Why Does Python Exclude the Stop Value in Range() and Slice() Functions?

Python's Exclusive Upper-Bound for Slices and Ranges

In Python, the stop value in both range() and slice() functions is not included in the generated sequence. This design choice stems from several factors.

Simplicity and Consistency:

By excluding the stop value, Python maintains simplicity and consistency in its slicing operations. For instance, s[:2] produces the first two characters of a string s, while s[2:] returns the remaining characters. This exclusivity ensures that slice operations always produce the expected results, regardless of the starting and ending indices.

Parity with C Syntax:

The range() function's exclusive upper-bound is also aligned with the C programming language's for loop idiom. In C, the loop statement for (i = start ; i < stop; i ) { ... } iterates over the values starting from start and ending before stop. Python's range() function replicates this behavior, enabling programmers to transition between the two languages with minimal mental overhead.

Looping Invariants:

The exclusion of the stop value in range() and slice() operations facilitates the creation of useful programming constructs. For example, the following invariant always holds: s[:i] s[i:] = s. This property simplifies certain string manipulation and data processing tasks.

Additionally, the length of a slice or range is calculated as the difference between the starting and ending indices, provided they are within bounds. This convention simplifies the calculation of the size of a sequence when using range() or slice() operations.

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