How do I add a method to a class from a third-party Python module without editing the original module

Question

I'm using the Basemap object from basemap module in the matplotlib toolkit (mpl_toolkits.basemap.Basemap). In basemap's __init__.py file (i.e. the mpl_toolkits.basemap.__init__ module), a method drawparallels is defined which draws latitudes on the map. I aim to duplicate that method to make a new method called drawmlat, making some adjustments in order to plot magnetic latitudes instead of geographic latitudes.

Ideally, I want the new drawmlat to be equivalent to the original drawparallel (a bound method of the instances of Basemap that I can call with using BasemapInstance.drawmlats()), and I do not want to modify the original file. How would I accomplish this?

I have tried variations of the "recipe" MyObj.method = MethodType(new_method, None, MyObj), but without placing anything in the original source file, the new method does not have access to globals etc. from the Basemap module (e.g. defined in its __init__.py).

If it seems I have misunderstood something, I probably have - I am more or less completely new to object-oriented programming.

Answer 1

Python is highly modifiable. Just add your function to the class:

from mpl_toolkits.basemap import Basemap


def drawmlat(self, arg1, arg2, kw=something):
    pass

Basemap.drawmlat = drawmlat

Now the Basemap class has a drawmlat method; call it on instances and self will be bound to the instance object. When looking up the method on instances, the function will automatically be bound as a method for you.

Anything defined in the Basemap.__init__ method that you need to care about are attributes on self.

Having looked over the mpl_toolkits.basemap.__init__ module, I do see that the drawparallel method relies on a few globals; you can import those from the module into your own namespace:

from mpl_toolkits.basemap import Basemap, _cylproj, _pseudocyl

This is no different from other imports you'd make; the original drawparallel method also relies on import numpy as np and from matplotlib.lines import Line2D, which make both np and Line2D globals in the original module.