bag = [1, 2, 3, 4, 5] 
for i in range(len(bag)): 
 bag[i] = bag[i] * 2

bag = [elem * 2 for elem in bag]

bag = [1, 2, 3, 4, 5] 
for i in range(len(bag)): 
 print(bag[i])

bag = [1, 2, 3, 4, 5] 
for i in bag: 
 print(i)

bag = [1, 2, 3, 4, 5] 
for index, element in enumerate(bag): 
 print(index, element)

a = 5 
b = 10
# 交换 a 和 b
tmp = a 
a = b 
b = tmp

a = 5 
b = 10 
# 交换a 和 b
a, b = b, a

bag = [] 
for _ in range(10): 
 bag.append(0)

bag_of_bags = [[0]] * 5 # [[0], [0], [0], [0], [0]] 
bag_of_bags[0][0] = 1 # [[1], [1], [1], [1], [1]]

bag_of_bags = [[0] for _ in range(5)] 
# [[0], [0], [0], [0], [0]]
bag_of_bags[0][0] = 1 
# [[1], [0], [0], [0], [0]]
“过早优化是万恶之源”

name = "Raymond" 
age = 22 
born_in = "Oakland, CA" 
string = "Hello my name is " + name + "and I'm " + str(age) + " years old. I was born in " + born_in + "." 
print(string)

name = "Raymond" 
age = 22 
born_in = "Oakland, CA" 
string = "Hello my name is {0} and I'm {1} years old. I was born in {2}.".format(name, age, born_in) 
print(string)

def binary(): 
 return 0, 1

result = binary() 
zero = result[0] 
one = result[1]

def binary(): 
 return 0, 1
zero, one = binary()

countr = {} 
bag = [2, 3, 1, 2, 5, 6, 7, 9, 2, 7] 
for i in bag: 
 if i in countr:
 countr[i] += 1
 else:
 countr[i] = 1

for i in range(10): 
 if i in countr:
 print("Count of {}: {}".format(i, countr[i]))
 else:
 print("Count of {}: {}".format(i, 0))

countr = {} 
bag = [2, 3, 1, 2, 5, 6, 7, 9, 2, 7] 
for i in bag: 
 countr[i] = countr.get(i, 0) + 1
for i in range(10): 
 print("Count of {}: {}".format(i, countr.get(i, 0)))

bag = [2, 3, 1, 2, 5, 6, 7, 9, 2, 7] 
countr = dict([(num, bag.count(num)) for num in bag])
for i in range(10): 
 print("Count of {}: {}".format(i, countr.get(i, 0)))

countr = {num: bag.count(num) for num in bag}

from collections import Counter 
bag = [2, 3, 1, 2, 5, 6, 7, 9, 2, 7] 
countr = Counter(bag)
for i in range(10): 
 print("Count of {}: {}".format(i, countr[i]))

bag = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 
for elem in bag[:5]: 
 print(elem)

bag = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 
for elem in bag[-5:]: 
 print(elem)

bag = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 
for index, elem in enumerate(bag): 
 if index % 2 == 0: 
 print(elem)

bag = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 
for elem in bag[::2]: 
 print(elem)
# 或者用 ranges
bag = list(range(0,10,2)) 
print(bag)