Glossaries

• Todo: study descriptor. Understanding descriptors is a key to a deep understanding of Python because they are the basis for many features including functions, methods, properties, class methods, static methods, and reference to super classes.

• Metaclass: The class of a class. Class definitions create a class name, a class dictionary, and a list of base classes.

What is a class dictionary?

• Packages are a way of structuring Python’s module namespace by using “dotted module names”.

• function parameters: ```python def f(pos1, pos2, /, pos_or_kwd, *, kwd1, kwd2): pass # ———– ———- ———- # | | | # | Positional or keyword | # | - Keyword only # – Positional only

def cheeseshop(kind, arguments, **keywords): matrix = [ [1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], ] list(zip(matrix)) # [(1, 5, 9), (2, 6, 10), (3, 7, 11), (4, 8, 12)]

* `list` can be used for `stack` or `queue`, but for `queue`, `pop(0)` is not efficient, so there is a `collections.deque` there for `queue`.
`deque.popleft()` and `deque.appendleft()` are for the purpose. `deque` may mean double-end queue.

* `modules` are files with functions defined in them.
* Without `__init__.py`, modules would not become packages. Packages are for importing purpose and used with `dotted`. like `sys.xxx`.
* A file could be a `module` or a `package`.
* importing a package, the code inside package `__init__()` will only be executed once.

```python
bugs = 'roaches'
count = 13
area = 'living room'
print(f'Debugging {bugs=} {count=} {area=}')
# Debugging bugs='roaches' count=13 area='living room'

• The str.rjust() method of string objects right-justifies a string in a field of a given width by padding it with spaces on the left.
• f = open('workfile', 'w', encoding="utf-8"), modes like w is write, r is read (default), a is append. modes default is text file, append b is binary file

with open('workfile', encoding="utf-8") as f:
    read_data = f.read()
# This is good practice since f is closed automatically.
# f.closed # test it, it returns True

with open('workfile', 'w', encoding="utf-8") as f:
    f.write('You are awesome.')

• Counter ```python from collections import Counter note1 = ‘aabbbbc’ c1 = Counter(note1) # Counter({‘b’: 4, ‘a’: 2, ‘c’: 1}) note2 = ‘ab’ c2 = Counter(note2) # Counter({‘b’: 1, ‘a’: 1}) c1 - c2 # Counter({‘b’: 3, ‘a’: 1, ‘c’: 1})

h1 = {‘b’: 3, ‘a’: 1, ‘c’: 1} h2 = {‘b’: 2, ‘c’: 1} Counter(h1) - Counter(h2) # Counter({‘b’: 1, ‘a’: 1})

* `JSON`
```python
import json

x = [1, 'simple', 'list']
k = json.dumps(x) # '[1, "simple", "list"]'
l = json.loads(k) # return list equals to `x`