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How to change python version in command prompt if I have 2 python version installed

I have installed Anaconda 2 & 3 in my system. Anaconda 2 contains python 2.7 & Anaconda 3 contains python 3.6.
I need to run my python code using command prompt & I need to use python 3.6
While I'm running python --version, I'm getting python 2.7.14. How do I change it to python 3.6?

As you can see, I have both Python2 and Python3 installed.
I hope you know that the path of the python executable has to be added to the PATH environment variable in Windows. As you can see, the path of Python2 is placed above the path of Python3 in my system.
How does the cmd run commands?
It searches for an executable with the same name in the directory the cmd has been opened in, then goes and searches for the command in the locations provided in the Windows PATH variable, from TOP to BOTTOM.
Which means, it gets the path to Python2 before it can get to the path of Python3. Therefore, every time you type python in your cmd, it runs Python2.
Both Python2 and Python3 executables have the same name in Windows so it never runs python3.
What seems like an obvious solution?
You might think, changing the name of python.exe to python3.exe for the Python3 executable will solve your problem. You are partially right, it will work. But you have to use python3 file.py or python3 --version, which I think, is understandable. But, pip will no longer work if you change the name of the original python executable. You will no longer be able to install external packages and modules.
How can this problem be solved?
You can create an alias for the Python3 executable called python3.bat.
.exe and .bat files can be called from the cmd directly without using their extension. We always write python filename.py instead of python.exe filename.py although both are correct. The same can be done with .bat files.
Go back to the first image and notice the python3.bat file below python.exe. That is my way of calling python3 without renaming my original python executable.
python3.bat
Create a new file using notepad or something and paste this %~dp0python %*
I don't fully understand how this works except that dp0 basically runs python from inside the same directory and %* passes all the arguments to the python executable. Place this file inside your Python3 installation directory and your problem will hopefully be solved.
python3 basically runs your python3.bat file, which in turn runs the python.exe from its folder and passes the arguments to it.
I hope this solves your problem.

You should have python3 also in your path. You could use python3 to run your script:
python3 <your_script>
But to answer your question, you could use alias and update it as python.
$ python --version
Python 2.7.6
$ alias python=python3
$ python --version
Python 3.6.4

If you want to run Python 3.6, then execute python3.6.
Otherwise, check with which python where the symbolic link to the actual python executable is. On my system, it's
/usr/bin/python
and when I ls -la /usr/bin/python, it gives
lrwxrwxrwx 42 user user 42 Aug 24 03:45 /usr/bin/python -> python2.7
If you are really sure that you want to do it, you can now just move the old symlink somewhere:
sudo mv /usr/bin/python /usr/bin/old_python
and create a new symlink:
sudo ln -s /usr/bin/python3.6 /usr/bin/python
Now you should get something like:
$ python --version
Python 3.6.5
I don't guarantee that it doesn't break everything in the world, especially if you have some distro that has a package manager that relies on a specific python version under python.
This seemed to work nicely on Linux Mint 18.

I made a small trick to solve this problem. For me
C:\Python27
and
C:\Python27\Scripts
were at the top of the PATH Environment variable list. I simply selected them and clicked on Moved Down to shift them to the end of the list.
Just make sure that your path of Python3 and its Script folder is above Python2 path.
And it solved the problem for me. Hope it would help you too.
Reason :
How does the cmd run commands?
It searches for an executable with the same name in the directory the cmd has been opened in, then goes and searches for the command in the locations provided in the Windows PATH variable, from TOP to BOTTOM. Which means, it gets the path to Python2 before it can get to the path of Python3. Therefore, every time you type python in your cmd, it runs Python2.
Both Python2 and Python3 executables have the same name in Windows so it never runs python3.
reason is taken from the accepted answer to this question.

In order to run your script with python 3 you can use
python3 <path to file>
P.S.: this should be a comment, but I do not have the required reputation. :)

If Anaconda 3's is the only python3 interpreter in the system you can run python3 instead of python in your command line and it should work.
You can alter PATH parameter inverting the positions of both interpreters.

I have two versions of the python which the first path is python 2(I am still working with python 2), but there are some scripts which should use python 3. This is what I have done:
I have create a bat file.
ex:
open Notepad++ and write :
#echo off
set PYTHONPATH=
"%~dpn0.py" %*
save it as 'yourscriptname.cmd'.
before my script(which should use python3) I just run this file in command line and then run my script. it is working.

Python 3 installation on windows running from command line

Just curious, is there a particular reason why Python 3.x is not installed on Windows to run default with the command line "python3", like it does on Mac OSX and Linux? Is there some kind of way to configure Python so that it runs like this? Thanks.
EDIT: Just to add, the reason I am asking is because I have both the Python 2 and 3 interpreter installed on my computer, and so it is ambiguous, as both are run using the command "python".

the reason I am asking is because I have both the Python 2 and 3 interpreter installed on my computer, and so it is ambiguous, as both are run using the command "python".
To run Python 2 executable:
C:\> py -2
To run Python 3 executable:
C:\> py -3
where py is a Python launcher that is bundled with your Python 3 installation.
py recognizes the shebang (e.g., #!/usr/bin/env python3 causes Python 3 executable to be run), it respects virtualenv (if you run py without specifying the explicit python executable version) i.e., run:
C:\> py your_script.py
and the correct python version is used automatically -- you don't need to specify the Python version on the command-line explicitly.
is there a particular reason why Python 3.x is not installed on Windows to run default with the command line "python3", like it does on Mac OSX and Linux?
OSX and Linux have python executable installed by default as a rule and it refers to Python 2 version in most cases at the moment that is why you need a separate python3 name there.
There is no Python on Windows by default. And therefore any version that you've installed is just python (I guess). The recommended way to manage multiple python versions is to use the Python launcher.
Is there some kind of way to configure Python so that it runs like this?
If you want to type python3 some_script.py instead of py some_script.py or even just some_script (assuming .py is in %PATHEXT% and Python launcher is configured to run Python scripts (check assoc .py and ftype Python.File) -- the default) then create a bat-file e.g., python3.cmd and put it in %PATH%:
"C:\path to\Python 3.X\python.exe" %*

You likely missed the checkbox at the bottom of the installer.
Full documentation here: https://docs.python.org/3/using/windows.html
Then, I think you just run python, not python3 from the Command Prompt. The reason Unix systems have python3 is because python defaults to Python2.x in many systems.

You have to add the python bin folder to your path. You can do it manually but when you install python i remember you have an option to do that.

I work with multiple Python 2.x and 3.x distros on Windows. Some of them are "portable" - i.e. not recorded in the Windows registry, and therefore not accessible by the version-selector py.exe delivered with Python 3.3+. To save my sanity, I wrote SelectPython.bat which is available on bitbucket. It configures the PYTHONHOME, PYTHONPATH and PATH variables according to the target you give it (a relative or absolute path to the parent directory of python.exe). You can do so in a way that is sticky for the rest of your command-line session:
> SelectPython C:\Path\To\Desired\Version\Of\Python
> python
or transiently, i.e. to call a particular python command without otherwise affecting the environment of the shell you're calling it from:
> SelectPython C:\Path\To\Desired\Version\Of\Python python -c "import sys;print(sys.version)"
You may find it helpful.

make python script use 2.7.8 instead of 2.6

I have 2 versions of python installed on my server.
Python 2.6 in /usr/bin/python
Python 2.7.8 in /usr/src
Python 2.7.8 install guide
How do I make python scripts use 2.7.8? instead of 2.6?
how do I set up an alias?
Thanks

Write:
#!<path to your python interpreter>
as the first line of the script your run. Then just execute it.
Other option:
<path to your python interpreter> your_script.py

There are multiple ways of doing this. First, I would suggest you check your $PATH environment variable to see if it holds the path to both of your python versions. If it doesn't, you can either add the path or simply make a symlink to it from /usr/bin/ and call it python2.7, for example.
Then you can use #Pavel's suggestion of specifying the path to the interpreter in the shebang of the file.
If you followed my previous instructions, your shebang should look something similar to the following.
#!/usr/bin/env python2.7
You could also use the second way #Pavel showed you, by simply calling the interpreter first in the terminal and pointing it to the script.
/usr/bin/python2.7 my_script.py
The best option you have, which I recommend, is using virtualenv as suggested before, but only if you want it for specific scripts. If you want it globally, I would suggest you read further.
Finally, if you want all your scripts to run under python 2.7 then I suggest you remove python 2.6 and install the 2.7 version from the package manager, the right way, and save yourself a lot of hassle.

setting/changing PYTHONPATH in OS Mavericks

Ok, I hate to start another of seemingly hundreds of other threads about this, but how do I set or tweak the PYTHONPATH? I'm very new to Mac and the only other time I've successfully done this was on Windows using Rapid Enviroment Editor. This looks like a whole different beast. I tried installing Django and it failed to do it for 2.7, while I need 3.4, so the current path must be for 2.7. Running which python in Terminal shows the /usr/bin/python directory, which I then can't cd into, let alone find by browsing. I see that my Python 3.4 directory has the Update Shell Profile file, but it has a lot of limitations. I also see other threads mention PYTHONPATH commands in IDLE and creating one of the bash profile type files for the Terminal. How can I set this and not worry about it anymore until I need to run a different version of Python? I'm on Mac 10.9.2.
"Explain like I'm five".

If you want to install packages for python 3.4 use: pip3 install django
When installing for python 2.7 just use: pip install django
To use python 3.4 type python3 in your shell.
To see where all installations of python are use: which -a python
Depending on how you installed the new versions of python you will see output like:
/usr/local/bin/python
/usr/bin/python
If you wanted to use the python in /usr/local/bin/python you can edit your .bashrc file and add export path="/usr/local/bin:$path".
Save, then type source .bashrc in your shell and when you type which python it will show something like /usr/local/bin/python
Don't screw around too much with different versions of python, you will end up causing yourself a lot of problems.
You should not have to change your PYTHONPATH, just specify which python or pip version you want to use and that will most likely be all you need to do.

To update PYTHONPATH you can run from the terminal:
export PYTHONPATH=$PYTHONPATH:/desired/path/to/add
Then to check the updated PYTHONPATH you can run:
echo $PYTHONPATH
I'm not sure if this completely answers your question, but this is one way to make sure modules are visible to python when you import them.

How to run python in different directory?

I am doing maintenance for a python code. Python is installed in /usr/bin, the code installed in /aaa, a python 2.5 installed under /aaa/python2.5. Each time I run Python, it use /usr/bin one. How to make it run /aaa/python2.5?
Also when I run Python -v; import bbb; bbb.__file__; it will automatically show it use bbb module under /usr/ccc/(don't know why), instead of use bbb module under /aaa/python2.5/lib
How to let it run python2.5 and use `/aaa/python2.5/lib' module? The reason I asking this is if we maintain a code, but other people is still using it, we need to install the code under a new directory and modify it, run it and debug it.

Change the shebang-line (if there is such a line)
Nomally
#!/usr/bin/env python
is used to start the python interpreter first found in the path, this is how virtualenv works, i.e. it changes the path so that the chosen interpreter is placed first and the line above simply works.
In your example, change i to
#!/aaa/python2.5
Note that the executable-flag has to be set for this to work.
Example:
$ chmod u+x script.py
$ ./script.py
See PYTHONPATH for a description of how python searches for modules.

Do /aaa/python2.5 python_code.py. If you use Python 2.5 more often, consider changing the $PATH variable to make Python 2.5 the default.

Add this to your .bash_profile file (create if non exist, it's a hidden file):
python25(){
/aaa/python2.5 "$1"
}
Now you can use Python (latest) and Python 2.5:
$ python my_file.py
$ python25 my_file.py
Good luck!