20, 2021

Python mechanize HTTP Error 403 request disallowed by robots.txt

When running following python script i get error “HTTP Error 403 request disallowed by robots.txt”

from mechanize import Browser

a = ['https://google.com', 'https://serverok.in', 'https://msn.com']

br = Browser() 

for x in range(len(a)):     
    br.open(a[x]) 
    print("Website title: ")
    print(br.title())
    print("\n")

To fix this, find

br = Browser()

Add below

br.set_handle_robots(False)
br.addheaders = [('User-agent', 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.77 Safari/537.36')]

set_handle_robots is used to disable robots.txt checking. Second line will set a User-agent, so remote won’t won’t block you as a robot.

See Python

June 1, 2020

error: Python.h: No such file or directory
When install dlib python module, i get error
```
ERROR: Failed building wheel for dlib fatal 
error: Python.h: No such file or directory
```
This is because missing python3-dev package. To fix, install
```
apt install python3-dev
```
If you are using python virtualenv, you will need to recreate the virtualenv after installing dev package.
```
virtualenv -p /usr/bin/python3 venv
```
February 16, 2020

Category: Python

How to start a Flask application?

How to run flask application on port 80

Default flask app name