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Not able to install python3-venv on Ubuntu

I'm using python 3.6.9 on Ubuntu 18.04
$ sudo apt-get install python3-venv
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
python3-venv : Depends: python3.6-venv (>= 3.6.5-2~) but it is not going to be installed
Depends: python3 (= 3.6.5-3) but 3.6.7-1~18.04 is to be installed
E: Unable to correct problems, you have held broken packages.

Why does it gives error while installing pip? [duplicate]

This question already has an answer here:
This is the result I get when trying to install pip3
(1 answer)
Closed 1 year ago.
I am getting error while installing pip. I have python 3.9.2 installed.
root#kali:~# sudo apt-get install python3-pip
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
python3-pip : Depends: python3-distutils but it is not going to be installed
Depends: python3-setuptools but it is not going to be installed
Recommends: python3-dev (>= 3.2) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
root#kali:~# ^C
enter image description here

Pip comes installed with Python 2 >=2.7.9 or Python 3 >=3.4.
First check it using:
python -m pip --version
To install it manually:
First, run this code to download get-pip.py using curl.
curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
Then, run this command in the folder that you have installed get-pip.py
python get-pip.py

It’s weird but I thought apt-get is supposed to retrieve any dependencies. Yet it tells you what it’s missing and refuses to install them. Have you tried to install the dependencies, then pip?
I’m not too familiar with apt-get but I recommend checking the error message to get to the bottom of why it refuses to install the dependencies. Your screen shot doesn’t have the whole message.
The last resort is the force install option. See if it understands --force as an argument. Note that this might put your package management system in a strange state if it isn’t already.
I’m not sure what “ig” is but I do not plan to link other social network applications to this one.

Installing MySQL Dev Client on Python

When I try running
sudo apt-get install python-pip python-dev libmysqlclient-dev
I get the following
Reading package lists...
Done Building dependency tree
Reading state information...
Done libmysqlclient-dev is already the newest
version.
Some packages could not be installed.
This may mean that you
have requested an impossible situation or if you are using the
unstable distribution that some required packages have not yet been
created or been moved out of Incoming.
The following information may
help to resolve the situation:
The following packages have unmet dependencies: python-pip : Depends:
python-setuptools (>= 0.6c1) but it is not going to be installed
Depends: python-requests but it is not going to be installed
Recommends: python-dev-all (>= 2.6) but it is not installable E: Unable to correct problems, you have held broken
packages

The Error says that you libmysqlclient-dev is already installed and on the newest version.
Please check that.
But it also says you need the following packages:
python-setuptools
python-requests
python-dev-all
Consider downloading/updating these via pip install and then try again :)

How to install python-numpy on OSX using apt-get?

so I have tried installing numpy using Homebrew. While it said on home-brew that I successfully installed it, the program I run couldn't detect it and it recommend using apt-get.
So I got apt-get through fink, but I couldn't install numpy like I wanted to.
The most relevant answer I found online is here:
http://mrprajesh.blogspot.hk/2009/11/e-couldnt-find-package-on-apt-get.html
But it only covers linux and I am not sure how to do the same on an OSX machine. Does anyone has experience with this?
Below is the error message. Any help is appreciated.
yings-mbp:madanalysis5 yvonne$ sudo apt-get install python-numpy
Reading Package Lists... Done
Building Dependency Tree... Done
E: Couldn't find package python-numpy
yings-mbp:madanalysis5 yvonne$ sudo apt-get install update
Password:
Reading Package Lists... Done
Building Dependency Tree... Done
E: Couldn't find package update
yings-mbp:madanalysis5 Sam$

In my opinion the best way to installing numpy and scipy, is just download Anaconda, even though its large, things are compiled and just work. If you need a smaller package then get miniconda, and run "conda install numpy". If you don't care for space, just get anaconda.

How to `pip install` a package that has non-Python dependencies?

Many python packages have build dependencies on non-Python packages. I'm specifically thinking of lxml and cffi, but this dilemma applies to a lot of packages on PyPI. Both of these packages have unadvertised build dependencies on non-Python packages like libxml2-dev, libxslt-dev, zlib1g-dev, and libffi-dev. The websites for lxml and cffi declare some of these dependencies, but it appears that there is no way to do figure this out from a command line.
As a result, there are hundreds of questions on SO that take this general form:
pip install foo fails with an error: "fatal error: bar.h: No such file or directory". How do I fix it?
Is this a misuse of pip or is this how it is intended to work? Is there a sane way to know what build dependencies to install before running pip? My current approach is:
I want to install a package called foo.
pip install foo
foo has a dependency on a Python package bar.
If bar build fails, then look at error message and guess/google what non-Python dependency I need to install.
sudo apt-get install libbaz-dev
sudo pip install bar
Repeat until bar succeeds.
sudo pip uninstall foo
Repeat entire process until no error messages.
Step #4 is particularly annoying. Apparently pip (version 1.5.4) installs the requested package first, before any dependencies. So if any dependencies fail, you can't just ask pip to install it again, because it thinks its already installed. There's also no option to install just the dependencies, so you must uninstall the package and then reinstall it.
Is there some more intelligent process for using pip?

This is actually a comment about the answer suggesting using apt-get but I don't have enough reputation points to leave one.
If you use virtualenv a lot, then installing the python-packages through apt-get can become a pain, as you can get mysterious errors when the python packages installed system-wide and the python packages installed in your virtualenv try to interact with each other. One thing that I have found that does help is to use the build-dep feature. To build the matplotlib dependencies, for example:
sudo apt-get build-dep python-matplotlib
And then activate your virtual environment and do pip install matplotlib. It will still go through the build process but many of the dependencies will be taken care of for you.
This is sort what the cran repositories suggest when installing R packages in ubuntu.

For most popular packages, There is a workaround for recent ubuntu systems. For example, I want to install matplotlib. When you order pip install matplotlib, it usually fails because of a missing dependency.
You can use apt-get install python-matplotlib instead. For python3, you can use apt-get install python3-matplotlib