Notes for Session 09¶
June 4, 2019¶
NOTE: These notes are “live” – they will change up until the class starts..
A collection of notes to go over in class, to keep things organized.
Issues that came up during the week.¶
sum()¶
You can use sum() for things other than numbers. Anything that you can add, you can use sum() for, but you need to give it a “start” value:
From trigrams:
In [14]: text
Out[14]:
['him mention her under any other name. In his eyes she eclipses',
'and predominates the whole of her sex. It was not that he felt',
'any emotion akin to love for Irene Adler. All emotions, and that']
In [15]: sum((line.split() for line in text), [])
Out[15]:
['him',
'mention',
'her',
'under',
'any',
'other',
'name.',
...
Except not strings:
In [17]: sum(["this", "that", "something"], "")
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-17-c882ae453790> in <module>
----> 1 sum(["this", "that", "something"], "")
TypeError: sum() can't sum strings [use ''.join(seq) instead]
That’s because you .join() is a lot more efficient.
Adding parameters to a subclass __init__¶
In general, when you override a method in a subclass, you want the method signature to be the same. That is – all the parameters should be the same.
However, sometimes, particularly with __init__, you may need a couple extra parameters. To keep things clean and extensible, you want to put the extra parameters at the beginning, before the super class’ parameters:
And this lets you use *args and **kwargs to pass along the usual ones.
class Base:
def __init__(self, par1, par2, par3=something, par4=something):
...
class Subclass(Base):
def __init__(self, newpar1, newpar2, *args, **kwargs):
self.newpar1 = newpar1
self.newpar2 = newpar2
super().__init__(*args, **kwarg)
Example: html_render Anchor tag:
class A(OneLineTag):
"""
anchor element
"""
tag = "a"
def __init__(self, link, *args, **kwargs):
kwargs['href'] = link
super().__init__(*args, **kwargs)
This becomes particularly important with super() and subclassing…
Testing Challenges¶
Any other html_render questions?¶
Brian: still up for a code review / debug session?
Lightning Talks¶
New Topics¶
sorting¶
maybe it’s a good idea to add a sort_key method to your classes?
see examples/Session09/sort_key.py
let’s try it on Circle….
classmethod¶
classmethod is really pretty simple to use, not much to talk about. But it can be a bit challenging to “get”.
The key point is that classmethods work for subclasses – like for alternate constructors.
Let’s look at that with my Circle solution:
solutions/Session08/circle.py
(and answer any other questions about Circle, while we are at it)
multiple inheritance and super()¶
super() is a mixed bag – it’s actually a pretty complex topic, but can be pretty easy to use – at least in the simple cases.
To get the hang of multiple inheritance, mix-ins, and super(), we’ll play around with object canvas:
See: examples/Session09/object_canvas.py