python乾貨整理之__str__

# Python乾貨整理之__str__

__str__是Python中類對象自帶的一個函數（通過其名字也能看出來，和__self__類似），那麼它有什麼作用呢？首先敲出下面的代碼運行一下看看結果是什麼：

class Person(object):
    def __init__(self, name = 'tom', age = 10):
        self.name = name
        self.age = age
        self.parent = None

tom = Person()
print(tom)

# 唔，在我這裏輸出了個看起來像是地址的東西
# >>> <__main__.Person object at 0x00000244B4A2EA90>

所以當我們打印一個實例化對象時，打印的其實是這個對象的地址。而通過自定義__str__()函數就可以幫助我們打印對象中相關的內容。

class Person(object):
    def __init__(self, name = 'tom', age = 10):
        self.name = name
        self.age = age
        self.parent = None
    
    def __str__(self):
        return "Your name is: " + self.name
    
tom = Person()
print(tom)

# 唔，果然輸出了我們自定義的內容
# >>> Your name is: tom

總結：在python中調用print()打印實例化對象時會調用__str__()函數，如果__str__()中有返回值，就會打印其中的返回值。

這個用法我是在刷題的時候看到的，來都來了那就賞你一道算法題吧，順便體會一下__str__函數的用途。

"""
Given a number n, generate all binary search trees that can be constructed with nodes 1 to n.
"""

# answer
class Node:
	def __init__(self, value, left=None, right=None):
		self.value = value
		self.left = left
		self.right = right

	def __str__(self):
		result = str(self.value)
		if self.left:
			result = result + str(self.left)
		if self.right:
			result = result + str(self.right)
		return result


def helper(low, high):
	if low == high:
		return [None]
	result = []
	for i in range(low, high):
		left = helper(low, i)
		right = helper(i + 1, high)
		for l in left:
			for r in right:
				result.append(Node(i, l, r))
	return result


def generate_bst(n):
	# Fill this in.
	return helper(1, n + 1)


for tree in generate_bst(4):
	print(tree)

# Pre-order traversals of binary trees from 1 to n.
# 123
# 132
# 213
# 312
# 321

#   1      1      2      3      3
#    \      \    / \    /      /
#     2      3  1   3  1      2
#      \    /           \    /
#       3  2             2  1