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Linux Mint Changing current version of Python

I have installed python 3.7 but still using and showing version of python 2.7 . I want to change it to 3.. I searched it but ı can't did it.

You should not change your syslink python to use python 3 because it is most likely that your system is using that syslink to python 2 for its own tasks and processes, if you change that, you may broke your system.
As Sammy said in a comment, you should use python3 to use that version.
On the pip side, it is probably that your python 3 does not have pip include (it should have it, but I have seen a lot of Python 3 without it). You can check if you have pip doing: python3 -m pip. The -m param is used to execute modules of python installed.
If you do not have pip installed, you can install it following this (which I recommend because always work): https://pip.pypa.io/en/stable/installing/
That is:
Download a script to install pip: curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
Execute the script in order to install pip (with sudo because you're going to modify your system): sudo python3 get-pip.py
Now you should have pip installed and doing python3 -m pip again should show you the help of pip.
If you have pip already installed and no pip3 command in your system, you can always create an alias to python3 -m pip with the name pip3 and problem solved. Also if you don't know or do not want to create alias or executables in the /usr/bin folder, you can always keep using python3 -m pip.
PS: It is highly recommended to use virtualenvs when developing with python. If you do not know what it is, here it is a link to the docs: https://virtualenv.pypa.io/en/latest/

Install Python of specific version system-wide with pyenv

I am researching possibility to upgrade to Python 3.6 in our project.
Right now we are using Python 3.5.2 from ppa:fkrull/deadsnakes on Ubuntu 14.04. The PPA doesn't have Python 3.6 yet and it's not clear when it will be available.
I don't want to install yet another PPA.
And I am trying to find a more general approach.
I found people suggesting to use pyenv which compiles Python from source, which sounds interesting, because I can upgrade Python any time without waiting until repo maintainer adds it. Also I can easily install other Python flavors like PyPy.
I am not ready to use pyenv as virtual environment yes, so I am wondering if it's possible to use it to compile and install Python globally so that I can just use it.

The documentation is a little confusing because there is no python-build binary added in PATH after installation.
python-build is a pyenv plugin (installed by default). Documentation and more info is here: https://github.com/pyenv/pyenv/tree/master/plugins/python-build.
How to install system-wide Python for all users: 1) Login as root and 2) install required Python version to /usr/local/python-X.Y.Z.
sudo ~/.pyenv/plugins/python-build/bin/python-build 3.6.1 /usr/local/python-3.6.1/
Now you can use this Python version as a normal user, for example you can create virtualenv for your project:
/usr/local/python-3.6.1/bin/python -m venv /var/www/my-app/.env/

https://github.com/yyuu/pyenv/wiki/Common-build-problems#installing-a-system-wide-python
Installing a system-wide Python
If you want to install a Python interpreter that's available to all
users and system scripts (no pyenv), use /usr/local/ as the install
path. For example:
sudo python-build 3.3.2 /usr/local/

I've contributed a package for python3.6 in deadsnakes for trusty / xenial :)
https://launchpad.net/~fkrull/+archive/ubuntu/deadsnakes/+packages?field.name_filter=python3.6&field.status_filter=published&field.series_filter=

By combining the hints from the other answers and reading through the documentation, I found a nice way to do exactly what you want that should work well in a CI system or in a Docker container or on a developer machine if they haven't already installed python3.x via Apt or Yum or Homebrew.
Assuming you have all the dependencies required to build your desired version of Python 3.x (anything above 3.4 requires some extra packages the pyenv-installer doesn't always warn you about), you can run the commands below to get a new system wide Python that should be executable by all users, which makes it easy to pass to virtualenv creations with python3.6 -m venv yourvenv.
curl https://pyenv.run | bash # or
wget -O - https://pyenv.run | bash
export PATH="$HOME/.pyenv/bin:$PATH"
$(pyenv which python-build) 3.6.10 /usr/local/
which python3.6
python3.6 --version
# If you get an error running the above commands, it probably means
# /usr/local/bin isn't in your PATH yet
# on Debian/Ubuntu and maybe others the /etc/environment or
# /etc/login.defs file puts it in the path when a user logs in
echo $PATH
export PATH="/usr/local/bin:$PATH"
python3.6 --version

Installing Python 2.x and python 3.x on the same computer

There are python 2.7 and python 3.2 on my computer. The default version is 2.7 because using python -V gives 2.7 as the version.
But when I use apt-get install numpy, scipy,pip why it install them into the python3.2 folder. After that I used pip to install the module into the 3.2 folder.
I also installed Theano this way but in the end it showed a message saying that there is no module named Theano installed although it is in the python 3.2 folder.

You haven't told on which OS you're running this, but it look likes a debian base linux, maybe ubuntu?
If so, I'd try with:
sudo apt-get install python3-numpy or
sudo apt-get install python2-numpy.
This would also work with python-pip2 and python-pip3.
After this, you could effectively use "pip2" or "pip3" to install your packages without having to go through the OS "prebuild" modules (but the os version of the packages are usually my prefered way to install them, if the exists in the repo)

Depending on what you're doing with python, it's often a good idea to run in a virtual environment, this lets you have several different versions of python with several different sets of installed packages on the same system. . .
See http://docs.python-guide.org/en/latest/dev/virtualenvs/ for the details.

You can also use Anaconda for maintaining two versions of Python. Anaconda consists of various libraries so you don't have to install them and after switching it to the different version of Python you can easily install them :
Download Anaconda for both Python versions
Open .bashrc
Add the path to new Anaconda you have installed for, e.g.:
export PATH="/home/paras/anaconda3/bin:$PATH"
Now there will be 2 export paths: one for Python 2 and one for Python 3. Comment the one which you don't want.

First install different versions of python or whichever python version you would like to use
sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.6 10
sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 20
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.3 30
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.4 40
Now create virtualenv like this and give the path of python version you want to use inside the virtualenv.
virtualenv -p /usr/bin/python3.6 <foldername>
virtualenv -p /usr/bin/python3.4 <foldername>

Upgrade python without breaking yum

I recently installed Python 2.7.3 on a CentOS machine by compiling from source. Python 2.7.3 is installed at /opt/python2.7 and when I installed it I just changed /usr/bin/python to point to the new version. This apparently is wrong though because when I did it it broke yum. I would get the following.
There was a problem importing one of the Python modules
required to run yum. The error leading to this problem was:
No module named yum
Please install a package which provides this module, or
verify that the module is installed correctly.
It's possible that the above module doesn't match the
current version of Python, which is:
2.7.3 (default, May 15 2012, 17:45:42)
[GCC 4.4.4 20100726 (Red Hat 4.4.4-13)]
If you cannot solve this problem yourself, please go to
the yum faq at:
http://yum.baseurl.org/wiki/Faq
I changed /usr/bin/python to point back to the python 2.6.6 but now 2.6.6 is the default version of python. Any idea how to fix this?

I have written a quick guide on how to install the latest versions of Python 2 and Python 3 on CentOS 6 and CentOS 7. It currently covers Python 2.7.13 and Python 3.6.0:
# Start by making sure your system is up-to-date:
yum update
# Compilers and related tools:
yum groupinstall -y "development tools"
# Libraries needed during compilation to enable all features of Python:
yum install -y zlib-devel bzip2-devel openssl-devel ncurses-devel sqlite-devel readline-devel tk-devel gdbm-devel db4-devel libpcap-devel xz-devel expat-devel
# If you are on a clean "minimal" install of CentOS you also need the wget tool:
yum install -y wget
The next steps depend on the version of Python you're installing.
For Python 2.7.14:
wget http://python.org/ftp/python/2.7.14/Python-2.7.14.tar.xz
tar xf Python-2.7.14.tar.xz
cd Python-2.7.14
./configure --prefix=/usr/local --enable-unicode=ucs4 --enable-shared LDFLAGS="-Wl,-rpath /usr/local/lib"
make && make altinstall
# Strip the Python 2.7 binary:
strip /usr/local/lib/libpython2.7.so.1.0
For Python 3.6.3:
wget http://python.org/ftp/python/3.6.3/Python-3.6.3.tar.xz
tar xf Python-3.6.3.tar.xz
cd Python-3.6.3
./configure --prefix=/usr/local --enable-shared LDFLAGS="-Wl,-rpath /usr/local/lib"
make && make altinstall
# Strip the Python 3.6 binary:
strip /usr/local/lib/libpython3.6m.so.1.0
To install Pip:
# First get the script:
wget https://bootstrap.pypa.io/get-pip.py
# Then execute it using Python 2.7 and/or Python 3.6:
python2.7 get-pip.py
python3.6 get-pip.py
# With pip installed you can now do things like this:
pip2.7 install [packagename]
pip2.7 install --upgrade [packagename]
pip2.7 uninstall [packagename]
You are not supposed to change the system version of Python because it will break the system (as you found out). Installing other versions works fine as long as you leave the original system version alone. This can be accomplished by using a custom prefix (for example /usr/local) when running configure, and using make altinstall (instead of the normal make install) when installing your build of Python.
Having multiple versions of Python available is usually not a big problem as long as you remember to type the full name including the version number (for example "python2.7" or "pip2.7"). If you do all your Python work from a virtualenv the versioning is handled for you, so make sure you install and use virtualenv!

vim `which yum`
modify #/usr/bin/python to #/usr/bin/python2.4

Put /opt/python2.7/bin in your PATH environment variable in front of /usr/bin...or just get used to typing python2.7.

pythonz, an active fork of pythonbrew, makes this a breeze. You can install a version with:
# pythonz install 2.7.3
Then set up a symlink with:
# ln -s /usr/local/pythonz/pythons/CPython-2.7.3/bin/python2.7 /usr/local/bin/python2.7
# python2.7 --version
Python 2.7.3

ln -s /usr/local/bin/python2.7 /usr/bin/python

Alright so for me, the error being fixed is when there are different versions of python installed and yum can't find a certain .so file and throws an exception.
yum wants 2.7.5 according to the error.
which python gives me /usr/bin/python
python --version gives me 2.7.5
The fix for me was append /lib64 to the LD_LIBRARY_PATH environment variable.
The relevant content is /lib64/python2.7 and /lib64/python3.6.
export LD_LIBRARY_PATH=/lib64:$LD_LIBRARY_PATH
Fixed the yum error for me with multiple python versions installed.

Daniel's answer is probably the most ideal one as it doesn't involve changing OS files. However, I found myself in a situation where I needed a 3rd party program which invoked python by calling usr/bin/python, but required Python 2.7.16, while the default Python was 2.7.5. That meant I had to make usr/bin/python point to a Python version of 2.7.16 version, which meant that yum wouldn't work.
What I ended up doing is editing the file /usr/bin/yum and replacing the shebang there to use to the system default Python (in my case, that meant changing #! /usr/bin/python to #! /usr/bin/python2). However, after that running yum gave me an error:
ImportError: No module named urlgrabber.grabber
I solved that by replacing the shebang in /usr/libexec/urlgrabber-ext-down the same way as in /usr/bin/yum. I.e., #! /usr/bin/python to #! /usr/bin/python2. After that yum worked.
This is a hack and should be used with care. As mentioned in other comments, modifying OS files should be last resort only.

I recommend, instead, updating the path in the associated script(s)
(such as /usr/bin/yum) to point at your previous Python as the interpreter.
Ideally, you want to upgrade yum and its associated scripts so that they
are supported by the default Python installed.
If that is not possible, the above is entirely workable and tested.
Change:
#!/usr/bin/python
to whatever the path is of your old version until you can make the
above yum improvement.
Cases where you couldn't do the above are if you have an isolated machine,
don't have the time to upgrade rpm manually or can't connect temporarily
or permanently to a standard yum repository.

If you want to try out rpm packages, you can install
binary packages based on the newest Fedora rpms, but recompiled
for RHEL6/CentOS6/ScientificLinux-6 on:
http://www.jur-linux.org/download/el-updates/6/
best regards,
Florian La Roche

I read a piece with a comment that states the following commands can be run now. I have not tested myself so be careful.
$ yum install -y epel-release
$ yum install -y python36

switch versions of python

Story:
One of the app that i have works on python 2.4 and other on 2.6. I tried to do a sym link of python2.4 to python and things started to break loose on ubuntu jaunty.
Now i am downloading every dependency of 2.4 and installing it using python2.4 setup.py install. The dependencies seem to be endless.
Question1: How will i tell any framework that go and use version so and so pf python like day django to use 2.6 and say mjango to use 2.4? Something like we say use database databasename kinda syntax.
Question2: Is there more elegant way to switch between version as my hack of symlinking was a virtual disaster?
Question3: Can I download a deb for say hardy and make jaunty believe its for her?

Use Virtualenv.
There is more information here: Working with virtualenv.
Using virtualenv you can create a new virtual python environment with whatever version of Python you want for each project or application. You can then activate the appropriate environment when you need it.
To expand on my answer:
You can install multiple versions of Python on your computer (I have 2.4, 2.5, 2.6 and 3.1 on my machine - I install each from source). I use a Mac, and keep my system Python as whatever OS X sets as the default.
I use easy_install to install packages. On ubuntu you can get easy_install like this:
sudo apt-get install python-setuptools
To install virtualenv then do:
easy_install virtualenv
I tend to create a new virtualenv for each project I'm working on and don't give it access to the global site-packages. This keeps all the packages tight together and allows me to have the specific versions of everything I need.
virtualenv -p python2.6 --no-site-packages ~/env/NEW_DJANGO_PROJECT
And then whenever I am doing anything related to this project I activate it:
source ~/env/NEW_DJANGO_PROJECT/bin/activate
If I run python now it uses this new python. If I use easy_install it installs things into my new virtual environment.
So, virtualenv should be able to solve all of your problems.

Pythonbrew is a magical tool. Which can also be called as Python version manager similar to that of RVM-Ruby version manager but Pythonbrew is inspired by Perlbrew.
Pythonbrew is a program to automate the building and installation of Python in the users $HOME.
Dependencies – curl
Before Installing the Pythonbrew, Install “curl” in the machine, to install curl use the below command in the terminal, give the the password for the user when prompted.
$sudo apt-get install curl
After Installing the curl, Now Install Pythonbrew, copy and paste the following commands in the terminal and type the password for the user when prompted.
Recomended method of installation - Easy Install
$ sudo easy_install pythonbrew
To complete the installation, type the following command
$pythonbrew_install
Alternate method of installation:
Use curl command to download the latest version of pythonbrew from github.
curl -kLO http://github.com/utahta/pythonbrew/raw/master/pythonbrew-install
After downloading, change “pythonbrew-install” to “executable”
chmod +x pythonbrew-install
Then, run the pythonbrew-install in the terminal
./pythonbrew-install
Now the Pythonbrew has been installed in the “Home Directory” i.e., /home/user/.pythonbrew
Next, copy and paste the following line to the end of ~/.bashrc
*NOTE: change “user” to your user name in the system
source /home/user/.pythonbrew/etc/bashrc
Thats it! Close the terminal.
Steps to Install different versions of Python:
Open a new terminal, type the following command or copy and paste it.
$pythonbrew install 2.6.6
This will install Python 2.6.6 and to install Python 2.7 or Python 3.2, change the version number in the previous command.
$pythonbrew install 2.7
or
$pythonbrew install 3.2
Update: If you get error while Installing then Install using the below command.
$pythonbrew install --force 2.7
or
$pythonbrew install --force 3.2
How to manage different versions of Python installed in system
For instance, if Python 2.6.6, Python 2.7 and Python 3.2 is installed in your system, switching between the versions can be done as follows:
By default, Python 2.6.6 will be active and in order to switch to Python 2.7 use the below command
$pythonbrew switch 2.7
The default Python is changed to Python 2.7.
Now, to switch to Python 3.2 change the version number in the previous command.
$pythonbrew switch 3.2
Use the below command to check or list the installed Python versions
$pythonbrew list
Use the below command to check or list the available Python Versions to install
$pythonbrew list -k
To uninstall any of the installed Python version (for example to uninstall Python 2.7), use the below command.
$pythonbrew uninstall 2.7
Use the below command to update the Pythonbrew
$pythonbrew update
Use the below command to disable the Pythonbrew and to activate the default version
$pythonbrew off
Enjoy the experience of installing multiple versions of Python in single Linux / ubuntu machine!

I find http://github.com/utahta/pythonbrew much easier to install and use than any other solution.
Just install it and you'll have these options:
pythonbrew install 2.7.2
pythonbrew use 2.7.2 # use 2.7.2 for a current terminal session
pythonbrew switch 2.7.2 # use 2.7.2 by default system wide
pythonbrew uninstall 2.7.2
Note: if you're using a Linux-based operating system with preinstalled Python, switching (system wide) to another version can make things go wrong, so be careful.

A more grassroot approach than Virtualenv is the side-by-side installation of two Python versions.
If there is an existing installation, and you want a second installation into the same root path (e.g. /usr/local), use this target when making install:
make altinstall
When your second installation is Python 2.6, this will leave you with a /usr/local/bin/python2.6 alongside the old /usr/local/bin/python.
A simple way to switch between these two versions is using a shell alias (alias python=/usr/local/bin/python2.6) on the shell where you invoke the interpreter. But this won't work across sub-shells and she-bang invocations.

pyenv is yet another Python manager. The README.md at that link has a good set of instructions, but they basically are:
$ cd
$ git clone git://github.com/yyuu/pyenv.git .pyenv
Then set up your $PATH.
$ echo 'export PYENV_ROOT="$HOME/.pyenv"' >> ~/.bash_profile
$ echo 'export PATH="$PYENV_ROOT/bin:$PATH"' >> ~/.bash_profile
$ echo 'eval "$(pyenv init -)"' >> ~/.bash_profile
Install the desired versions of Python:
$ pyenv install 2.7.8
After installing you need to run this:
$ pyenv rehash
Then switch to the version of Python you want to run, for the shell:
$ pyenv shell 2.7.8

"Question1: How will i tell any framework that go and use version so and so pf python like day django to use 2.6 and say mjango to use 2.4?"
You simply run them with the specific python version they need. Run mjango with /usr/bin/python2.4 and django with /usr/bin/python2.6. As easy as that.
"Question2: Is there more elegant way to switch between version as my hack of symlinking was a virtual disaster?"
Yes, see above. Have two separate installs of Python, and run explicitly with the different versions.
"Question3: Can I download a deb for say hardy and make jaunty believe its for her?"
That generally works. If it doesn't, it's because it has dependencies that exist in Hardy, and does not exist in Jaunty, and then you can't.
And here is a Question 4 you didn't ask, but should have. ;)
"Is there an easier way to download all those Python modules?"
Yes, there is. Install setuptools, and use easy_install. It will not help you with library dependecies for those Python modules that have C code and need to be compiled. But it will help with all others. easy_install will download and install all the Python dependencies of the module in question in one go. That makes it a lot quicker to install Python modules.

Move to the project directory :
Create an environment :
virtualenv -p python2.7 --no-site-packages ~/env/twoseven
Then activate your source :
source ~/env/twoseven/bin/activate