读书笔记

9 Python‘s build-in Functions

dir function

dir using the variable popcorn containing a string value, and you get the list of functions provided by the string class (all strings are members of the string class):
>>> popcorn = 'I love popcorn!'
>>> dir(popcorn)

['__add__', '__class__', '__contains__', '__delattr__', '__doc__',
'__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__',
'__getnewargs__', '__gt__', '__hash__', '__init__', '__iter__',
'__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__',
'__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__',
'__rmul__', '__setattr__', '__sizeof__', '__str__',
'__subclasshook__', 'capitalize', 'center', 'count', 'encode',
'endswith', 'expandtabs', 'find', 'format', 'format_map', 'index',
'isalnum', 'isalpha', 'isdecimal', 'isdigit', 'isidentifier',
'islower', 'isnumeric', 'isprintable', 'isspace', 'istitle',
'isupper', 'join', 'ljust', 'lower', 'lstrip', 'maketrans', 'partition',
'replace', 'rfind', 'rindex', 'rjust', 'rpartition',
'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip',
'swapcase', 'title', 'translate', 'upper', 'zfill']

At this point, you could use help to get a short description of any function in the list. Here’s an example of running help against the upper function:
>>> help(popcorn.upper)
Help on built-in function upper:
upper(...)
S.upper() -> str
Return a copy of S converted to uppercase.
The information returned can be a little confusing, so let’s take a closer look. The ellipsis (...) means that upper is a built-in function of the string class and, in this case, takes no parameters. The arrow (->) on the next line means that this function returns a string (str). The last line offers a brief description of what the function
does.

10 Useful Python Modules

shallow copy and deep copy

>>> harry = Animal('hippogriff', 6, 'pink')
>>> carrie = Animal('chimera', 4, 'green polka dots')
>>> billy = Animal('bogill', 0, 'paisley')
>>> my_animals = [harry, carrie, billy]
>>> more_animals = copy.copy(my_animals)
>>> print(more_animals[0].species)
hippogriff
>>> print(more_animals[1].species)
chimera

what happens if we change the species of one of our Animal objects in the original my_animals list (hippogriff to ghoul). Python changes the species in more_animals, too.
>>> my_animals[0].species = 'ghoul'
>>> print(my_animals[0].species)
ghoul
>>> print(more_animals[0].species)
ghoul

That’s odd. Didn’t we change the species in my_animals only? Why did the species change in both lists?
The species changed because copy actually makes a shallow copy, which means it doesn’t copy objects inside the objects we copied.
Here, it has copied the main list object but not the individual objects inside the list. So we end up with a new list that does not have its own new objects—the list more_animals has the same three objects inside it.

By the same token, if we add a new animal to the first list (my_animals), it doesn’t appear in the copy (more_animals). As proof, print the length of each list after adding another animal, like this:
>>> sally = Animal('sphinx', 4, 'sand')
>>> my_animals.append(sally)
>>> print(len(my_animals))
4
>>> print(len(more_animals))
3

As you can see, when we append a new animal to the first list, my_animals, it isn’t added to the copy of that list, more_animals. When we use len and print the results, the first list has four elements and the second has three. 

Another function in the copy module, deepcopy, actually creates copies of all objects inside the object being copied.

12 Tkinter

from tkinter import *
tk = Tk()
canvas = Canvas(tk, width=400, height=400)
canvas.pack()
canvas.create_polygon(10, 10, 10, 60, 50, 35)
def movetriangle(event):
 if event.keysym == 'Up':
 canvas.move(1, 0, -3)
 elif event.keysym == 'Down':
 canvas.move(1, 0, 3)
 elif event.keysym == 'Left':
 canvas.move(1, -3, 0)
 else:

 canvas.move(1, 3, 0)

canvas.bind_all('<KeyPress-Up>', movetriangle)
canvas.bind_all('<KeyPress-Down>', movetriangle)
canvas.bind_all('<KeyPress-Left>', movetriangle)
canvas.bind_all('<KeyPress-Right>', movetriangle)