Argh!! Can't wrap my head around this Python stuff!

[bartc]

On 26/11/2017 09:09, Greg Tibbet wrote:
> 
> I'm an old timer, have programmed in Fortran, C, C++, Perl, and a bit
> of Java and trying to learn this new-fangled Python language!
> 
> I've got a small program that uses PIL to create an image, draw some
> primitives (rectanges, ellipses, etc...) and save it.  Works fine...
> no issues.
> 
> I've found in the past, the best way to "really learn" the language
> was to "dig into the guts" and understand it,.. I thought I was making
> progress, but when looking into the PIL library to see what's going on
> behind the scenes, I find the following code in ImageDraw.py
> 
> def ellipse(self, xy, fill=None, outline=None):
>          """Draw an ellipse."""
>          ink, fill = self._getink(outline, fill)
>          if fill is not None:
>              self.draw.draw_ellipse(xy, fill, 1)
> <...snipped...>
> 
> ellipse() uses the method  self.draw.draw_ellipse()   Okay, fine...
> but WHERE is draw_ellipse defined??  What magic is happening there?
> I've searched the entire PIL directory tree, and the ONLY two places
> draw_ellipse is mentioned are right there in the ellipse() function...
> WHAT am I missing??

Python isn't a very pure language in that much of the functionality that 
looks like it should be written in Python (because you imported a module 
just like you import any Python module), actually is written in 
something else, as has been pointed out.

It's reasonable that some things need to be implemented using some 
foreign functions. But the boundary between Python and non-Python is 
blurred.

Take this program:

  import sys

and try and find sys.py in your installation.

(This is an obstacle if, for example, you're thinking of implementing a 
Python interpreter. In theory, once you have it working, it should run 
any .py program. But the critical modules it needs don't have .py source 
code. And the interface to those non-Python functions isn't defined with 
special byte-code instructions.

(It will be done /via/ those instructions, but the magic needed is on 
the other side of them. Calling into sys.fn() uses the same 
CALL_FUNCTION byte-code as calling into a regular Python function.)

As I said, it's not pure. More of a jungle as you've found out.)

-- 
bartc