Time: Wed 10/4/17 3 pm
Similar to the elementary data abstraction, just that we can change values in the encapsulation now somehow.
nonlocal? And why is it useful in this case? Can we just bind some variable in the parent frame to some new values without using this keyword? Do we need to have something in a nonlocal frame with the name beforehand?
Here's something really bad to do as an example:
def make_bike(): # The color of the bike color = "red" def get_color(): return color def change_color(new_color): # Change the color of the bike color = new_color # However now you thought you forgot to use nonlocal # But note that it doesn't hoist nonlocal color # Invalid syntax # Python will just give up with your code :( # ... and literally halt here return [get_color, change_color] [get_color, change_color] = make_bike() # Make a bike change_color("blue") # Make it blue, don't like it being red
And a slightly better example (in which there is no syntax error, but logically incorrect):
def make_factory(): # The color of the bike by default factory_color = "green" def make_bike(): # Use the factory color as default color = factory_color def get_color(): return color def change_color(new_color): # Someone hacked into the script and changed the following lines nonlocal factory_color factory_color = new_color return [get_color, change_color] return make_bike make_bike = make_factory() # Make a new bike [get_0_color, change_0_color] = make_bike() # Inspect its color get_0_color() # green # And now we want to repaint the bike change_0_color("blue") # And when we try to inspect the bike again... get_0_color() # green # Uhm it is still green :/ # Make another bike [get_1_color, change_1_color] = make_bike() # Try to inspect its color... which should be green get_1_color() # blue # Oh well, the factory settings are modified... uh oh
Can we still implement something with a same interface that does exactly the same without
There's something called
global; however, Python doesn't act as picky as it does to
nonlocal. When it sees something undefined paired up with
global, it just doesn't complain. More tears...
Can we use
global rather than
nonlocal for everything? Please don't.
Primitively, they are just syntactical sugars for data abstraction. What are methods, bound methods and static methods?
A quick example of each:
class balloon(): @staticmethod def describe(): print("Balloon!") def grow(self): print("The balloon grew larger...") b = balloon() balloon # <class '__main__.balloon'> b # <__main__.balloon object at ...> # Static method balloon.describe # < balloon.describe at ...> balloon.describe() # Balloon! b.describe # < balloon.describe at ...> b.describe() # Balloon! # Unbound method balloon.grow # < balloon.grow at ...> balloon.grow(b) # The balloon grew larger... # Bound method b.grow # <bound method balloon.grow of <__main__.balloon object at ...>> b.grow() # The balloon grew larger...
Class variables and instance variables are different. Class variables are shared between instances; however, instance variables are just different from instance to instance.
class sandbag(): weight = 10 # Someone goes to buy a sandbag s = sandbag() # He weighs the sandbag... s.weight # 10 # And finds that it is of 10 kilos # And another person comes into the story and does something terrible sandbag.weight = 5 # The next day, whoever that bought the sandbag tries to re-weigh it s.weight # 5 # And somehow... It became lighter :(
How do you make custom constructors for a class?
There are some goodies and complications that come with objects too. Multiple inheritance is a beast--check out the diamond problem--and it can bring ambiguities in programming. There are ways to probe into the method resolution order.
super, and why do we use it?
They are naturally sustainable! Oh well, think about how
trees are conceptually related to
pairs. Define the terminologies around trees--tree, root, label, branch, leaf (the language we use may vary in different places).
Consider the following implementation:
def tree(label, branches=): return [label] + branches def label(tree): return tree def branches(tree): return tree[1:]
There are multiple ways to get started on playing with such tree structure! As a few examples: