SetUp Python virtual environment

What is virtual environment?

A tool to create an isolated environment to run the different projects with their own dependence.

For example: If you want to run 2 django or flask projects in the same machine with different package version. You have to isolate them into different environments.
django1.2 in venv1
django2.2 in venv2

virtualenv

run this to install

pip install virtualenv
pip install virtualenv
pip install virtualenv

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check it by running this in terminal
this will list out the available commands

virtualenv
or
virtualenv --help
virtualenv
or
virtualenv --help
virtualenv
or
virtualenv --help

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syntax to create a virtual environment

 Usage: virtualenv [OPTIONS] DEST_DIR
 -p PYTHON_EXE, --python=PYTHON_EXE
 Usage: virtualenv [OPTIONS] DEST_DIR
 -p PYTHON_EXE, --python=PYTHON_EXE
 Usage: virtualenv [OPTIONS] DEST_DIR
 -p PYTHON_EXE, --python=PYTHON_EXE

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PYTHON_EXE is for which version of python you are going to create a virtual environment

Create a virtual environment with name as venv, this will create it in the current working director

 virtualenv --python=python3.7 venv
 virtualenv --python=python3.7 venv
 virtualenv --python=python3.7 venv

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to create at a specific directory you can go to the path or else you can run the command with the path of venv

 virtualenv --python=python3.7 test/venv
 virtualenv --python=python3.7 test/venv
 virtualenv --python=python3.7 test/venv

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Virtual environment is created but we dont have the access for it, need to be activated

 source venv/bin/activate
 source venv/bin/activate
 source venv/bin/activate

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(venv) mac@macLBP %
this will activate the environment, you can see the venv in terminal and to disabled it just run

deactivate
deactivate
deactivate

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to create environment for python 2, by default python will be taken a python2(in my machine its python 2 for you it may vary)

 virtualenv --python=python testenv1
 virtualenv --python=python testenv1
 virtualenv --python=python testenv1

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to create environment for python 3.7

 virtualenv --python=python3.7 testenv2
 virtualenv --python=python3.7 testenv2
 virtualenv --python=python3.7 testenv2

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to create environment for python 3.8

 virtualenv --python=python3.8 testenv3
 virtualenv --python=python3.8 testenv3
 virtualenv --python=python3.8 testenv3

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finally you can install any packages as you want, package dependence in one project will not affect others

virtualenv --python=python3 test/venv
cd test 
source venv/bin/activate 
python -V
    Python 3.7.3
pip freeze
pip install flask
pip freeze
    click==7.1.2
    Flask==1.1.2
    itsdangerous==1.1.0
    Jinja2==2.11.2
    MarkupSafe==1.1.1
    Werkzeug==1.0.1
deactivate
virtualenv --python=python3 test/venv
cd test 
source venv/bin/activate 
python -V
    Python 3.7.3
pip freeze
pip install flask
pip freeze
    click==7.1.2
    Flask==1.1.2
    itsdangerous==1.1.0
    Jinja2==2.11.2
    MarkupSafe==1.1.1
    Werkzeug==1.0.1
deactivate
virtualenv --python=python3 test/venv
cd test 
source venv/bin/activate 
python -V
    Python 3.7.3
pip freeze
pip install flask
pip freeze
    click==7.1.2
    Flask==1.1.2
    itsdangerous==1.1.0
    Jinja2==2.11.2
    MarkupSafe==1.1.1
    Werkzeug==1.0.1
deactivate

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原文链接：SetUp Python virtual environment