Revert to original version of python

Question

I am trying to install a package called nmoldyn in my system. This package caused havoc in my system. It required some packages in python, so what it precisely did was install an older version of python (v 2.5.4) and tried to install other packages along with it. But somehow it was not able to install matplotlib (may be it was trying to install an old version and it was not available from the repository). The major trouble it has caused is that it has changed my path variable to newer version of python (i.e. v 2.5.4) while I earlier had (v 2.7.2). How do I revert back to old path and environment setting ??

EDIT 1:

harish@harish-lap:~$ apt-cache policy python
python:
  Installed: 2.7.5-5ubuntu3
  Candidate: 2.7.5-5ubuntu3
  Version table:
 *** 2.7.5-5ubuntu3 0
        500 http://in.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages
        100 /var/lib/dpkg/status
harish@harish-lap:~$ type python
python is /usr/local/bin/python

EDIT 2:

harish@harish-desktop:~/dl_class_1.9_serial/execute$ env python
Python 2.5.4 (r254:67916, Sep 16 2015, 16:39:34) 
[GCC 4.8.4] on linux3
Type "help", "copyright", "credits" or "license" for more information.
>>>         
harish@harish-desktop:~/dl_class_1.9_serial/execute$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

EDIT 3 :

harish@harish-desktop:~/dl_class_1.9/execute$ sudo find /usr -type f -name 'python'
[sudo] password for harish: 
/usr/share/lintian/overrides/python
/usr/share/bash-completion/completions/python
/usr/local/bin/python
harish@harish-desktop:~/dl_class_1.9/execute$ command -v python
/usr/local/bin/python

Solution

Your path looks ok. Therefore a first shot:

Reinstall the original Python package via

sudo apt-get install --reinstall python

to overwrite the previous installation.

But your last edit (Edit 3) gives the solution. Remove the binary in /usr/local/bin via

sudo rm /usr/local/bin/python

or rename the file

sudo mv /usr/local/bin/python /usr/local/bin/python-2.5.4

Comments

[A.B.]

Edit your question and add the output of apt-cache policy python and the output of type python

[kos]

How did you install the old version? (Python 2.7.4)

[A.B.]

Edit your question and add the output of env python and echo $PATH

[A.B.]

Edit you question and add the output of find /usr -type f -name 'python'

[A.B.]

And the output of command -v python

[Ravan]

Since you have accepted the answer,award bounty manually by clicking +50 blue just below tick mark,(you will get benefactor as well),

[user35952]

Nope, still doesn't work, python still looks at python 2.5 only

[user35952]

Won't removing /usr/local/bin/python affect anything so much ??

[A.B.]

No, and if so, then we fix the problem too.

[user35952]

may be if I just rename that directory as some thing like unnamed, isn't it equivalent to delete it

[A.B.]

Yes, I have added the command.

[user35952]

Thank you so much, I will just accept the answer in a while, but just wanted to ask, this didn't completely solve the problem, as am not able to run nMoldyn as such, I have to recompile it differently with different paths I think (earlier it was given as /usr/local/bin, now when I use /usr/bin its saying python is not installed, which means it will install it and do all ruckus again, any reason why its not detecting python. I understand this is different question, but just pardon the inconvenience.

[A.B.]

check the used python binary with command -v python. It should be /usr/bin/python

[A.B.]

Which moldyn, this? github.com/arcan1s/moldyn

[user35952]

It still doesn't agree !! No, nMoldyn by ILL

[A.B.]

Let us continue this discussion in chat.

[Ravan]

Since you have accepted the answer,award bounty manually by clicking +50 blue just below tick mark,(you will get benefactor as well),