Why use sys path append path instead of sys path insert 1 path

Question

Edit  based on a Ulf Rompe s comment  it is important you use  1  instead of  0   otherwise you will break sys path   I have been doing python for quite a while now  over a year   and I am always confused as to why people recommend you use sys path append   instead of sys path insert     Let me demonstrate   Let s say I am working on a module named PyWorkbooks  that is installed on my computer   but I am simultaneously working on a different module  let s say PyJob  that incorporates PyWorkbooks   As I m working on PyJob I find errors in PyWorkbooks that I am correcting  so I would like to import a development version   There are multiple ways to work on both  I could put my PyWorkbooks project inside of PyJob  for instance   but sometimes I will still need to play with the path   However  I cannot simply do a sys path append   to the folder where PyWorkbooks is at   Why   Because python will find my installed PyWorkbooks first   This is why you have to do a sys path insert 1  path to dev pyworkbooks   In summary   sys path append path to dev pyworkbooks  import PyWorkbooks   does NOT import dev pyworkbooks  imports installed one   or   sys path insert 1  path to dev pyworkbooks    based on comments you should use   1 not 0   import PyWorkbooks   imports correct file   This has caused a few hangups for me in the past  and I would really like it if we  as a community  started recommending sys path insert 1  path   as if you are manually inserting a path I think it is safe to say that that is the path you want to use   Or do I have something wrong   It s a question that sometimes bothers me and I wanted it in the open

User · Accepted Answer

If you have multiple versions of a package / module, you need to be using virtualenv (emphasis mine):

virtualenv is a tool to create isolated Python environments.

The basic problem being addressed is one of dependencies and versions, and indirectly permissions. Imagine you have an application that needs version 1 of LibFoo, but another application requires version 2. How can you use both these applications? If you install everything into /usr/lib/python2.7/site-packages (or whatever your platform’s standard location is), it’s easy to end up in a situation where you unintentionally upgrade an application that shouldn’t be upgraded.

Or more generally, what if you want to install an application and leave it be? If an application works, any change in its libraries or the versions of those libraries can break the application.

Also, what if you can’t install packages into the global site-packages directory? For instance, on a shared host.

In all these cases, virtualenv can help you. It creates an environment that has its own installation directories, that doesn’t share libraries with other virtualenv environments (and optionally doesn’t access the globally installed libraries either).

That's why people consider insert(0, to be wrong -- it's an incomplete, stopgap solution to the problem of managing multiple environments.

User · Answer

If you really need to use sys path insert  consider leaving sys path 0  as it is   sys path insert 1  path to dev pyworkbooks    This could be important since 3rd party code may rely on sys path documentation conformance      As initialized upon program startup  the first item of this list    path 0   is the directory containing the script that was used to   invoke the Python interpreter

User · Answer

you are confusing the concept of appending and prepending  the following code is prepending   sys path insert 1   thePathToYourFolder      it places the new information at the beginning  well  second  to be precise  of the search sequence that your interpreter will go through  sys path append   puts things at the very end of the search sequence    it is advisable that you use something like virtualenv instead of manually coding your package directories into the PYTHONPATH everytime  for setting up various ecosystems that separate your site-packages and possible versions of python  read these two blogs     python ecosystems introduction bootstrapping python virtual environments   if you do decide to move down the path to environment isolation you would certainly benefit by looking into virtualenvwrapper  http   www doughellmann com docs virtualenvwrapper

[python] Why use sys.path.append(path) instead of sys.path.insert(1, path)?

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