Pass Variable On Import

Question

Let\'s say you have some time-consuming work to do when a module/class is first imported. This functionality is dependent on a passed in variable. It only needs to be done

Answer 1

If there is only one configuration item to set, then I have found overriding the python __builtin__ to work just fine, but it is an example of "Monkey patching" which is frowned on by some.

A cleaner way to do it which is very useful for multiple configuration items in your project is to create a separate Configuration module that is imported by your wrapping code first, and the items set at runtime, before your functional module imports it. This pattern is often used in other projects.

myconfig/__init__.py :

PATH_TO_R_SOURCE   = '/default/R/source/path'
OTHER_CONFIG_ITEM  = 'DEFAULT'
PI                 = 3.14

mymodule/__init__.py :

import myconfig

PATH_TO_R_SOURCE = myconfig.PATH_TO_R_SOURCE
robjects.r.source(PATH_TO_R_SOURCE, chdir = True) ## this takes time

class SomeClass:
  def __init__(self, aCurve):
    self._curve = aCurve

if myconfig.VERSION is not None:
  version = myconfig.VERSION
else:
  version = "UNDEFINED"

two_pi = myconfig.PI * 2

And you can change the behaviour of your module at runtime from the wrapper:

run.py :

import myconfig

myconfig.PATH_TO_R_SOURCE = 'actual/path/to/R/source'
myconfig.PI = 3.14159
# we can even add a new configuration item that isn't present in the original myconfig:
myconfig.VERSION="1.0"

import mymodule
print "Mymodule.two_pi = %r" % mymodule.two_pi
print "Mymodule.version is %s" % mymodule.version

Output:

> Mymodule.two_pi = 6.28318
> Mymodule.version is 1.0