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2. Detecting different versions of Python installed :
For Windows :
It is explained in PEP 514 by Steve Dower. 
Windows has registry keys which can be accessed by winreg or _winreg module.
Priority of registry keys :
HKEY_CURRENT_USER > HKEY_LOCAL_MACHINE
On 64 bit Windows there is another registry key for 32 bit programs,
HKEY_LOCAL_MACHINE\Software\Wow6432Node
Therefore using registry keys we can know about Python packages installed by enumerating on :
1. HKEY_CURRENT_USER\Software\Python\<Company>\<Tag>\<InstallPath>
2. HKEY_LOCAL_MACHINE\Software\Python\<Company>\<Tag>\<InstallPath>
3. HKEY_LOCAL_MACHINE\Wow6432Node\Python\<Company>\<Tag>\<InstallPath>
<Tag> will tell the version number of Python, and in <InstallPath> key there are two more subkeys which will tell the location of Python executable :
1. <Executable Path>
2. <WindowedExecutablePath>
For Linux :
We can find Python installations in /usr/bin/pythonXYZ
For MAC OS :
It can be searched for in 
/Library/Frameworks/
/System/Library/Frameworks
If Python was installed by Homebrew :
Then the location can be in 
/usr/local