Develop Reference

Python is installed but is not working in terminal

I installed Python 3 using the Cygwin terminal. There were no issues during installation.
I get this:
$ which python3
/usr/bin/python3
and
$ whereis python3
python3: /usr/bin/python3 /usr/bin/python3.6 /usr/lib/python3.6 /usr/lib/python3.8
/usr/include/python3.6m /usr/include/python3.8 /usr/share/man/man1/python3.1.gz
But neither of
$ python3 $ python3 main.py $ python3 --version
give any output. I have done this the same way before on the same computer and it worked fine. What did I do wrong this time? Or forgot to do?

Try to create an alias to attach python to the common name "python3". In your Cygwin terminal write:
$ alias python3=$(which python3.6)

python3 should be a link managed by alternatives pointing to python3.8
$ alternatives --display python3
python3 - status is auto.
link currently points to /usr/bin/python3.8
/usr/bin/python3.6 - priority 36
/usr/bin/python3.7 - priority 37
/usr/bin/python3.8 - priority 38
Current `best' version is /usr/bin/python3.8.
that belongs to python38 package
$ cygcheck -f /usr/bin/python3.8
python38-3.8.7-3
verify that python38 is correctly installed
$ cygcheck -c python38
Cygwin Package Information
Package Version Status
python38 3.8.7-3 OK
and that there are no issue on the dependency
$ cygcheck /usr/bin/python3.8.exe
D:\cygwin64\bin\python3.8.exe
D:\cygwin64\bin\cygwin1.dll
C:\WINDOWS\system32\KERNEL32.dll
C:\WINDOWS\system32\ntdll.dll
C:\WINDOWS\system32\KERNELBASE.dll
D:\cygwin64\bin\libpython3.8.dll
D:\cygwin64\bin\cygintl-8.dll
D:\cygwin64\bin\cygiconv-2.dll
D:\cygwin64\bin\cyggcc_s-seh-1.dll
More likely you need to re-install python38 or one of the needed libraries
$ cygcheck -f /usr/bin/cygwin1.dll /usr/bin/cygintl-8.dll /usr/bin/cygiconv-2.dll /usr/bin/cyggcc_s-seh-1.dll
cygwin-3.1.7-1
libgcc1-10.2.0-1
libiconv2-1.16-2
libintl8-0.19.8.1-2
be sure to stop all Cygwin processes before re-installing the packages; specially any running service

create the python soft link fails

I have multiple Python versions, and thus also include multiple Python binary executables.
ls /usr/bin/python*
shows the python environments in my ubuntu 18.04
/usr/bin/python /usr/bin/python2.7 /usr/bin/python2-pbr /usr/bin/python3 /usr/bin/python3.5m /usr/bin/python3.6-config /usr/bin/python3.6m-config /usr/bin/python3m
/usr/bin/python2 /usr/bin/python2-jsonschema /usr/bin/python2-wsdump /usr/bin/python3.5 /usr/bin/python3.6 /usr/bin/python3.6m /usr/bin/python3-config /usr/bin/python3m-config
in order to satisfy the PyFlink requirement regarding the Python environment version, i have to choose to soft link python to point to your python3 interpreter:
ln -s /usr/bin/python3 python
however when I use the command, it tells me
ln: failed to create symbolic link 'python': File exists
so i wonder do I need to delete the /usr/bin/python first and then use the command to create the soft link?

Command ln -s /usr/bin/python3 python creates link ./python pointing to /usr/bin/python3. You probably executed it multiple times so ./python already exists. You could overwrite it by providing -f flag to ln.
You definitely should not delete /usr/bin/python.
The whole idea with manually creating links to Python interpreter to install a package seems very weird. I suggest one of the following options:
just use the full path to the interpreter: /usr/bin/python3.6 -m pip install <package>; if the package adds any scripts globally usable from command line, like pyspark, they will be installed with hashbang pointing to the interpreter you installed them with
use a virtual environment: /usr/bin/python3.6 -m venv ~/.env-py36; source ~/.env-py36/bin/activate; python -m pip install <package>
use something like pyenv

How to set Python 3.8 as my default python version on kali linux

I installed python 3.8 in my kali linux OS, but when I type python in kali linux terminal it showing like this
> Python 2.7.18 (default, Apr 20 2020, 20:30:41)
How can I set Python 3.8 as my default?

Hope this answers your question.
The easiest way would be to run alias python=python3 .
So, now if you run the command python it will load python3 instead of python while being under the same alias/command python.

What I usually do is install non-default versions of programs under /usr/local. Then I configure the file $HOME/.bash_profile so that the $PATH variable has /usr/local/bin first, before /usr/bin and /bin (which might actually be the same physical directory on your system).
$HOME/.bash_profile:
# If not running interactively, don't do anything
case $- in
*i*) ;;
*) return;;
esac
# PATH starts with...
PATH="/usr/local/bin"
# add $HOME/bin if it exists...
if [ -d "${HOME}/bin" ]; then
PATH="${PATH}:${HOME}/bin"
fi
# These should exist, add them as fall back....
PATH="${PATH}:/bin:/usr/bin:/sbin:/usr/sbin"
# Some custom locations last
if [ -d /usr/local/arm-elf/bin ]; then
PATH="${PATH}:/usr/local/arm-elf/bin"
fi
if [ -d /opt/local/bin ]; then
PATH="${PATH}:/opt/local/bin"
fi
This has the effect of making anything located in /usr/local/bin the default when I am using an interactive shell, and letting the system defaults remain in effect otherwise.

first, check if you have python3.8 installed
use
ls /usr/bin/python*
then check for an alternative python version using
update-alternatives --list python
if the output of python3.8 is set to 2
To set the default python command to run python3.8
use this to create a persistent symlink
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.8 2
the 2 depends on your python alternatives
then
python --version

First remove python 2.7.18
Type >> sudo apt purge -y python2-minimal
install python latest version
Type >> sudo apt install -y python3-pip
latest version of python is installed!

zsh: abort python error when I try to run the app in venv

I setup Python 3.6 using pyenv so I could manage multiple Python versions (e.g. 3.7 and 3.8) in the future. I didn't use Homebrew to install Python since it changes the system version. It's my first time to use zsh shell since it's the default shell in Catalina OS. Currently, I use 3.6.8 version for my existing project.
So here's my current setup:
% pyenv versions
result:
system
* 3.6.8 (set by /Users/macbook/.python-version)
3.7.3
% python -V results to Python 3.6.8
% which python results to /Users/macbook/.pyenv/shims/python
% echo $PATH results to /Users/macbook/.pyenv/shims:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
The content of my ~/.zshrc is PATH=$(pyenv root)/shims:$PATH
I created a virtual env using % python -m venv venv, installed all the necessary packages, and when I activate it and get the python path,
(venv) % python -V
Python 3.6.8
(venv) % which python
/Users/macbook/python-project/venv/bin/python
(venv) % echo $PATH
/Users/macbook/python-project/venv/bin:/Users/macbook/.pyenv/shims:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
Finally, when I try to run the app, I always get zsh: abort error:
(venv) % python app.py
zsh: abort python app.py
(venv) % export FLASK_APP=app.py
(venv) % flask run
zsh: abort flask run
I don't know what else is still missing or are there anything wrong with my python path?
Thanks!

After searching through the web, I think this is a common issue with the latest MacOS or Homebrew. This thread fixed the issue.
brew update && brew upgrade && brew install openssl
copy the two files from /usr/local/Cellar/openssl#1.1/1.1.1g to /usr/local/lib/
cd /usr/local/Cellar/openssl#1.1/1.1.1g/
sudo cp libssl.1.1.1.dylib libcrypto.1.1.1.dylib /usr/local/lib/
add symlink to missing openssl libs
cd /usr/local/lib
sudo ln -s libssl.1.1.1.dylib libssl.dylib
sudo ln -s libcrypto.1.1.1.dylib libcrypto.dylib

For me, the below worked:
Python 3.6.9
MacOs Catalina 10.15.7
cd /usr/local/Cellar/openssl#1.1/1.1.1h/
cp lib/libssl.1.1.dylib lib/libcrypto.1.1.dylib /usr/local/lib
cd /usr/local/lib
sudo ln -s libssl.1.1.dylib libssl.dylib
sudo ln -s libcrypto.1.1.dylib libcrypto.dylib
Thanks to the answer by Zhanrah

Installing distribute in Python 3.3 venv (OS X/Homebrew)

I've been trying to get up and running with the built-in "venv" module of Python 3.3 on my OS X machine. I've installed Python 3.3 using Homebrew.
As per the docs, creating and switching virtual environment works as you'd expect:
$ python3 -m venv myvenv
$ source myvenv/bin/activate
And I've tested something like this:
$ echo "YEAH = 'YEAH!'" > myvenv/lib/python3.3/site-packages/thingy.py
$ python
>>> import thingy
>>> print(thingy.YEAH)
'YEAH!'
But when I try to install distribute, it simply won't go in the proper place. For some reason, it insists on trying to install into /usr/local/lib/python3.3/site-packages/, which fails with the following messages:
No setuptools distribution found
running install
Checking .pth file support in /usr/local/lib/python3.3/site-packages/
/Users/victor/myvenv/bin/python -E -c pass
TEST FAILED: /usr/local/lib/python3.3/site-packages/ does NOT support .pth files
error: bad install directory or PYTHONPATH
You are attempting to install a package to a directory that is not
on PYTHONPATH and which Python does not read ".pth" files from. The
installation directory you specified (via --install-dir, --prefix, or
the distutils default setting) was:
/usr/local/lib/python3.3/site-packages/
and your PYTHONPATH environment variable currently contains:
''
This happens regardless if I try to install using distribute_setup.py or using the source distribution directly. I've even tried using --prefix=/Users/victor/myenv but it still tries to put everything in my "global" site-packages.
I can't figure out why this happens, but it's consistent on two of my machines. Note that sys.prefix reports the correct path (the virtual environment).
Is this a problem with Homebrew? OS X? Python 3.3? venv? Me?

This has been an issue with Homebrew, yes, but it is working now since https://github.com/mxcl/homebrew/commit/0b50110107ea2998e65011ec31ce45931b446dab.
$ brew update
$ brew rm python3 #if you have installed it before
$ brew install python3
$ cd /tmp
$ which python3
/usr/local/bin/python3
$ python3 -m venv myvenv
$ source myvenv/bin/activate
$ wget http://python-distribute.org/distribute_setup.py # may need brew install wget
$ python3 distribute_setup.py
...
Finished processing dependencies for distribute==0.6.45
After install bootstrap.
Creating /private/tmp/myvenv/lib/python3.3/site-packages/setuptools-0.6c11-py3.3.egg-info
Creating /private/tmp/myvenv/lib/python3.3/site-packages/setuptools.pth
You see that distribute install successfully into the /tmp dir.

This happens because homebrew installs distutils config file:
$ brew cat python3 | grep "Tell distutils" -A5
# Tell distutils-based installers where to put scripts
(prefix/"Frameworks/Python.framework/Versions/#{VER}/lib/python#{VER}/distutils/distutils.cfg").write <<-EOF.undent
[install]
install-scripts=#{scripts_folder}
install-lib=#{site_packages}
EOF
$ mv ~/.local/Frameworks/Python.framework/Versions/3.3/lib/python3.3/distutils/distutils.cfg ~/tmp/
$ cat ~/tmp/distutils.cfg
[install]
install-scripts=/Users/gatto/.local/share/python3
install-lib=/Users/gatto/.local/lib/python3.3/site-packages
$ . venv/bin/activate
(venv) $ python distribute-0.6.36/distribute_setup.py
(venv) $ ls venv/lib/python3.3/site-packages/
distribute-0.6.36-py3.3.egg easy-install.pth setuptools-0.6c11-py3.3.egg-info setuptools.pth
See "distutils.cfg Can Break venv" issue at bugs.python.org.