So I decided to learn python - why? because it is used by Mercurial. And there is at least one of the gatekeeping scripts which I needed to hack for the nfs41-gate.

I bought Learning Python, Third Edition by Mark Lutz because the local Borders did not have Programming Python, Third Edition. From some reviews I read, it would probably have been a better fit for me.

I know I can find most of what I want on the net, but I wanted a printed resource.

Anyway, I had a question right off the bat - about whether if file a imports modules b and c, what happens if c also imports b? Deeper in the book that I've read, it does state that an import is equivalent to load the file if it is not already loaded. But that doesn't help me learn the language. :->

The following example is quite simple, but effective in answering the question for me:

a.py

> cat a.py
#!/usr/bin/python

title = "This is the file a.py!"

print title

print "importing b from a"
import b

print "importing c from a"
import c

b.py

> cat b.py
#!/usr/bin/python

title = "This is the file b.py!"

print title

c.py

> cat c.py
#!/usr/bin/python

import b

title = "This is the file c.py!"

print title

Test 1

> ./a.py
This is the file a.py!
importing b from a
This is the file b.py!
importing c from a
This is the file c.py!

So we see that if a has loaded it, then c will not. How about the other way?

a2.py

> cat a2.py
#!/usr/bin/python

title = "This is the file a.py!"

print title

print "importing c from a"
import c

print "importing b from a"
import b

print "and b's title is"
print b.title

Test 2

> ./a2.py
This is the file a.py!
importing c from a
This is the file b.py!
This is the file c.py!
importing b from a
and b's title is
This is the file b.py!

We see that c loads b and that b's attributes are visible from a.

What would really help me here is if b could state the call stack of what is importing it.

Test 3

A simple change fails:

> cat b.py
#!/usr/bin/python

title = "This is the file b.py!"

print title

print "Called from", __file__

but it does show the effect of byte code compilation:

> ./a.py
This is the file a.py!
importing b from a
This is the file b.py!
Called from /home/tdh/python/b.py
importing c from a
This is the file c.py!
> ./a2.py
This is the file a.py!
importing c from a
This is the file b.py!
Called from /home/tdh/python/b.pyc
This is the file c.py!
importing b from a
and b's title is
This is the file b.py!

I can see the "nesting" if I pop into an interactive session:

> python
>>> import a2
This is the file a.py!
importing c from a
This is the file b.py!
Called from b.pyc
This is the file c.py!
importing b from a
and b's title is
This is the file b.py!
>>> a2.__dict__.keys()
['c', 'b', 'title', '__builtins__', '__file__', '__name__', '__doc__']
>>> a2.c.__dict__.keys()
['b', 'title', '__builtins__', '__file__', '__name__', '__doc__']
>>> a2.c.b.__dict__.keys()
['__builtins__', '__name__', '__file__', '__doc__', 'title']

But this doesn't answer my question of how to figure this out recursively. I.e., I guess I am looking for a parent "pointer" and I could walk it to get my answer.

But I've still learned more than just reading the book linearly.