Python, have a condition in a for loop

Question

Someone came with this example to me (python2):

num = int(input("num")) 
den = int(input("den"))

quot = 0
rest = den
i = num

for i in range(i,i>den, -den):
  quot = quot + 1
  rest = i - den

print quot
print rest

The code runs fine, does what it needs to be doing, and doesn't produce errors.

I don't understand why. To me, range() requires a lower and an upper limit, and to me, i would be the lower value, while i>den should evaluate to a boolean?

The context is a tutorial function which implements division with a for loop.

can you add a bit more of the surrounding code? as written we don't know all the preconditions (like the initial values of i and num). — aruisdante
– aruisdante, Commented Mar 9, 2015 at 1:33
You can have a condition in a for loop but not a range function. — Malik Brahimi
– Malik Brahimi, Commented Mar 9, 2015 at 1:34
@MalikBrahimi No, you can, because True evaluates to 1 and False to 0. But it certainly wouldn't behave like a C-like's for statement. It's just setting the stop point of iteration to either 0 or 1. — aruisdante
– aruisdante, Commented Mar 9, 2015 at 1:35
Looks to me that whoever wrote this code got Python for-loop syntax mixed up with the for-loop syntax in C, Java, JS, etc. Even if it is technically correct, it is absolutely not pythonic. — pzp
– pzp, Commented Mar 9, 2015 at 1:50

Brionius · Accepted Answer · 2015-03-09 01:35:31Z

3

This is an application of "duck typing". In this case, i>num evaluates to a boolean value of True or False, but in the context of the range function, True is equivalent to the integer 1, and False is equivalent to the integer 0.

So, for example, if i>num is False, then that code is equivalent to

for i in range(i, 0, -num):
  #do stuff

answered Mar 9, 2015 at 1:35

Brionius

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10 Comments

aruisdante Over a year ago

Having said that, there is almost certainly a more clear way to write out the conditions for the iteration than using a clever trick like this, which in Python 2.X is technically relying on an implementation detail.

Brionius Over a year ago

For sure - it's a weird bit of code, and not one I'd care to debug or maintain.

aruisdante Over a year ago

Gah, seeing the OP's update it's even crazier, since i = num entering the loop so it will always be False. WTF?

unsafe_where_true Over a year ago

The update opened my eyes: as i gets initialized to num, then the condition i > num evaluates to True. The range(i, True) evaluates to for example (16, 1), which is an empty range []. BUT, as we have the step change -dev, if dev is 2 for example, the range results in [16, 14, 12, 10, 8, 6, 4, 2] and that's why the code does what it needs to do!

Veedrac Over a year ago

@aruisdante What implementation detail?

|

Community · Accepted Answer · 2017-05-23 12:06:25Z

2

This is because Python considers True == 1 and False == 0.

See here for more information: Is False == 0 and True == 1 in Python an implementation detail or is it guaranteed by the language?

edited May 23, 2017 at 12:06

CommunityBot

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answered Mar 9, 2015 at 1:34

thedayturns

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