1. List comprehensions provide a simple way to create lists from sequences. Usually the application program applies some operations to each element of a certain sequence, and uses the result obtained as the element to generate a new list, or creates a sub-sequence according to certain judgment conditions.
Every list comprehension is followed by an expression.
v=[2,4,6]
print([3*x for x in v])
print([[x,x*3] for x in v])
print([3*x for x in v if x>3])
print([3*x for x in v if x<2])
freshfruit = [' banana', ' loganberry ', 'passion fruit ']
print([weapon.strip() for weapon in freshfruit]) 
freshfruit = ['banana', 'loganberry ', 'passion fruit']
print(freshfruit)#这一行的作用和下一行一样
print([x.strip() for x in freshfruit]) #随便定义的，strip()函数是把列表中的输出来 
2. There can be zero to more for or if clauses. The return result is a list generated from the following for and if contexts based on the expression. If you want the expression to deduce a tuple, you must use parentheses.
v1=[2,4,6]
v2=[4,3,-9]
print([x*y for x in v1 for y in v2]) #x和y分别相乘，不是对应位置相乘
print([x+y for x in v1 for y in v2]) #x和y分别相加，不是对应位置相加
print([v1[i]*v2[i] for i in range(len(v2))]) #指定下标就是对应位置相乘 
3. List comprehensions can use complex expressions or nested functions:
print([str(round(355/113,i)) for i in range(1,6)]) 
The round() method returns the decimal point of x rounded to n digits.