• Posts
  • RSS
  • ◂◂RSS
  • Contact

  • Importing Python By Path

    February 17th, 2011
    tech, python  [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:

    The Limit of Circles in the Suburbs

    In dense urban cores, it’s valuable to run circular rail lines. They connect dense near-center neighborhoods to one another without going through the more congested center, and help make transferring between parallel lines more efficient, again through av…

    via Pedestrian Observations September 6, 2020

    Collections: Bread, How Did They Make It? Addendum: Rice!

    As an addendum on to our four-part look at the general structures of the farming of cereal grains (I, II, III, IV) this post is going to briefly discuss some of the key ways that the structures of rice farming differ from the structures of wheat and barle…

    via A Collection of Unmitigated Pedantry September 4, 2020

    Notes on “Anthropology of Childhood” by David Lancy

    I read David Lancy’s “The Anthropology of Childhood: Cherubs, Chattel, and Changelings” and highlighted some passages. A lot of passages, it turns out. [content note: discussion of abortion and infanticide, including infanticide of children with disabilit…

    via The whole sky August 27, 2020

    more     (via openring)


  • Posts
  • RSS
  • ◂◂RSS
  • Contact