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Installing packages in virtual environment using anaconda for different projects

Do we need to install same conda packages repeatedly for different virtual environments? Is there a way to reuse python packages from one virtualenv to other? Is there a way to use global conda packages within a virtual environment?

You can copy the packages in one environment to other using the following command..
conda create -n --clone
For example , default environment is "base" and i want to copy it to a new env "new" , then...
conda create -n new --clone base
For more options , type ...
conda create -h

Is there a way to list all python virtual environments created using the venv module?

Conda allows me to list all virtual environments as shown here. The commands are:
conda info --envs OR conda env list
I want to do that using pip. Does pip have any option to list all virtual environments created by me ? I have created a virtual environment on my desktop but I cannot figure out a way to list it along with the base environment.

No, not in an easy way.
Python virtual environments created with venv, virtualenv and most other python-only virtual environments can be stored anywhere on the disk. And AFAIK they are not indexed, they are truly isolated (after all you can just remove venv directory and be done with it, you don't need to do anything special). They are also unmanaged by an environment manager. So that would require entire disk scan. Which potentially can be done (you can search for all Python executables for example) but is rather painful.
It works with miniconda because miniconda manages other packages and files that it installs, so it places venvs in concrete path, e.g. /home/username/miniconda/envs/.

One indirect way is to run below command and see the directories where the python executable resides. Some of the paths will be python virtual environments and you can verify it by listing the files in those paths.
$ whereis python

You can use conda to create and manage venv and virtualenv environments and other packages installed using pip.
First create a conda environment with CONDA AND PIP installed into it, e.g.,
conda create --name core --channel conda-forge python=3.9 conda pip
Here I created the conda environment named "core" and installed Python 3.9, conda, and pip into it. So now the 'core' conda environment functions like an administrative environment shell. By installing conda into the conda environment, conda will track packages installed by pip into that environment. You must use "pip install" INSIDE this new conda environment, so conda will index and track those pip package installations. However, conda will still not index and centrally manage the venv environments, like it does for its own conda environments.
Here is a very good detailed guide that explains how and why to use conda and pip for virtual environments. It covers the important aspect of using conda and pip together.
Why You Need Python Environments and How to Manage Them with Conda
Create virtual environments for python with conda

Install packages with Conda for a second Python installation

I recently installed Anaconda in my Windows. I did that to use some packages from some specific channels required by an application that is using Python 3.5 as its scripting language.
I adjusted my PATH variable to use Conda, pointing to the Python environment of the particular program, but now I would like to use Conda as well for a different Python installation that I have on my Windows.
When installing Anaconda then it isn't asking for a Python version to be related to. So, how can I use Conda to install into the other Python installation. Both Python installations are 'physical' installations - not virtual in any way.

Uninstall the other python installation and create different conda environments, that is what conda is great at.
Using conda from your anaconda installation to manage packages from another, independent python installation is not possible and not very feasible.
Something like this could serve your needs:
Create one env for python 3.5 conda create -n py35 python=3.5
Create one env for some other python version you would like to use, e.g. 3.6: conda create -n py36 python=3.6
Use conda activate py35, conda deactivate, conda activate py36 to switch between your virtual environments.

Importing a package installed with anaconda in virtual environment

I want to work with the python package holopy. Apparently you have to use conda to install it, so I first installed Anaconda 4.2.0 (since I'm using Python 3.5). I opened the virtual environment I normally use and installed holopy as they recommend on the official site:
conda install -c conda-forge holopy
Afterwards, when in the virtual environment I type conda list, holopy shows up. But when I type python3 and then import holopy, it says package not found. It does however work when I leave the virtual environment. I need it in the virtual environment though, how can I do that?

I'm not sure how well anaconda and virtual environments i.e.venv work together. If you're using anaconda anyway then I highly recommend using anaconda environments. Please go through this short tutorial about anaconda environments - you won't regret it.
Why it didn't work for you?
The conda command is available only in the base anaconda environment. So when you run the command - conda insall -c conda-forge holopy, it installed holopy in the base anaconda environment and it won't be available to you in your venv.
After looking at the documentation of holopy it seems probable that when they said virtual environment they actually meant anaconda virtual environment. Therefore the solution is to first create an anaconda virtual environment called holopy-env and then run the command conda install -n holopy-env -c conda-forge holopy.
A better way of doing things with Anaconda
I will also give you a quick and clean example of how to create an environment using anaconda. If you're using Anaconda then it would be wise to use it's environment management tools. Create an environment.yml file with the following contents:
environment.yml using conda-forge/holopy & python 3.6
name: holopy-env # any name for the environment
channels:
- conda-forge
dependencies: # everything under this, installed by conda
- python=3.6
- holopy
- pip: # everything under this, installed by pip
- future
How to install the environment?
conda create --force -f environment.yml
How to activate the environment?
source activate opencv-env
After activating the environment
You should be able to import holopy
Install pip packages using pip install <package>
Install conda packages using conda install -n holopy-env -c CHANNEL <package>

conda is a packaging tool and installer that aims to do more than what pip can do; handle library dependencies outside of the Python packages as well as the Python packages themselves. Conda also creates a virtual environment, like virtualenv does. For creating virtualenv with conda, use the following command:-
conda create -n yourenvname python=x.x anaconda
Use the following to activate the virtualenv in conda
source activate yourenvname
Then, you can install the packages in virtualenv using conda as:-
conda install -n yourenvname [package]
To Deactivate use:-
source deactivate
And to delete a no longer needed virtualenv, use :-
conda remove -n yourenvname -all

I know this is a bit late, but you don't need to use conda to install HoloPy. This is just the least technical option. Otherwise, you need to be able to compile HoloPy's fortran components yourself, which is fairly straightforward on Unix-based systems but complicated on Windows. Instructions can be found in HoloPy's documentation at https://holopy.readthedocs.io/en/latest/users/dev_tutorial.html.
We are also working on putting together a singularity container distribution of HoloPy. Let me know if this is of interest to you and I will make it a priority.

create anaconda python environment with all packages

I want to create an anaconda python environment with all of the packages that Continuum includes in its default Anaconda installer. Based on some internet search I used the following command:
conda create -n env_full python=3
However, only handful of packages would be installed. Please see the screen shot.
Kindly guide me to use correct commands.
Right now I am trying to do this on a desktop computer, but I would like to apply the same principles to the cluster facility.

To install all of the packages that Continuum includes in its default Anaconda installer, the simplest command is this:
conda create -n env_full anaconda
This will install the latest version of the anaconda package set, as compiled for your default version of Python (the one you used to install Anaconda originally). If you'd like to create an environment with a different version of Python, then just add that to the command line; e.g.
conda create -n env_full anaconda python=2.7
conda create -n env_full anaconda python=3.5

Anaconda ships with a root env, this is named as base. You can use this as it is or clone a new environment from it.
if you just want a environment with all the packages for day to day then you can use the base enviornment itself.
you can list the all available conda env on your machine as follows
conda info --env
you will see a enviornment name base, activate it to use it
source activate base
You can verify all the packages available in the env with following command ( This work with any env created with conda)
conda list -n base
As I said above if you want a different env then you can clone base using following command
conda create --name <env_name> --clone base

When I run this command:
conda create -n env_full anaconda>
I get a PackageNotFoundError. So I create a simple environment:
conda create -n env_full
and use this command to install all the anaconda default packages:
conda install anaconda