• Posts
  • RSS
  • ◂◂RSS
  • Contact

  • Importing Python By Path

    February 17th, 2011
    python, tech  [html]

    Update 2012-01-06: Warning: this post is what you get if you don't know about the function __import__ and try to reinvent it.

    Imagine I have two different python files that are drop-in replacements for each other, perhaps for some kind of plugin system:

      $ cat a/c.py
      def d():
         print "hello world a"
      $ cat b/c.py
      def d():
         print "hello world b"
      
    And futher imagine I want to use both of them in the same program. I might write:
      import c
      c.d()
      import c
      c.d()
      
    But that has no chance of working. Python doesn't know where to find 'c'. So I need to tell python where to look for my code by changing sys.path:
      sys.path.append("a")
      import c
      c.d()
      sys.path.append("b")
      import c
      c.d()
      
    This will work, ish. It will print "hello world a" twice. Part of this is that the path "a" is sys.path before "b". I really only want this sys.path change to last long enough for my import to work. So maybe I should do:
      class sys_path_containing(object):
         def __init__(self, fname):
             self.fname = fname
    
         def __enter__(self):
             self.old_path = sys.path
             sys.path = sys.path[:]
             sys.path.append(self.fname)
    
         def __exit__(self, type, value, traceback):
             sys.path = self.old_path
    
      with sys_path_containing("a"):
         import c
    
      c.d()
    
      with sys_path_containing("b"):
          import c
    
      c.d()
      
    This is closer to working. I define a context manager so that code run with sys_path_containing will see a different sys.path. So my first "import c" will see a sys.path like ["foo", "bar", "a"] and my second import will see ["foo", "bar", "b"]. Each is isolated from the other and from other system changes. Unfortunately, it still won't work, because python remembers what it has imported before and doesn't do it again, this will still only print "hello world a" twice. Switching the second "import c" to a "reload(c)" does fix this problem, but at the expense of you already having to know whether something is loaded. Switching to "del sys.modules['c']" and using __import__ would work, though. Let's make that change and put it all into a context manager that does most of the work for us:
      class imported(object):
         def __init__(self, fname):
             self.fname = os.path.abspath(fname)
    
         def __enter__(self):
             if not os.path.exists(self.fname):
                 raise ImportError("Missing file %s" % self.fname)
    
             self.old_path = sys.path
             sys.path = sys.path[:]
    
             file_dir, file_name = os.path.split(self.fname)
             sys.path.append(file_dir)
    
             file_base, file_ext = os.path.splitext(file_name)
             module = __import__(file_base)
             del sys.modules[file_base]
    
             return module
    
         def __exit__(self, type, value, traceback):
             sys.path = self.old_path
    
      with imported("a/c.py") as c:
         c.d()
    
      with imported("b/c.py") as c:
         c.d()
      
    This will print "hello world a" and then "hello world b". Yay!

    Recent posts on blogs I like:

    Governance in Rich Liberal American Cities

    Matt Yglesias has a blog post called Make Blue America Great Again, about governance in rich liberal states like New York and California. He talks about various good government issues, and he pays a lot of attention specifically to TransitMatters and our …

    via Pedestrian Observations November 19, 2020

    Collections: Why Military History?

    This week, I want to talk about the discipline of military history: what it is, why it is important and how I see my own place within it. This is going to be a bit of an unusual collections post as it is less about the past itself and more about how we st…

    via A Collection of Unmitigated Pedantry November 13, 2020

    Misalignment and misuse: whose values are manifest?

    Crossposted from world spirit sock puppet. AI related disasters are often categorized as involving misaligned AI, or misuse, or accident. Where: misuse means the bad outcomes were wanted by the people involved, misalignment means the bad outcomes were wan…

    via Meteuphoric November 13, 2020

    more     (via openring)


  • Posts
  • RSS
  • ◂◂RSS
  • Contact