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?
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
len with mylen is a very convincing argument. Users coming from other languages, where compiler optimizations are able to do much more than is possible in Python, might reasonably expect a builtin length call on an immutable object to get optimized.
len(...). I wanted to show Python behavior. :)s and len are not changing using a static analysis at compile time. In reality it does not bother, of course, but that would be conceivable for developers more familiar with languages that have farther reaching compiler optimizations. And it's something that would obviously not be tractable with an arbitrary user-defined function in place of the built-in len.Python will not cache the value in any way. len(s) will be called on every iteration.
In such looping constructs, there's no point to create a separate variable to store a length, since the calculation is fast for all built-in container types. Doing so would be called a premature optimization.
it recalculates
len(s)
because the while loop needs to check on each iteration if its condition or conditions are True.
s[i]with that index, thenfor c in s:and never checking length would likely be better.