Possible mixed indentation in Python?

Question

Looking at this question, I tried OP's the code on my machine. Here are a text version and a screenshot:

What just happened? This supposed to be a square function, and it is implemented correctly. To be sure, I copy-pasted the code, and tried it again:

Well, I can't see any difference between these versions of square, but only the latter works.

The only reason I can think of is that I may have mixed tabs and spaces, so the return statement is actually indented, and so the loop is executed exactly once. But I could not reproduce it, and it looks like an unbelievable flaw in the interpreter's mixed-indentation-check. So I have two questions, or maybe three:

1. What do I miss?
2. If this is a mixed indentation thing, what it may be, exactly?
3. If this is a mixed indentation thing, why wasn't it caught by the interpreter? Obviously the whole idea of indentation in python (and in general) is to avoid such problems. And it's too important to let such things slip.

Answer 1

Easy!

def square(x):     runningtotal = 0     for counter in range(x):         runningtotal = runningtotal + x <tab>return runningtotal 

First, tabs are replaced (from left to right) by one to eight spaces such that the total number of characters up to and including the replacement is a multiple of eight <...>

So this tab at the last line is replace with 8 spaces and it gets into the loop.

Answer 2

For Python indenting - a tab is counted as equivalent to 8 spaces

Since people almost never have their tab width set to 8 spaces, it's never a good idea to mix the two.

Like many people, I used to prefer tabs for indenting, but found that it is a constant source of confusion when emailing code or posting in forums, etc. Which is what has happened here

The most common thing these days is to just have the tab key in your editor insert 4 spaces.

The point is that Python has to respect tabs because of backward compatibility, but it's not a good idea to use them anymore.

As mentioned by @Fredrik, there is the -t option from the man page

       -t     Issue a warning when a source file mixes  tabs  and  spaces  for               indentation  in a way that makes it depend on the worth of a tab               expressed in spaces.  Issue an error when the  option  is  given               twice. 

Here the return runningtotal has a tab.

$ python -tt Python 2.7.4 (default, Apr 19 2013, 18:28:01)  [GCC 4.7.3] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> def square(x): ...     runningtotal = 0 ...     for counter in range(x): ...         runningtotal = runningtotal + x ...     return runningtotal   File "<stdin>", line 5     return runningtotal                       ^ TabError: inconsistent use of tabs and spaces in indentation