How to dynamically load a Python class

Question

Given a string of a Python class  e g  my package my module MyClass  what is the best possible way to load it   In other words I am looking for a equivalent Class forName   in Java  function in Python  It needs to work on Google App Engine   Preferably this would be a function that accepts the FQN of the class as a string  and returns a reference to the class   my class   load class  my package my module MyClass   my instance   my class

User · Answer

In Google App Engine there is a webapp2 function called import string  For more info see here https   webapp-improved appspot com api webapp2 html  So   import webapp2 my class   webapp2 import string  my package my module MyClass     For example this is used in the webapp2 Route where you can either use a handler or a string

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If you re using Django you can use this  Yes i m aware OP did not ask for django  but i ran across this question looking for a Django solution  didn t find one  and put it here for the next boy gal that looks for it     It s available for v1 7    https   github com django django blob stable 1 7 x django utils module loading py from django utils module loading import import string  Klass   import string  path to module Klass   func   import string  path to module func   var   import string  path to module var     Keep in mind  if you want to import something that doesn t have a    like re or argparse use   re     import    re

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def import class cl       d   cl rfind          classname   cl d 1 len cl       m     import   cl 0 d   globals    locals     classname       return getattr m  classname

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Here is to share something I found on   import   and importlib while trying to solve this problem   I am using Python 3 7 3   When I try to get to the class d in module a b c   mod     import    a b c     The mod variable refer to the top namespace a   So to get to the class d  I need to  mod   getattr mod   b    mod is now module b mod   getattr mod   c    mod is now module c mod   getattr mod   d    mod is now class d   If we try to do  mod     import    a b c   d   getattr mod   d     we are actually trying to look for a d   When using importlib  I suppose the library has done the recursive getattr for us  So  when we use importlib import module  we actually get a handle on the deepest module   mod   importlib import module  a b c    mod is module c d   getattr mod   d    this is a b c d

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module     import    my package my module   the class   getattr module   MyClass   obj   the class

User · Answer

If you happen to already have an instance of your desired class  you can use the  type  function to extract its class type and use this to construct a new instance   class Something object       def   init   self  name           self name   name     def display self           print self name   one   Something  one   one display   cls   type one  two   cls  two   two display

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import importlib  module   importlib import module  my package my module   my class   getattr module   MyClass   my instance   my class

User · Answer

From the python documentation  here s the function you want   def my import name       components   name split          mod     import   components 0       for comp in components 1            mod   getattr mod  comp      return mod   The reason a simple   import   won t work is because any import of anything past the first dot in a package string is an attribute of the module you re importing   Thus  something like this won t work     import    foo bar baz qux     You d have to call the above function like so   my import  foo bar baz qux     Or in the case of your example   klass   my import  my package my module my class   some object   klass     EDIT   I was a bit off on this   What you re basically wanting to do is this   from my package my module import my class   The above function is only necessary if you have a empty fromlist   Thus  the appropriate call would be like this   mod     import    my package my module   fromlist   my class    klass   getattr mod   my class

User · Answer

OK  for me that is the way it worked  I am using Python 2 7    a     import    file to import   globals    locals           -1  b   a MyClass     Then  b is an instance of class  MyClass

User · Answer

If you don t want to roll your own  there is a function available in the pydoc module that does exactly this   from pydoc import locate my class   locate  my package my module MyClass     The advantage of this approach over the others listed here is that locate will find any python object at the provided dotted path  not just an object directly within a module   e g  my package my module MyClass attr   If you re curious what their recipe is  here s the function   def locate path  forceload 0          Locate an object by name or dotted path  importing as necessary         parts    part for part in split path       if part      module  n   None  0     while n  lt  len parts           nextmodule   safeimport join parts  n 1         forceload          if nextmodule  module  n   nextmodule  n   1         else  break     if module          object   module     else          object     builtin       for part in parts n            try              object   getattr object  part          except AttributeError              return None     return object   It relies on pydoc safeimport function   Here are the docs for that      Import a module  handle errors  return None if the module isn t found   If the module  is  found but an exception occurs  it s wrapped in an ErrorDuringImport exception and reraised   Unlike   import    if a package path is specified  the module at the end of the path is returned  not the package at the beginning   If the optional  forceload  argument is 1  we reload the module from disk  unless it s a dynamic extension

[python] How to dynamically load a Python class

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