python - python 是否缓存或重新计算 while 循环操作上的函数调用

Question

鉴于以下情况:

s = '1234567'
i = 0
while (i < len(s)):
    i += 1

Python 是否在每个循环中重新计算 len(s)，还是只计算一次？换句话说，我应该将上面的 len(s) 移动到循环上方的变量中，还是放在原来的位置就可以了？

Answer 1

https://docs.python.org/3/reference/compound_stmts.html#the-while-statement

while_stmt ::=  "while" assignment_expression ":" suite
               ["else" ":" suite]

This repeatedly tests the expression and, if it is true, executes the first suite;

通过将 len 替换为客户长度函数，您可以验证行为。

>>> def mylen(s):
...     print('mylen called')
...     return len(s)
... 
>>> s = '1234567'                                                                                       
>>> i = 0 
>>> while i < mylen(s):
...     i += 1
... 
mylen called
mylen called
mylen called
mylen called
mylen called
mylen called
mylen called
mylen called

顺便说一句，如果您想顺序迭代序列(在本例中为字符串)，为什么不使用简单的 for 循环。

如果确实需要使用索引，可以使用enumerate:

>>> for i, character in enumerate(s):
...     print(i, character)
... 
0 1
1 2
2 3
3 4
4 5
5 6
6 7