Using a python class without calling __init__ method?

Question

I am learning python, step-by-step. Today is about Object Oriented Programming. I know how to create and use simple classes, but something bugs me. Most of the objects I use in python do not require to call a constructor

How can this works? Or is the constructor called implicitly? Example:

>>> import xml.etree.ElementTree as etree    
>>> tree = etree.parse('examples/feed.xml')  
>>> root = tree.getroot()                    
>>> root
<Element {http://www.w3.org/2005/Atom}feed at cd1eb0>

(from http://www.diveinto.org/python3/xml.html#xml-parse)

I would have gone this way (which actually works):

>>> import xml.etree.ElementTree as etree 
>>> tree = etree.ElementTree() # instanciate object
>>> tree.parse('examples/feed.xml')

I'd like to use this way of programming (do not call constructor, or at least call it implicitly) for my own project, but I can't get how it really works.

Thanks

Answer 1

etree.parse is a Factory function. Their purpose is mainly to be convenient ways of constructing objects (instances). As you can easily verify by looking at the source, the parse function does almost exactly as you do in your second example except it ommits a line of code or two.