Does python cache or recalculate a function call on a while loop operation

Question

Given the following:

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

Does python recalculate the len(s) on every loop, or does it only calculate it once? In other words, should I move the len(s) above into a variable above the loop or is it fine where it is?

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;

By replacing len with customer length function, you can verify the behavior.

>>> 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

BTW, if you want to iterate the sequence (string, in this case) sequentially, why don't you use simple for loop.

If you really need to use index, you can use enumerate:

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