Chapter 1c: Python as a Computer Language

There should be one-- and preferably only one --obvious way to do it. - < The Zen of Python >

1. Built-in Collections (Python)

# List
array = [ None ] * n

# alternative to array map 
array = [ x*2 for x in range(i, j) ]  

# 2D Array, dont use [[0]*cols] * rows as 
# it will use the same object for all subarrays  
_2d_array = [ [0]*cols for x in range(rows) ] 
flattened = [ ele for subarr in _2d_array for ele in subarr]

# Use List as a stack
array.append(x)
curr = array.pop()

# Use List as a queue
queue = deque(array)
queue.append(x)
curr = queue.popleft()

# Use List as heap
heap = heapq.heapify(array)
heapq.heappush(heap, x)
curr = heapq.heappop(heap)

# hash set
hash_set = set()            
hash_set = {'a', 'b', 'c'}


# hash table
hash_table = {}
hash_table = Counter()

2. Mutate String

String 是Immutable的，因此要对String 进行修改时，可以用l = list(str) 将其转换为char array然后用所有array的方法进行操作，再进行" ".join(l)

3. Iterators and Iterable

Reference: https://opensource.com/article/18/3/loop-better-deeper-look-iteration-python

Python的for loop并非C-style的for loop，与forEach一样是访问iterable内的每一个元素，因此for x in range(0,n) 的最终值为n-1

4. Decimal Precision Limitation

Python中的小数都不是完全精确的，根本原因是因为用二进制来表示小数的limitation: https://docs.python.org/3/tutorial/floatingpoint.html

5. Inline Function

需要把问题的一些子功能拆分出来时，可以用inline function来定义，这样免去了需要母函数需要传递很多参数给子函数的麻烦，使得code的逻辑更清晰。注意如果我们需要在inline function的scope内更改这些变量的话，要用nonlocal关键词，类似于C++中的pass-by-reference。

def foo():
    x = 1
    def bar():
        nonlocal x
        x = 2
    
    bar()
    print(x)    # => 2

Inline function普遍使用于递归，因为递归函数的输入输出，经常和问题需要的输入输出不同。

def quickSort(arr: List[int]) -> None:
    # pass start and end index instead of subarray to save space 
    def impl(start: int, end: int) -> None:
        if start >= end:
            return

        pivot = arr[start] + (arr[end] - arr[start]) / 2

        l, r = start, end
        while l <= r:
            while arr[l] < pivot: l += 1
            while arr[r] > pivot: r -= 1
            if l <= r:
                arr[l], arr[r] = arr[r], arr[l]
                l += 1
                r -= 1
        
        impl(start, l-1)
        impl(r+1, end)

    impl(0, len(arr)-1)

6. Instance variable

Python不存在private的instance variable（从convention来说可以在他们的命名前加上__ )，但是可以用@property 和 @setter去 override 读取和写入变量的behavior，这样保证了一件事情只有一种做法。instance variable可以在instance method中初始化，而并不一定要用__init__。

class myClass(baseClass):
    def __init__(self,var1,var2):
        self.var1 = var1
        self.var2 = var2

    @property
    def var2(self):
        return self.var2
    
    @var2.setter
    def var2(self, val):
        self.var2 = val
    
    # init variable in instance method
    def func(self):
        self.var3 = None