# Thinking in python

Magnus Lie Hetland in his Beginning python wrote a code to replicate the range() built-in.

def interval(start, stop=None, step=1):
if stop is None:
start, stop = 0, start
result = []
i = start
while i < stop:
result.append(i)
i += step
return result


I re-wrote it like this.

def interval(*args):
if  len(args)<1:
raise TypeError('range expected at least 1 arguments, got 0')
elif len(args)>3 :
raise TypeError('range expected at most 3 arguments, got %d' % len(args))
else:
if len(args)==1:
start = 0
stop = args[0]
step = 1
elif len(args)==2:
start=args[0]
stop=args[1]
step=1
elif len(args)==3:
start=args[0]
stop=args[1]
step=args[2]

result = []
while start < stop:
result.append(start)
start+=step

return result


I know my code is lengthier but don't you people think its easier to grasp/understand than Hetland's code? Different peoples' minds work differently. In my mind the code felt easier to understand cos I come from a C background. My emphasis is on code comprehension.

• You got Magnus Lie Hetland's version slightly wrong. The line that reads i = start is indented one level too far. – Omnifarious Dec 15 '11 at 8:55

I think your code is more straightforward, but its size is a bit daunting. I actually like the original better. The only real 'trick' it uses is multiple assignment. And while it requires you to think through the different cases of range a bit to realize it works, this isn't something I mind. In fact, with yours, if I were to verify its correctness I'd have to spend just as much time because it has so much more code.

I tend to prefer code that's a bit terse, but elegantly handles all the cases to code that laboriously checks each case and handles that case with case-specific code. And this is in any programming language.

• Thanks I get the point.less code if not complicated even if complex. – Jibin Dec 15 '11 at 8:24

    while start < stop:
result.append(start)
start+=step


Don't reuse start to be the index of the loop. It's confusing. It violates the meaning of start.

The lengthy parameter parsing is good, but can be made more clear. Don't waste time on checking for <1 or >3. Just use the if statement.

if len(args)==1:...
elif len(args)==2:...
elif len(args)==3:...
else:
raise TypeError( "Wrong number of arguments" )

• Well,the range() built-in shows the argument No in exception message like range expected at least 1 arguments, got 0.I wanted to emulate it. – Jibin Dec 30 '11 at 4:04
• @Jibin: "built-in shows the argument number". That doesn't mean it's good design. It should also be part of the else clause if it's so important to provide that kind of message. – S.Lott Dec 30 '11 at 13:29
• :got that.I think there is a psychological reason to 'why error/exception should be in else case'.Suppose a programmer is reading the code, for first few moments his mind is fresh so main logic should come first,error/exceptions can come last.What do you think? – Jibin Jan 3 '12 at 4:32
• "logic should come first,error/exceptions can come last." I'm unclear on how it can be otherwise. What's the alternative? – S.Lott Jan 3 '12 at 23:06
• As in my code - the exception comes first in the if clause and actual the calculation of range comes last in the else clause. – Jibin Jan 11 '12 at 6:57

You've replaced three named arguments with a variable-length list of anonymous arguments and three seperate ways of mapping list positions onto named variables ... I'd question why that seems clearer to you.

You're then manually enforcing constraints on the variable-length argument list which the interpreter handled for you in the original code.

And, finally, where you raise an exception, you don't need to put the following code in an else block: if you're coming from C, think of the raise more like an early return.

eg.

def interval(*args):
if  len(args)<1:
raise TypeError('range expected at least 1 arguments, got 0')
if len(args)>3 :
raise TypeError('range expected at most 3 arguments, got %d' % len(args))
if len(args)==1:
...


(although S.Lott's formulation is better still).

• The problem with the version that has three named arguments is that those names aren't necessarily indicative of what the argument does, i.e. if you call interval(3) the name of the first argument is start even though its semantics are stop. So for that reason alone I think the *args version is actually clearer - at least the user won't make any assumptions about what the arguments to without consulting the documentation first. – sepp2k Dec 16 '11 at 22:38
• OK, good point: the code itself still looks more readable to me, but might also be misleading. – Useless Dec 19 '11 at 16:31
• Ok,I agree that else & elif is not required,but they make sure that only one block gets executed for sure,though logically there is no possibility for multiple if blocks getting executed in your code cos exception is raised .Is there any performance penalty if I use elif or else than if – Jibin Dec 30 '11 at 4:23