[lower bound : upper bound : step size]
I- Convert upper bound and lower bound into common signs.
II- Then check if the step size is a positive or a negative value.
(i) If the step size is a positive value, upper bound should be greater than lower bound, otherwise empty string is printed. For example:
s="Welcome"
s1=s[0:3:1]
print(s1)
The output:
Wel
However if we run the following code:
s="Welcome"
s1=s[3:0:1]
print(s1)
It will return an empty string.
(ii) If the step size if a negative value, upper bound should be lesser than lower bound, otherwise empty string will be printed. For example:
s="Welcome"
s1=s[3:0:-1]
print(s1)
The output:
cle
But if we run the following code:
s="Welcome"
s1=s[0:5:-1]
print(s1)
The output will be an empty string.
Thus in the code:
str = 'abcd'
l = len(str)
str2 = str[l-1:0:-1] #str[3:0:-1]
print(str2)
str2 = str[l-1:-1:-1] #str[3:-1:-1]
print(str2)
In the first str2=str[l-1:0:-1], the upper bound is lesser than the lower bound, thus dcb is printed.
However in str2=str[l-1:-1:-1], the upper bound is not less than the lower bound (upon converting lower bound into negative value which is -1: since index of last element is -1 as well as 3).