The Parameters And Usage Of Ranges In Python

The aim of this page📝is to cover the little weirdness of ranges in python.

1. what is a range

• range is a sequence that represents an arithmetic progression of integers
• in Python it is created by range() built-in function — there is no literal form
• NOTE: the end value is one past the stop value of a sequence — same as in slicing, it is utilizing half open interval convention
• the start value is usually 0 — but can be optionally passed into the range constructor
• range is strange because it determines what its argument means by counting them
• i.e., arguments are not it is not positional
• and it also does not support keyword args (a bit un-pythonic as this is a special case breaking the rule)
• it counts the arguments and concludes the meaning as follows

ARG COUNT | ARG MEANING       | COMMENT
----------|-------------------|-------------------------------------
1         | stop              | counting from zero
2         | start, stop       | using the half-open interval (!)
3         | start, stop, step | an increment 

• as hinted above, to use step argument, you have to supply all 3 arguments

>>> list(range(0,18,3))
[0, 3, 6, 9, 12, 15]

2. why half-open range convention and the three times -1 range arguments to parse a list backward

• for working with consecutive ranges
• the end specified in one range is the beginning of the next range
• there is no overlap

>>> list(range(1,4))
[1, 2, 3]
>>> list(range(4,8))
[4, 5, 6, 7]

• but it can get a little weird
• e.g. in the linked leet code example you want simply to loop through indexes of a list in a decreasing order
• you have to have -1 as a stop parameter if you want to stop at index 0 (which is what you want with standard 0-based indexing), something like this:

>>> l = list(range(6))
>>> l
[0, 1, 2, 3, 4, 5]

# WRONG
>>> for i in range(len(l)-1,0,-1):print(i)
5
4
3
2
1

# CORRECT
>>> for i in range(len(l)-1,-1,-1):print(i)
5
4
3
2
1
0

3. typical usage: loop counters

• use range object to count loops

>>> for i in range(7): print(i)
...
0
1
2
3
4
5
6

• specifies the number of times that the suite should be executed (3 times in the loop below)
• if an item will not appear in any expression, it is conventional to use underscore (_) in for loop header

for _ in range(3):
    print("Go Nuts")

>>> Go Nuts!
>>> Go Nuts!
>>> Go Nuts!

4. usage: list/tuple constructor

• pass a range constructor into a list constructor to get the half-open range

>>> list(range(5))
[0, 1, 2, 3, 4]
>>> tuple(range(5))
(0, 1, 2, 3, 4)